GOD AND MLK

MARTIN LUTHER KING, Jr. spoke at Western Michigan University December 18, 1963, fifty years ago today.  I had the privilege of being in the audience.  His message, as always, recommended non-violence in making social changes and the importance and rights of all individuals as children of God.

He must have realized his position exposed him to violent racist attack.  But he continued his mission and message for peaceful eradication of racial discrimination and establishment of racial justice.  When someone gives their life to promote the love God has for all of us, they are considered martyrs and deserving of automatic sainthood.  So, in my mind he is a Saint, although my church has not to my knowledge any policy of declaring sainthood for those of other denominations.

Joseph G. Engemann        December 18, 2013

CREATIVITY: HONESTY HELPS

Honesty was a section in The Two Way Street, my 1970's unpublished manuscript on creativity, Chapter 4, A Creative Frame of Mind.  The section reads as follows-

     Honesty is essential to the maintenance of creativity.  Honesty with others will reduce self-deception.  Honesty does not mean that everything known must be used or told.  Honesty must be coupled with the proper evaluation to be complete.  Accurate evaluation will make it easy to determine the best use.
     When honesty has been practiced so long and with such diligence that it is automatic, judgments and estimates will be realistic.  The resultant accuracy can contribute to the accuracy of derivative hypotheses.
     The defense of honesty should include the careful labeling of humor.  The unrealistic aspect of humor should not blend with the real and confuse, rather it should delight by its contrast.  Thus the choice of humor should be determined by the audience, not the teller.  Perhaps a sophisticated listener can discriminate on a very fine scale and appreciate what would be uniform and bland to the less sophisticated.  So also, honesty is partially determined by the listener, be it self or someone else.  Is it honesty to state something true when we know it is or will be misunderstood?  If we are willing to state something it is pointless to say anything unless we are willing to say it clearly.

A cartoon to amplify the third sentence of the above quotation showed a woman and a man in their backyard, each talking over adjacent sides of the fence to another woman and another man.  The quotation under the cartoon could be attributed to either one talking, it reads "I wish what went in Pat's ear went out the other ear instead of the mouth."

Honesty makes life easier when you don't have to clutter up your mind with lies or misinformation.  It makes it easier to keep from believing what started out as a lie.  It makes it easier to believe others; but evaluating information as it comes in may make it easier to reject untrue information.

A basic faith in the honesty of scientists made me look beyond the conflict of the view Gans and Northcutt present, as noted in blogs 16 and 17, June 23 and 24, 2013, and make the major discovery of how the two main groups of animals are linked.  That view reestablishing the annelid theory of chordate origin has yet to be understood by my peers.

Joseph G. Engemann       November 11, 2013

SCIENCE: WHALES AND CLIMATE CHANGE

A MUST READ BOOK

Coming Climate Crisis? Consider the Past, Beware the Big Fix is a 2010 book written by Claire A. Parkinson.  Chapter 11, Compounding Social Pressures, should be required reading for all scientists and would be scientists as well as those in the publishing and grant awarding fields and members of Congress and their staffs.  It is also an excellent reference for those interested in an objective assessment of the global warming/cooling controversy.

A major social pressure affecting scientific output in some unfortunate instances is peer pressure.  The pressure is not only seen in the peer review outcomes of publication and grant results but in modified behavior by scientists as well.

WHALES AND THE OCEANS AS A CARBON DIOXIDE SINK

While reading her book it occurred to me that a carbon sink aspect exists in the role of sperm whales (see August 31, 2013 blog on the whale’s role in fisheries production) that I did not think of when originally working on the topic in the 1970’s.  It was not part of the discussion in the joint manuscript I was working on with Dr. Patrick Kangas in 1989 (unpublished).  But it could be inferred from the massive amounts of carbon (tens of millions of tons) in the primary production by marine algae resulting from the sperm whale’s ammonia recycling role.  Recovery of sperm whale numbers to pre-whaling levels would more than double the value of the existing marine sink functioning in atmospheric carbon dioxide reduction.

The gradual rise in atmospheric carbon dioxide during the whaling years may have been as much a result of whaling as it was from the industrial revolution.

The climate concerns aside, Parkinson’s book gives remarkable insights into the way science functions.  I also want to thank Dr. Charles Heller for making the book available to me and prompting me to read it.  At the time of writing her book, Dr. Parkinson was climatologist at NASA’s Goddard Space Flight Center.  One focus of her work has been the increase/decrease seen in Arctic and Antarctic sea ice.  If you know Dr. Parkinson, please convey my enthusiastic response to her book.

Joseph G. Engemann      November 4, 2013

EVOLUTION AND STEM CELL MORALITY

STEM CELLS AND MORALITY

Some contend that experimentation with embryonic stem cells violates the commandment prohibiting killing since the embryo destined to be an adult human being is destroyed in the process.  There is some support from evolutionary theory that it is also bad or wasteful science.  Experience in stem cell research also shows that embryonic stem cells have only the slimmest of chances of effecting cures whereas adult stem cells have produced a considerable number of successes.

The evolutionary process whereby organisms were produced involved many developmental steps prior to the adult stem cell production.  Those steps may have been modified in different ways in different species over time.  The same thing may be said for the process of producing the adult organ from the adult stem cell.  But the adult stem cell is programmed to do the right thing in the right place.  The embryonic stem cell is programmed to do everything, but it has been difficult to limit its development to just the job of one type of adult stem cells.  As a result, embryonic stem cell research for cures has mostly resulted in failures, many of which involved induction of tumors or cancers.  The failure is presumably due to the inability to turn off the potential of the embryonic cells in all directions but the desired one.

There is optimism in the recent discovery of methods to produce the equivalent of adult stem cells from some adult cells.  This can be potentially the best stem cell research line for the following reason.  The stem cells produced from this method could come from the individual needing treatment and as a result be free from the danger of rejection of non-compatible donor cells.

For the non-religious or those who have no objections on moral grounds, embryonic stem cell research should not be promoted or receive tax dollars because it is money wasted.  The value all research has for training new scientists and development methods is not unique to embryonic stem cell research and can as readily be had by doing adult stem cell research.

Joseph G. Engemann     October 27, 2013

EVOLUTION AND MORALITY

NATURAL LAW AND SEXUAL REPRODUCTION

Natural law is a foundation for the morality of some people, probably both those that are atheists and those that are deeply religious.  For those that are religious and believe that God created the world and is the author of natural law it would seem that they would find evolution an aide if they realized God is the author of evolution.

I recently started thinking about this topic when a friend of my wife was discussing with her the relative contributions of men versus women to genetic flaws found in offspring.  The friend thought women get blamed for the preponderance of genetic flaws such as trisomy 21 which occurs with a higher frequency when the mother is older.  I wasn't too quick to point out that males always pass on flaws of their X-chromosome to their daughters and of their Y-chromosome to their sons.

Of course, mitochondria are another matter since they are only passed on through the mother's egg.  So pick an energetic mother and don't worry about having a lethargic father if you want to have plenty of energy.  But it is not that simple.

From an evolutionary standpoint, in vitro fertilization is a terrible mistake.  Old-fashioned sex is the best way we have to deliver healthy sperm to an egg.  Aging and/or defective sperm may not have the energy to reach the egg and be the first to donate their chromosomes to the egg.  How important that is can be demonstrated by the higher percentage of males conceived.  Carrying a large X-chromosome must slow them somewhat compared to carrying a small Y-chromosome.  Not all deleterious genes will be expressed in egg or sperm but those that are can perhaps be selected out by the competition of sperm to reach and fertilize the egg.  The sperm also have to have some teamwork by helping to disperse some of the cells surrounding the egg so one can fertilize the egg and immediately cause the fertilization membrane to form and prevent other sperm from entering.  It makes it seem like males participate in team sports from day 1.

So it would seem that the fertility clinics may be unwittingly defeating the natural selection that helps maintain the fertility of the human population.

Who wins the battle of the sexes in a genetic sense?  I have no idea, it seems pretty even to me.  But sex-linked recessive genes behave as dominate genes when in a male.  For that reason red-green color deficiency in vision is many times more common in men than in women.

Joe Engemann     October 22, 2013

SPERM WHALE OIL AND DIVING

OIL and its multiple functions

A large sperm whale can have as much as ten thousand gallons of oil stored in a large organ above the upper jaw and in front of the skull.  The oil was dipped out by whalers of the past several hundred years until whaling was stopped due a great reduction in sperm whale numbers.  Even more oil was stored in the blubber in a thick inner layer of the skin.  The oil was a valuable commodity leading to a major maritime industry in the United States during its developmental years.

ECHOLOCATION, FEEDING, AND COMMUNICATION

Smaller toothed relatives such as dolphins and porpoises have a similar, but much smaller, organ called a melon in a similar position on their heads.  They are thought to function in echolocation by focusing sound waves sent out to produce echoes that enable the sender to identify the objects in their environment.  Some sounds sent out may be used to stun or disorient prey as well.  Baleen whales are better known for their many sounds, but both must be able to use sound for communicating with others of their species over vast distances. The sperm whale is said to produce the loudest sound of any animal.  It is thought to have a small structure, at the rear of the anterior air chamber where the connecting duct to the posterior chamber is located, that is where the sound originates.

ENERGY STORAGE, INSULATION, AND BUOYANCY

The blubber of all the large whales is important for storing energy for long trips away from their feeding areas.  In the sperm whale it serves as insulation when diving into deep waters where temperatures stay only a few degrees above freezing.  Oil in blubber also helps balance the weight of bones to give closer to neutral buoyancy.  The buoyancy provided by air in the lungs and passageways would be greatly reduced by pressure.  It would have about one-half the volume at thirty feet of depth, one-quarter at ninety feet, one-eighth at 210 feet, and less than 1% of the original volume on the deepest dives.

ORIENTATION SENSING

The large oil store in the head of the sperm whale has some connective tissue and two air chambers with a connecting duct that could possibly serve as a mechanism for providing information of orientation in the water more reliably than the minute inner ear mechanism of most other animals. If residual air in the lungs were transferred to the air sacs during deep dives it would provide less vascular surface area for nitrogen to enter the blood.  What really happens is unknown.  But it would seem that the uppermost chamber would have the most air and signal the direction of the surface; a survival mechanism that would be useful to a whale groggy from too long at great depth.  When the chambers are in a vertical position relative to each other, they could shift the center of gravity appropriately to help keep the orientation maintained on course when either diving or returning to the surface as well as aiding the sensory cues involved.

Nitrogen is much more soluble in sperm oil than it is in blood.  So it could help protect from nitrogen bubbles forming when return to the surface occurs from great depths.  The small quantity of air taken down with them may also help.  But exchange is not likely to have enough blood circulated close to the oil as the circulatory system shuts down less essential areas during a dive to make this function of much importance.

The large head oil storage organ bulges like an enormous nose that could also provide some shock absorber protection if they bumped into a rock bottom or submerged objects.  Evolution can lead to multiple functions developing for the same feature.  So energy storage, flotation, echolocation, communication, adjustment of center of gravity, and insulation, and mechanical protection can all be compatible uses for the oil stored in a sperm whale's head.

Sperm whales have a few large conical teeth on the lower jaw that fit in toothless sockets on the upper jaw when the mouth is closed.  The teeth may attract squid by mimicking a school of fish in dim bioluminescence of the deep sea when the lower jaw is open. 

The Great Sperm Whale, a book by Richard Ellis (2011, University Press of Kansas), is a interesting account of the sperm whale’s natural history and the story of the whaling industry from the perspective of an author with a love of whales and service on the International Whaling Commission to the time a moratorium was placed on sperm whale hunting.  Hal Whitehead is one of the more prolific sources of information on sperm whales in recent decades.

Joseph G. Engemann   September 16, 2013

SPERM WHALES CAN AIDE OCEAN FISHERIES

SPERM WHALE INTERVENTION IN MARINE NUTRIENT RECYCLING

Available nitrogen is a limiting factor in marine food chains.  Nitrogen is needed by the algae at the base of open ocean food chains.  Atmospheric nitrogen can be fixed into available nitrogen by some marine microorganisms, but not in quantities needed to support a thriving food chain.  Ammonia or some other form of nitrogen can be recycled in the food chain when it is released by protein metabolism of other organisms.

The majority of the open ocean is like a biological desert because of the limiting factor of low available nitrogen.  What is there can be removed by - sea birds depositing guano on land, commercial and sport fish harvesting, and detritus and dead organisms carrying it to deep water and eventual loss to geologic deposits on the sea bed.

The first two sources of removal are somewhat balanced by return to the sea by rivers containing fertilizer and food nitrogenous components.  That makes coastal waters more productive than the open sea.  The second source of removal  can also be balanced by return due to up-welling currents producing high productivity in some localized regions in response to some cyclic weather induced currents.  Once in the deep sea deposits it is mostly trapped for long periods of geologic time.  But before it is trapped, a significant amount can be returned to surface waters by sperm whale intervention.

Sedimentation of small organic particles in the open ocean is very slow.  They can be recycled in intermediate level food chains before the component nutrients reach the seabed.  One particularly effective avenue for recycling is the intervention of ammoniacal squid that live at considerable depths in the ocean and accumulate ammonia from protein metabolism as a flotation material.  A portion of that accumulated ammonia will be returned to surface waters by sperm whales feeding on those deep water squid as well as giant squid.

The great reduction of sperm whale numbers has probably been a major factor in decline of some marine fisheries.  The reduced growth of phytoplankton diminished both the amount of food to those higher in the food chain and their ability to recover from over-fishing.

Fortunately, very little harvesting of sperm whales is occurring today.  But recovery to former numbers is very slow because so few are left, and their well-being in terms of learned culture passed on socially may have been impaired.  I tried to alert congress and our representative to the United Nations (in the 1970's) of the need to protect sperm whales to save fisheries.  About the same time there developed an international consensus to ban sperm whaling.  But Japan did not join the consensus and continued to harvest some for sperm whale research for a while.  I wrote a paper for a Japanese newspaper competition for submissions on environmental matters, but it did not get accepted.  

I thank Dr. Patrick C. Kangas, then at Eastern Michigan University, later at the University of Maryland, who alerted me to much of the quantitative contribution aspect of sperm whale intervention.  

Living giant squid have recently been shown on television for the first time.  An introduction to some of the data used above can be found in Berzin (1972) and Clarke (1977).

References

Berzin, A. A.  1972.  The Sperm Whale.  Israel Program for Scientific Translations.  374 pp.

Clarke, M. R.  1977.  Beaks, nets and numbers.  Symp. Zool. Soc. Lond., 38:89-126.

Joseph G. Engemann     August 31, 2013.

SPERM WHALES

Whales are impressive animals.  The largest are said to be even larger than the largest dinosaurs.  My specialty deals with mostly very small animals, the invertebrates.  But the largest invertebrate, the giant squid, is a  food item for the sperm whale, the largest of the toothed whales.  During the time when sperm whales were relentlessly hunted for their oil, stomach analyses showed that squid were a major part of their diet.  Perhaps the most abundant squids in their diet were ammoniacal squid which live at great depths and retain high levels of ammonia compounds from their food.  The stored ammonia is thought to serve as a flotation device reducing the squid's need to swim to avoid sinking.

Longevity

Records of sperm whales from whaling days showed the average size was considerably larger than today.  Some of the largest males had deformed teeth suggested greater age than the largest sperm whales of today.  Tooth layering suggests the large ones in recent years approach one hundred years in age.  I think it may have been my search for long-lived animals triggered by my findings about extreme longevity in the deep sea (evolutioninsights 6/22/2013 post) that triggered my interest in sperm whales.

Deep diving

The large males dive deeper and stay down longer than any other mammal.  Perhaps the pressure is an additional factor in their ability to stay submerged for so long in addition to the known factors of high levels of myoglobin for oxygen storage in muscle and circulatory shifts to keep essential organs functioning.

The deep dives go into water only a few degrees above freezing in temperature.  The layer of blubber on the body not only insulates them from the cold, but because fat is much lighter than sea water it balances some of the weight of the heavier than sea water bones.  Much of the volume of the enormous head is filled with oil containing organs which counterbalances the weight of the bones of the skull and jaws.

Baleen whales

The blue whale is the largest whale and is one of the baleen whales that feed extensively on krill.  Krill are small crustaceans, but very numerous in some colder parts of the oceans.  Baleen is the filter on the whale's jaw that sifts out the krill from enormous quantities of water.  The blue whale may be able to select a layer of water where the krill are concentrated.  Some baleen whales are known to concentrate food organisms by circling a cluster until they are in a dense column that they can take much of into their enormous mouth cavity and filter them from the water.

Whales seem most closely related to some ancient hoofed mammals.  They branched off from our evolutionary line millions of years ago.  I will have one or two more posts on aspects of sperm whales not generally covered in discussions of their natural history.  One is their unusually important role in marine food chains, the other is an overlooked function of the oil storage organs in the head.

Joseph G. Engemann   August 29, 2013

EVOLUTION OF MOLLUSKS

The big three

Annelids, arthropods, and mollusks are three major invertebrate phyla, with coeloms and blood vascular systems, that have successfully expanded from probable marine origins into freshwater and terrestrial environments.  Their close relationship was suspected for over a century because of the annelid cross.  The annelid cross was a peculiar relationship of four cells found in at least some of the early embryos of all three phyla.

Annelids and arthropods are easily seen to be closely related because of their obvious metamerism, cuticles, and general relationships of some systems.  But all but some recently discovered mollusks lack obvious metamerism.  So which came first, mollusks or metamerism?

Annelid origin of mollusks

        Ventral view of the external anatomy of Neopilina

The discovery (Lemche an Wingstrand, 1959) of a living member of the Monoplacophora, Neopilina, in 1952 helped answer the question.  It had obvious remnants of metamerism in paired nerves, blood vessels, and muscles.  The single pair of ventricles straddled the posterior portion of the gut and helps illustrate the way the unusual perirectal ventricle of bivalves evolved to enclose that portion of the gut.

McAlester (1964) provided further evidence of the monoplacophoran-bivalve connection with the intermediate fossil, Babinka.  Figure 9-41 (above), on page 479 of

Invertebrate Zoology, 3^rd ed., by Engemann and Hegner (1981) illustrates the transition of muscle scars those of modern bivalves such as the clam, Anodonta.

The fossil studied by Sutton et al. (2001) helped fill the gap between polychaete annelids and mollusks.  They reported it had a polychaete-like body with 7 small calcareous dorsal valves spaced along the dorsal surface.   They named it Acaenoplax hayae gen. nov. and sp. nov. in Phylum Mollusca, and the fossil they “interpret as a plated aplacophoran.”  Structure was determined by computerized reconstruction of serially ground sections.  The only internal structure was a tube in some sections thought to be a gut.  A space may indicate the position of an undeveloped valve.  The posterior 7^th valve has a ventral portion as well as a dorsal portion.  About 18 ridges circling top and sides bear setae and give a polychaete-like appearance in their photos.

   

The molluscan shell as a cause of loss of metamerism

The success of the cone-shaped shell and the muscular foot may well have made the survival value of repeated structures less valuable for survival.  Hence the loss of appendages and segmentation was promoted by the protective shell.  

It is easy to make a transitional series of gastropod shells from limpets to elongated and spiral shells arising from the monoplacophoran type.  From gastropods with siphons to cephalopods with their jet propulsion is less clear but quite likely.

References

Engemann, Joseph G., and Robert W. Hegner.  1981.  Invertebrate Zoology, 3^rd ed.  Macmillan Publishing Co., New York.  746 pp.

Lemche, H., and K. G. Wingstrand.  1959.  The anatomy of Neopilina galatheae, 1957.  Galathea Report, 3:9-72. +56 Pl.

McAlester, A. Lee.  1964.  Transitional Ordovician bivalve with both monoplacophoran and lucinacean affinities.  Science, 146:1293-1294.  Babinka has muscle scars intermediate between the monoplacophoran, Neopilina, and modern bivalves.   

Sutton, Mark D., Derek E. G. Briggs, David J. Siveter and Derek J. Siveter.  2001.  An exceptionally preserved vermiform mollusc from the Silurian of England.  Nature, 410:461-463.

Joseph G. Engemann   August 3, 2013

ACOELOMATE EVOLUTION, 3, FLATWORMS

Flatworms: planarians, flukes, tapeworms

The above sounds less pretentious than Platyhelminthes: Turbellaria, Trematoda, and Cestoda.  For purposes of showing the ancestral links from protozoa through sponges, corals, jellyfish, flatworms and an unknown intermediate (probably a ribbon worm) link to annelids then deuterostomes, the list is more comprehensible.  Side shoots from the ancestral line to chordates leave far more passed than are listed. 

Planaria

Planarians are the turbellarians most like the ancestral link of interest.  Flukes and tapeworms are of interest too, but are all parasites and neither is a link to any other major group.  The common planarian has been an example of flatworms for many students because it is abundant world-wide. 

Marie Jenkins (1963) made an observation of planarians that may be instructive.  She found ones that were cultured in a slippery container could not divide by pulling themselves apart to regenerate two new planarians.  They just kept growing longer until they eventually formed a second head at the distant tail end.  Apparently, the head releases some chemical messenger that inhibits head formation until it is too far away to be in an effective concentration.

Other turbellarians much smaller than planaria have an anterior mouth, are not flattened, and have new mouths and a fission plane develop before separation.  The preparations for division can produce a chain of connected potential individuals up to 16 in number.

Possible descendent groups

Small flatworms may have given rise to Aschelminthes, including a branch becoming the nematodes and another branch, the rotifers.  An intermediate group, the Gnathostomulida, have some flatworm characteristics, and like most aschelminths are adapted to life in sediments of the sea and freshwater. 

The central selective action shaping the aschelminths was their adaptation to the interstitial water (water filling the spaces between sand grains and other small particles of the bottom of aquatic habitats and beaches) where small size enabled their movement while excluding slightly larger predators.  Besides their minute size similar to large protozoans, they often have a forked posterior with each short branch having adhesive glands.  In the nematodes the posterior toes are missing but some have a pair of gland cells and can attach temporarily to the substrate.  Many parasitic nematode species are much larger than their microscopic free-living relatives.

A feature of aschelminths that make it very unlikely they were ancestral to any mainline animals is the fact that they lack the ability to regenerate, probable because they adapted to miniaturization by reduction of chromosomal material as cells of the embryo differentiate into the adult.  A specific number of cells and or nuclei are found in adults of some smaller aschelminths.  Nematodes are unusual in lacking cilia; having only longitudinal muscles in the body wall; and having those muscles enervated by muscle cell processes (the processes lack muscle fibers) that reach either the dorsal or ventral nerve to receive the nerve impulses.  Rotifers are very numerous in lakes and their sediments and beaches.  Pennak (1978) describes in his introductory material the importance of the interstitial habitat as a route for some smaller organisms as they adapt and invade fresh water.

And the likely link

The nemerteans are thought to be descendants of flatworms also because some of them have rhabdites in their ciliated epidermal cells.  But nemerteans, like most aschelminths, have added an anal opening to the digestive system.  Nemerteans also have a blood vascular system so it is possible they were part of an ancient complex derived from flatworms that served as intermediates on the way to annelids at an early stage in the evolution of higher animals.  The living nemerteans do not have clear evidence of an ancestral role, but they are most representative of living animals approximating an intermediate form.  In my unpublished 2010 manuscript, Evolution Insights, I refer to the putative ancestor as a protonemertean.

The hypothetical protonemertean may have used a central branch of the turbellarian gastrovascular system to complete the digestive system with the posterior opening never disappearing after the individuals divided.  Lateral branches of the gastrovascular system may have lost their connection to the gut and become modified into blood vessels.  The benefit of a long body may have been the selective force keeping individuals attached as they evolved coordination as one organism, becoming the annelid worm central to the remaining major phyla evolution.

The soft body of this step in evolution may not have left a fossil record.  The steps along the way may not have anatomically instructive living descendants.  The answer may be in carefully targeted molecular phylogeny studies.  I don’t expect to have another eureka event like the one that made me see the pogonophorans were the missing link between annelids and the deuterostomes.

References

Jenkins, M. M.  1963.  Bipolar planarians in a stock culture.  Science, 142:1187.

Pennak, R. W.  1978.  Fresh-Water Invertebrates of the United States. 2^nd Ed.  Wiley, New York.

Joseph G. Engemann    July 22, 2013      minor editing November 7, 2014; also this note that the features noted in the post about nematodes, almost certainly precludes nematodes from an ancestral role in the Ecdysozoa, as also noted in other posts. 

ACOELOMATE EVOLUTION 2 CNIDARIANS

Cnidaria: corals, anemones, hydroids, jellyfish

The Cnidaria are characterized by production and use of nematocysts.  The nematocysts are complex organelles contained in certain cells of cnidarians.  The previous post suggests how they may have developed from modifications around the spicules inherited from ancestral sponges. 

Two main body types are found in the sexual stage of cnidarians.  The polyp or hydroid body type is the original type if the findings of Kazmierczak (1984) are accepted.  Generations of biologists assumed something like the simple hydra was one of the earliest cnidarians.  The new evidence makes an extinct coral the likeliest candidate for the ancestral origin of cnidarians.  Corals and other in the class Anthozoa have the polyp stage predominating, and no medusa (or jellyfish) stage.  Those in the class Hydrozoa usually have both polyp and medusa stages.  In the class Scyphozoa the jellyfish stage dwarfs other stages.  The medusa stage is the sexual adult stage in cnidarians having a medusa. 

Tentacles, furnished with many nematocysts, and partitions or tubes in the digestive cavity (gastrovascular cavity) in considerable variety are often part of structural diversity of cnidarians.  The medusa stage was recognized by early biologists as having a very similar structure to an inverted polyp stage.  So, thinking of a coral-like polyp stage as the starting point in pre-Cambrian seas, it is easy to imagine an early extinction event making life for the coral so difficult that its polyp, released from the coral skeleton, managed to survive a marginal extinction event to preserve genes enabling such a release.  Ultimately, repeating the process eventually produced forms with medusae dominating the life cycle.

Besides the microscopic features such as the spicule-nematocyst connection, gross features providing a base for determining selection for the sponge-cnidarian transition involved a number of events. (1) The attached bottom dwelling lifestyle was conducive to retaining radial symmetry.  (2)  The upward facing osculum of the sponge provided an opening for gradual evolution of a mouth and transition of the spongocoel to a gastrovascular cavity as adaptations for acquiring larger particulate food developed.  (3) Spicule deposition shifted from the generalized sponge skeletal elements to the external cup-like coral skeleton.  Adaptations for muscles, nerve, and other new and useful soft structure elements optimized for size made vegetative growth of colonies by budding a suitable solution.

Extinct tetracorals were common early fossil corals.  Prior to or along with their radiation into the vast range of anthozoan hard corals, soft corals, and anemones, it is likely that they led to the hydrozoan medusae that were the ancestral hydrozoans.  The four radial gastrovascular canals and related parts may be due to the square cups of tetracoral skeletons affect on selection/development of soft parts.

The hydra is one of the hydrozoan polyps.  Several nematocyst types are found in hydrozoa, about four kinds in hydra.  Hydra is specialized for fresh water existence by loss of the medusa stage.  The freshwater jellyfish retained the medusa stage but the polyp stage does not have tentacles; their polyps bud from a connection in the sediment, some polyps bud off medusae but most have a mouth for feeding.  Similar hydrozoan medusae are found in salt water species.

Tracing vertebrate roots through cnidarians

Just as sponges underwent much diversification after giving rise to cnidarians, cnidarians gave early rise to the forerunner of the flatworms.  The prevailing opinion that phylum Cnidaria begin at a rudimentary stage is incorrect.  The structure of the hydrozoan medusa needed relatively little modification beyond elongation to produce the ancestor of the common planarian.  The centrally located manubrium of the jellyfish is positioned similarly to the proboscis of planaria.  The four branches of the gastrovascular cavity are reduced to the three in the planaria; the fourth was eventually lost due to inability to develop in the compressed space above the proboscis.  The sensory complexes were lost in all locations except where the head developed at the anterior or forward branch of the gastrovascular cavity. 

Hadzi (1963) noted the similarities of flatworm and medusa and proposed the flatworms as intermediate between protozoans and cnidarians, a view that has not been accepted.  Ax (1963) seemed to think the long evolutionary history of existing groups precluded any from being ancestors of any other major group.  But successful adaptations, in my opinion, could very likely persist as their variations give rise to great diversification and other phyla.  The spin-off of new groups was easier before adaptations became well fixed or stable.  Such a series of events in the early history of animal diversification seems more compatible with the Cambrian “explosion” of animal groups.

As the jellyfish body flattened and elongated for bilateral and mobile life as a flatworm, the outer longitudinal muscle fibers and inner circular muscle fibers are now known and positioned as outer circular and inner longitudinal muscle layers.

The nematocysts took on more degenerated and/or restricted function as the rhabdites of the flatworm epidermis.  Other developments added to the complexity of flatworms.

References

Ax, P.  1963.  Relationships and phylogeny of the Turbellaria.  Pp. 191-224 in E. Dougherty.  The Lower Metazoa.  Univ. of California Press, Berkeley.  478 pp.

Hadzi, Jovan.  1963.  The Evolution of the Metazoa.  Macmillan, New York.  499 pp. 

Kazmierczak, Jozef. 1984.  Favositid tabulates: evidence for poriferan affinity.  Science, 225:835-837. 

Joseph G. Engemann    July 19, 2013  

ACOELOMATE EVOLUTION, 1, SPONGES

The evolutionary lineage of our post-protozoan ancestors can be followed through three phyla that left many existing related groups.  The related groups diversified into forms, most of which are not in our ancestral line.

PORIFERA, THE SPONGES

The first post-protozoan phylum is the sponges.  They are so different from other animals that many have thought they were an evolutionary dead end or side-shoot that was not in the mainstream of evolution.  The peculiar amphiblastula larva of some sponges was so different that it seemed to preclude them being in the mainstream.  But Bergquist (1978), in her book on sponges, illustrates a wide variety of sponge larvae, some of which have great resemblance to the planula larvae of some cnidarians.  The mainstream position of sponges was proposed by Tuzet (1963).

If we do not consider the sponges as in the mainstream, it would be necessary to postulate a similar organism as an intermediate form between protozoans and cnidarians.  For more on the intermediate nature of sponges you can find some discussion in the introduction to sponges in the second (1968) and third (1981) editions of Invertebrate Zoology which I edited.  I later found research describing spicules in nematocysts of some cnidarians and it made sense in terms of a spicule, nematocyst, and rhabdite transition found in sponges, cnidarians, and flatworms.

Spicules

Spicules, both calcareous and siliceous ones, found in an early fossil anthozoan (Kazmierczak, 1984) show cnidarians are most likely derived from a sponge most like the Sclerospongea which also have both types of spicules.  The Calcarea, but not the Hexactinellida or Demospongea, may have had an earlier ancestral role, but it is just as likely a primitive member of the Sclerospongia was in that ancestral role.  Hexactinellida have only siliceous spicules, Demospongea have only siliceous spicules if spicules are present (bath sponges in this group lack spicules).

As sessile (attached) animals, sponges were dependent for survival on the selection of things that made them unpalatable to new and mobile predators.  A diversity of toxic substances are found in today’s sponges.  Spicules also may deter predators.  Spicules also served as skeletal elements preventing collapse of the canal systems essential to sponge growth to larger forms.

Protruding spicules increased the sponge’s ability to passively deliver adhering toxins.  Improvements in partially enclosing the spicule in a tube of poison allowed selection for increasingly more effective delivery as the evolution to cnidarians continued.  The nematocysts of most cnidarians may have lost the need for the spicule once the tubular delivery system for poison was effective.

Nutrition

Sponge nutrition involves trapping of small food particles by the collar of the flagellated cells lining some of the chambers of the sponge.  The particles are taken into the cells for digestion.  Water transporting the food particles passes in through small openings (the ostia) on the surface and passes through the chambers with the flagellated cells before exiting through a larger chamber and a large opening, (the osculum).  Sponges may have symbiotic, photosynthetic microorganisms in their tissues.  These were especially important for nutrition during the early evolution into cnidarians before the capacity for feeding on larger living organisms developed. 

Other features

As indicted in a previous post, sponge tissue structure is much like loose connective tissue of vertebrates.  Rudimentary muscular and nervous cellular structure has been observed.  Sexual reproduction is present and eggs produce ciliated or flagellated larvae.  Asexual reproduction occurs in several ways.  Internal buds are produced by some sponges, fragments can regenerate new sponges, and bath sponges can regenerate from a portion left attached to the sea bottom.

The sponges of today are mostly specialized in ways that do not indicate the ancestral position of an unknown ancient member of the Sclerospongea, already specialized in its own way.  Living Sclerospongea were discovered in the last half of the last century; they live hidden away in cavities in coral reefs and are presumably quite different from the ones that evolved into cnidarians.

After listing seven points supporting the ancestral role of sponges, but before the 1984 report by Kazmierczak, I made the following statement in the 1981 invertebrate text:  “In view of the preceding it is reasonable to anticipate further evidence of an important phylogenetic position for ancestral sponges with hopes for clarification or resolution of this issue.”  The discovery of a spicule at the apex of some nematocysts is further new evidence.  The conclusions presented are based in part on hypothetical interpretations of the role of spicules in the absence of any other reasonable hypothesis.

References

Bergquist, P. R.  1978. Sponges.  Univ. of California Press, Berkeley.  268 pp.

Engemann, J. G., and R. W. Hegner.  1981.  Invertebrate Zoology, 3^rd ed.  Macmillan, N. Y.  746 pp.

Kazmierczak, Jozef. 1984.  Favositid tabulates: evidence for poriferan affinity.  Science, 225:835-837.
Tuzet, O.  1963.  The phylogeny of sponges according to embryological, histological, and serological data, and their affinities with the Protozoa and the Cnidaria.  pp. 129-148.  In E. Dougherty, The Lower Metazoa.  Univ. of California Press, Berkeley.  478 pp.

Joseph G. Engemann     July 18, 2013

EVOLUTION PROTOSTOME-DEUTEROSTOME LINK

EVOLUTION – ANNELID THEORY BIBLIOGRAPHY a supplement to earlier post today.  

I will not feel bad if you don’t read this, even if you are a biologist.  If you have a specific interest in annelid theory of chordate origin, the references may be helpful.

The references below are some that I found useful in understanding the evolution of the animal phyla, especially as pertains to the annelid theory.  They tend to focus on molecular aspects if I thought they shed light on the problem.  The bibliography is a partial one and could have been greatly expanded if for example, I included all those that did not make a link to both protostomes and deuterostomes [Akam, Michael, 1998, Biol. Bull., 195:373-374 deals with shifts in Hox gene expression in segments during evolution of arthropods] or my database was defective [a 1978 invertebrate collagens article in Science, 202:591-598 has an obvious defect in the author’s name(s)].  Since disappointment with the defects [noted in my 5/31/13 post] of some phylogenetic studies over a decade ago, I have not been very attentive to subsequent reports.

Arendt, D., and K. Nübler-Jung.  1994.  Inversion of dorsoventral axis?  Nature, 371:26. 

Arendt, Detlev, Ulrich Technau, and Joachim Wittbrodt.  2001.  Evolution of the bilaterian larval foregut.  Nature, 409:81-85.

De Robertis, E. M., and Yoshiki Sasai.  1996.  A common plan for dorsoventral patterning in Bilateria.  Nature, 380:37-40.

Eakin, Richard M. 1979.  Evolutionary significance of photoreceptors: in retrospect.  Am. Zool., 19:647-653.    Fig. 1 shows great similarity of photoreceptors of annelids and cephalochordates although he puts them near the tips of separate lines

Field, Katharine G., Gary J. Olsen, David J. Lane, Stephen J. Giovannoni, Michael T. Ghiselin, Elizabeth C. Raff, Norman R. Pace, and Rudolf A. Raff.  1988.  Molecular phylogeny of the animal kingdom.  Science, 239:748-753.  “Coelomates are thus monophyletic, and they radiated rapidly into four groups: chordates, echinoderms, arthropods, and eucoelomate protostomes.”

Gardiner, Stephen L., and Meredith L. Jones.  1985.  Ultrastructure of spermiogenesis in the vestimentiferans tube worm Riftia pachyptila (Pogonophora: Obturata).  Trans. Am. Microsc. Soc., 104(1):19-44.

Gould, James L.  1985.  How bees remember flower shapes.  Science, 227:1492-1494.  “presumptive vertebrate-invertebrate dichotomy is false”  bees use search and memory process similar to vertebrates.

Lull, Richard Swann.  1945.  Organic Evolution.  Macmillan, New York.  744pp. [Revised edition, 1929, earlier 1917] a paleontologist at Yale.  Fig. 123, page 429 after Wilder of annelid theory of vertebrate origin

Meurling, Patrick.  1967.  The vascularization of the pituitary in elasmobranchs.  Sarsia, 28:1-104. 

Miyamoto, Michael M., Jerry L. Slightom, and Morris Goodman.  1987.  Phylogenetic relations of humans and African apes from DNA sequences in the ψη-globin region.  Science, 238:369-373.  “. . the slowdown in the rate of sequence evolution evident in higher primates is especially pronounced in humans.” 

Moore, Richard C., and Michael D. Purugganan.  2003.  The early stages of duplicate gene evolution.  Proc. Natl. Acad. Sci. USA, 100:15682-15687.  “Gene duplications are one of the primary driving forces in the evolution of genomes and genetic systems.”

Pellettieri, Jason, and Geraldine Seydoux.  2002.  Anterior-posterior polarity in C. elegans and Drosophia-PARallels and differences.  Science, 298:1946-1950. “par” genes important in polarization for C. elegans embryo; homologs were discovered in mammals, this study looks at fruit fly

Peterson, Kevin J., Steven Q. Irvine, R. Andrew Cameron, and Eric H. Davidson.  2000.  Quantitative assessment of Hox complex expression in the indirect development of the polychaete annelid Chaetopterus sp.  Proc. Natl. Acad. Sci. USA, 97:4487-4492.  they found a similar Hox complex utilization in cells for adult body plan in the polychaete to that process described in the sea urchin

Ritzmann, Roy E., Martha L. Tobias, and Charles R. Fourtner.  1980.  Flight activity initiated via giant interneurons of the cockroach: evidence for bifunctional trigger interneurons.  Science, 210:443-445.  “command or trigger interneurons have been identified . . . .  including annelids, arthropod, mollusks, and turtle (1).” 

Romer, Alfred Sherwood.  1962.  The Vertebrate Body, 3^rd edition.  Saunders, Philadelphia.  627 pp.    Illustration of annelid theory of chordate origin is on page 25 (same as Lull one but different caption) see pages 298-299 for transition in position of structures leading to pituitary – p. 298

Ruppert, Edward E., and Elizabeth J. Balser.  1986.  Nephridia in the larvae of hemichordates and echinoderms.  Biol. Bull., 171:188-196. 

Sarnat, Harvey B.  1984.  Muscle histochemistry of the planarian Dugesia tigrina (Turbellaria: Tricladida): implications in the evolution of muscle.  Trans. Am. Microsc. Soc., 103(3):284-294. 

Schwenk, Kurt, and Günter P. Wagner.  2001.  Function and the evolution of phenotypic stability: connecting  pattern to process.  Amer. Zool., 41:552-563. 

Smith, Peter R., Edward E. Ruppert, and Stephen L. Gardiner.  1987.  A deuterostome-like nephridium in the mitraria larva of Owenia fusiformis (Polychaeta, Annelida).  Biol. Bull., 172:315-323.

Southward, Alan J., and Eve C. Southward.  1982.  The role of dissolved organic matter in the nutrition of deep-sea benthos.  Amer. Zool., 22:647-658.  

Stein, Elizabeth A., and Edwin L. Cooper.  1983.  Inflammatory responses in annelids.  Am. Zool., 23:145-156.  inflammation of vertebrates and annelids shows related factors – histamine, agglutinins, lysins, etc.  also have amoeboid phagocytic cells   

Stoichet, Sarah A., Talat H. Malik, Joel H. Rothman, and Ramesh A. Shivdasani.  2000.  Action of the Caenorhabditis elegans GATA factor END-1 in Xenopus suggests that similar mechanisms initiate endoderm development in ecdysozoa and vertebrates.  Proc. Nat. Acad. Sci., USA, 97:4076-4081.

Terwilliger, R. C., and N. B. Terwilliger.  1987.  Are pogonophoran and annelid extracellular hemoglobin structures similar to one another?  Am. Zoologist, 27(4):32A, abstract #152.  Yes for Vestimentifera which also have a smaller Hb similar to one found in Perivata 

Tiplady, Brian, and Morris Goodman.  1977.  Primitive haemoglobin.  J. Mol. Evol., 9:343-347.  “The variations in nucleotide substitution rates were interpreted in terms of Darwinian selection, the emergence of a new function being followed by a rapid rate of evolution, which then slows down once the molecule has been optimized.” 

Tomarov, Stanislav I., Patrick Callaerts, Lidia Kos, Rina Zinovieva, Georg Halder, Walter Gehring, and Joram Piatigorsky.  1997.  Squid Pax-6 and eye development.  Proc. Natl. Acad. Sci. USA, 94:2421-2426. (March 1997)  “Pax-6 in vertebrates and its homolog eyeless in Drosophila are known to be essential for eye development.”

Wagner, Gunte P., Chris Amemiya, and Frank Ruddle.  2003.  Hox cluster duplications and the opportunity for evolutionary novelties.  Proc. Natl. Acad. Sci. USA, 100:14603-14606.  “Hox genes play a key role in animal body plan development.  These genes tend to occur in tightly linked clusters in the genome.  Vertebrates and invertebrates differ in their Hox cluster number, with vertebrates having multiple clusters and invertebrates usually having only one.”

Joseph G. Engemann    June 30, 2013

EVOLUTION - SINGLE ORIGIN OF COELOMATES

MONOPHYLETIC ORIGIN OF COELOMATES

Segmentation as a starting point

Annelids are the animals most like the ancestral form of all coelomates.  Their segmentation enabled the speedy evolution of diversity.  That diversity includes forms that show little evidence of the remnants of segmentation.  Metamerism, of chordates and some other phyla, is the repetition of organs or structure along the length of a body that no longer has segmentation.

The serial repetition of structures enables regional modification of appendages and other structures from a relatively complex base with minimal genetic changes.  This is illustrated most clearly by arthropod appendages.  It is evident in our own pectoral and pelvic appendages, although not so obviously having an early stage of origin in segmentation.  Our early embryo has some of the more convincing evidence in myotomes; adults have vertebrae with paired nerves, blood vessels, and muscles that may also be convincing.

Reduction of segmentation in the chordate line was a result of its loss by pogonophorans in the portion of the body retained in hemichordate evolution.  Most mollusks lost evidence of metamerism as a result of the shell removing benefits of obvious metamerism. A vermiform fossil shows evidence of the annelid origin of mollusks (Sutton et al. 2001); and a fossil, Neopilina, shows a more molluscan intermediate stage (Lemche, 1957).   Arthropods have striking evidence of metamerism externally in most groups, both with skeletal segmentation and appendages

Just as segmentation was modified beyond easy recognition, the coelom was also greatly modified as higher animals diversified.  Coeloms, found in both the protostome line and the deuterostome line, but not in animals earlier than annelids, provide evidence of the annelid origin of coelomates.

Nephridia

Water and salt regulation is an important function of our kidney.  The annelid nephridium, with pairs in most of their segments, was modified by evolution into the nephrons of our kidneys; an intermediate connection is shown by Ruppert and Balser, 1986.  The oviduct also seems to be a nephron modification.  The water-vascular system of echinoderms may be derived from nephridia; the water-vascular system stays open to the exterior, indicating the echinoderms have probably lost the benefits of osmoregulation and thus never were able to survive in fresh water.

Blood

The central position of annelids in the ancestry of coelomates may be illustrated by blood pigments.  Polychaete worms have as many as four different oxygen transporting pigments in the blood.  Higher forms typically have one of those pigments, in our case, hemoglobin.

Other Molecular Evidence

Once I thought the Pogonophora were the deuterostome connection to protostomes (1983), I found a lot of supporting molecular evidence.  One of the first was the finding of Lipman and Pearson (1985) that crayfish Trypsin I is high-scoring for similarity to bovine trypsinogen.

The interpretation of molecular evidence is complicated by the fact that vertebrates tend to have some important genetic features in family clusters of genes whereas other animals typically have only one from a family.  Also, the same molecules may act in somewhat similar but different ways in different groups.  Hobmayer et al. (2000) found the WNT signaling pathway that had been found in nematodes, insects, and vertebrates (all with bilateral symmetry) also acted in axis formation of a radially symmetric cnidarian.

The protostome-deuterostome transition via pogonophorans may be returned to at some later date.  I expect to end the topic with my next post of a few additional references, some annotated, from my reference file that deal in some way with the topic, most about molecular evidence.

In summary, a wide variety of evidence points to the basic truth of the annelid theory of chordate origin.

References

   Hobmayer, Bert, Fabian Rentzsch, and 6 others.  2000.  WNT signaling molecules act in axis formation in the diploblastic metazoan Hydra.  Nature, 407:186-189.

   Lemche, H.  1957.  A new living deep-sea mollusk of the Cambro-Devonian class Monoplacophora.  Nature, 179:413-416.

   Lipman, David J., and William R. Pearson.  1985.  Rapid and sensitive protein similarity searches.  Science, 227:1435-1441.

   Ruppert, Edward E., and Elizabeth J. Balser.  1986.  Nephridia in the larvae of hemichordates and echinoderms.  Biol. Bull., 171:188-196.

   Sutton, Mark D., Derek E. G. Briggs, David J. Siveter and Derek J. Siveter.  2001.  An exceptionally preserved vermiform mollusc from the Silurian of England.  Nature, 410:461-463.

Joseph G. Engemann    June 30, 2013

EVOLUTION of systems inversion

EVOLUTION – ORIGIN OF DEUTEROSTOME SYSTEM INVERSION

Several major steps were involved in the inversion of systems as polychaete annelids gave rise to the chordate line of deuterostomes. 

First a branch of tube-dwelling, bottom-dwelling polychaetes evolved with some reduction of clear segmentation; development of a plume of tentacles for feeding and/or respiration probably was occurring as well.  A variety of marine species having a similar intermediate condition still exist.

The second major step involved the complete adaptation to life in abyssal sediments as pogonophoran worms.  This included (1) a loss of dorso-ventral distinctions common to species ancestral to groups with radial symmetry, (2) reduction of development of the gut, (3) increased reliance on passive absorption of nutrients, and (3) retention of the circulatory system to provide oxygen to the portion of the worm embedded in low oxygen sediments. 

The embryological changes noted in the previous post were occurring simultaneously with these changes.  They now are the pogonophorans, well-adapted to survive the extinction events of pre-Cambrian times.  The major feature identifying them as annelid descendants was the extreme lower segmented and setae bearing portion that was not noticed in specimens of early collections made by dredges that did not retrieve whole worms; the deeply embedded part presumably was left in the ocean bottom.

The third major step in the protostome-deuterostome journey was the sequence of changes as descendants moved to shallower seas during the period following the intense asteroid bombardment (see May 11 post).  Those moving from their tubes to reach particulate food more abundant on shallower sea sediments found it less jarring to the nervous system to emerge with the previous ventral nervous system of the annelids positioned so it was nearer the upper surface.  In such a position, a remnant of the gut, perhaps more substantial because of the greater abundance of food, put endodermal and ectodermal tissue closer together to induce the mouth formation associated with such an event.

A further consequence of this new position of the mouth enabled fusion of ganglia and connectives to form a brain without encircling the esophagus.  These and other changes above were facilitated by natural selection of those with genetic modifications better serving the processes.

The culmination of this process needed very little fine tuning to make a hemichordate, probably the closest annelid derivative to the chordate line of deuterostomes.  The larval stages of pogonophorans and hemichordates are very similar.  Correspondence of the anterior of a hemichordate and the upper portion of a certain pogonophorans is quite similar.  The lowest segmented portion of the pogonophoran degenerated to leave three body regions some think are characteristic of chordates.  Previous discussions indicated the speed of loss of features not contributing to survival by the greater energy left over for reproduction is a common feature in evolutionary events.

Clearly, pogonophorans are excellent candidates for the missing link clarifying the inversion of systems suggested by the annelid theory of chordate origin.  

BRING BACK THE ANNELID THEORY.

Joseph G. Engemann    June 28, 2013

EVOLUTION - THE PROTOSTOME-DEUTEROSTOME LINK

ORIGIN OF DEUTEROSTOME EMBRYOLOGY

Annelid theory as a working hypothesis

Since Gans and Northcutt (1983) provided evidence of a close relative of annelids having some features of development resembling deuterostomes, as noted in the previous post, it is reasonable to evaluate other evidence.  Inversion of systems, the primary evidence supporting the annelid theory, has been deemed inadequate by those who comment.  But much other evidence is available.  The biggest impediment had been the drastic embryological differences between protostomes and deuterostomes.

Embryological evidence

My doctoral thesis research included a comparison of development of two species of isopods with very different life cycle rates of development as well as a new embryological structure in one species.  The rapidly developing egg of the Michigan species was smaller, had a thinner egg shell, and two appendages on the egg.  The Tasmanian one had a thicker egg shell and no appendage; but on the each side of the embryo within the egg was a yolk filled bulge in the position from which the other ones had their egg appendages develop.

Otherwise, both embryos packed the egg fully.  They both developed in a folded position, legs outermost.  But the flattening differs so the Tasmanian one filled up space with the yolk filled bulge.  The Tasmanian species has less change from ancestral crustacean features; they also lack the abundant source of food from deciduous tree leaves as available for the Michigan ones.

The main point of this isopod egg observation is that the evolution of a new feature in the egg goes counter to what many biologists think was perhaps a valid portion of the discredited “biogenetic law”.  The law is not absolute, especially as my observation showed me, evolution can occur by the development of new features in the earliest life stages of an organism.  Clearly, embryological stages do not faithfully repeat steps in the evolution of the organism. 

Since the pogonophorans are likely candidates as intermediates, in spite of the general opinion that they were an evolutionary dead-end, what do they contribute to the story?  Well, they have lost the annelid digestive system in the adult, their segmentation has nearly disappeared, and they live in an abyssal world with a very low rate of input of food.  Such a regime would drastically select options or mutations that save energy. 

Why protostome spiral cleavage became deuterostome radial cleavage

The thinning of the egg shell would not constrain the early dividing cells into the packed spiral pattern of ancestral protostomes but it would conserve resources otherwise used for a strong egg shell.  Consequently, the loss of structural integrity of the egg shell would not impose the constraints for efficient use of space as in spiral cleavage.

Limited energy and resulting low reproductive potential puts survival at a premium for the individual.  Thus, although the first cell divisions (cleavage) of protostome eggs end the potential of the daughter cells to each develop into individuals, it is possible for each of the early dividing cells of the deuterostome egg.  Injury or death of one of the first few cells of a deuterostome egg would not necessarily result in death.  In fact, identical twins, triplets, and other genetically identical individuals could not have developed if deuterostomes had retained the features of spiral cleavage.  The survival advantage of this feature for pogonophoran species in the nutrient poor abyss should be an obvious benefit.

In both cases it is a loss, loss or reduced production of shell, loss of control of early developmental specification.  As discussed in an earlier post, loss can occur more rapidly than gain of a feature.  The rates are relative to other factors such as food supply, generation time, and value of the features for survival.  But clearly, pogonophorans are excellent candidates for the missing link connecting embryological features of protostomes and deuterostomes.  The conclusion is hypothetical.  The event described was undoubtedly a Pre-Cambrian occurrence.  But the conclusion is based on comparative evidence consistent with similar conclusions that will be presented for other evidence.

Interesting, but somewhat irrelevant to the current discussion is the fact that the Michigan isopod egg appendage had the cellular appearance of adult respiratory tissue and must aide their relatively rapid development.

References

Engemann, Joseph G.  1963.  A Comparison of the Anatomy and Natural History of Colubotelson thomsoni Nicholls, a South Temperate, Fresh-water Isopod and Asellus communis Say, a North Temperate, Fresh-water Isopod.  Ph.D. Thesis, Michigan State University, East Lansing.  146 pp.

Gans, Carl, and R. Glenn Northcutt.  1983.  Neural crest and the origin of vertebrates: a new head.  Science, 220:268-274.

Acknowledgements

A United States Fulbright Grant for study in Australia, aide of staff and use of facilities at the University of Tasmania and Michigan State University, as well as a Faculty Research Grant at Western Michigan University, were instrumental in my making many observations involved in this series of blogs.  Many individuals deserve my thanks as well.

Joseph G. Engemann     June 24, 2013