Pogonophora - Central position in evolution

Pogonophora: Their spectacular role in animal evolution

A spectacular connection of the two main higher animal groups

Early embryology of the two main lines of animals

     The Pogonophora show how the radial and indeterminate cleavage of deuterostomes (echinoderms, hemichordates and vertebrates) came about from pre-Cambrian annelids in the deep sea while other annelids retained the spiral and determinate cleavage of protostomes as they were giving rise to arthropods and mollusks.

     The deep sea had low input of nutrients that put a premium on not investing in heavy egg shells that protected eggs of protostomes as they were confined in the spiral pattern of cleavage, a confinement that was released by less confining outer membranes that allowed a more direct radial cleavage pattern.  At the same time radial cleavage did not produce the immediate cell fate into a particular tissue, thus making possible more than one viable embryo from a single egg as each of the early cleavage cells retained all of the development potential of the original egg.  The ability to complete development without all the original cleavage products had high survival value in the rigorous deep-sea environment, although details are speculative.

    My comparative study of Tasmanian and Michigan isopods included observation of the impact of the egg membrane’s role in confining development, leading to a unique egg appendage, a clear indication that evolution can occur in developmental stages independent of adult development.  (see more on “Origin of deuterostome embryology” in blog post dated 6/24/2013)

Systems inversion

A simple process

The inverted position of blood vessels, nerves, oral openings of deuterostomes as compared to protostomes (especially the ancestral annelids of both groups) was proposed as evidence of the central role of annelids in evolution of higher animal phyla.  The original reason for rejecting the theory was the drastic difference in early embryology of the two lines of animals.  The next reason thought to negate the annelid theory as well as the embryological argument against it was the use of nucleotide and other molecular data.  Such data must be realigned taking into consideration the effect of the astronomically slow rate of genetic change in the pogonophorans ancestors.

Inversion, the first step

The inversion of annelids began with certain polychaetes that began living vertically in tubes they secreted, as seen in Sabella and many other shallow water polychaetes.   As those growing in progressively deeper water, with less food, became dependent on absorption of nutrients in pore water of the sediments they outcompeted those wasting energy on producing a mouth and some bilateral structure.  This stage is still found in abyssal pogonophorans. 

Inversion, the second step

The return of descendants to shallow seas occurred once the worst episode of pre-Cambrian asteroid bombardment eased.  As they arrived in shallower more nutrient rich areas, they reformed the remnants of their digestive system with a new mouth on the former dorsal side which became the ventral side, as they groped around the sediment surface near their tube, finding food particles that pushed the epidermal and gut layer together triggering mouth formation on the former dorsal surface without the restriction of the nervous system that originally had encircled the esophagus.  Other clues to this step are presented by the parallels of endocrine hormone function, transport, and structural similarities of vertebrates, arthropods, and annelids.

Protostome-Deuterostome links

Segmentation/Metamerism

The segmentation of annelid type was found on a short portion of the most deeply embedded part of some pogonophorans; it included setae that are a very annelid like characteristic.  A few anterior regions are noticeable but without the posterior segmented region it would be difficult to make an annelid connection.  The metamerism of chordates such as ourselves seen in bone, muscle, nervous system and blood vessels is now easily understandable with the intermediate stage of pogonophorans.

The transition from pogonophorans to chordates is best shown by the larval stage comparisons of pogonophorans and hemichordates.

Molecular evidence

Molecular features of several types show greater similarity between deuterostomes and advanced protostomes than their earliest variants found in more ancient protostomes once thought to be the closest common ancestors at the protostome-deuterostome split.

DNA/RNA studies of evolutionary relationships at the phylum level need reevaluation because major ones have ignored the mutation rate differences associated with generation times.  Many well focused studies have shown generation time does affect evolution rates.  One impact has been the Pogonophora showing up in many odd places in phylogenetic trees because they are almost unchanged since their divergence from major groups that have diverged even more from more recent relatives.

Six other posts, from June 17 to June 20, 2013 have additional clarification of the points made above.  The second June 30^th post of that year is an annotated bibliography that has some emphasis on protostome/deuterostome comparisons.

Joseph Engemann   

Emeritus Professor of Biology, Western Michigan University, Kalamazoo, Michigan    May 19, 2019

ENVIRONMENT AND NATURAL SELECTION

HOW THE ENVIRONMENT AFFECTS NATURAL SELECTION

The environment makes no conscious effort to select traits of an animal or plant beneficial to itself or the organisms resident in its habitats.  If the location is in a suitable part of the range the animal occupies, it can contribute to survival and reproduction of the animal, especially of genetic variants having the genetic features best adapted to the environment.  An animal may select the environment, although it may not be a conscious act.  Foraging to new areas is less likely to occur when food and housing are abundant so a search does not take the organism to new areas.

Some examples of how the environment passively results in natural selection of animal features you are already familiar with-
      the streamlined shape of fish (and birds), tapered at each end so resistance to movement through water (air) is minimized.  The obvious exceptions, as in baleen whales, occur when feeding or some other benefit is more important in their lives.
     coloration blending with the visual background.  Again, exceptions occur when attracting a mate or warning predators of your toxic properties is more beneficial to survival.
     fur or feathers aiding temperature regulation at the same time providing physical protection such as abrasion resistance, flotation, and a mechanism for seasonal color change.
     appendage modification improving function for climbing (claws), digging, feeding, and locomotion to name a few.

THE LUNG AND BONY SKELETON CONNECTION

A less familiar example, unless  you have a background in evolution and marine biology, is the difference between the cartilaginous and modern bony fishes and the role of air breathing fish in the evolution of bony skeletons.  Survival of fish in anoxic ponds or ones that temporarily dried up, selected fish having more mouth surface able to extract oxygen from water or air.  Ultimately, a pocket becoming the lung of lung fishes, then of amphibians and eventually reptiles, birds, and mammals in one line and from the fishes the early lung was adapted to the air bladder/swim bladder enabling evolution of the modern bony fishes.

It is interesting that the bony fish in the ocean had their origin in fresh water.  Sharks and other cartilaginous fish do not have fresh water fish in their ancestry.  The flotation provided modern fish by the air bladder offsetting the extra weight produced by bone, as compared to the weight of sea water, is partially provided by oil and/or fat stores for cartilaginous fish for excess of weight of bodies heavier than sea water.  Some compensate by continuous swimming providing "lift" from wing-like pectoral fins or body shape.

SUBTERRANEAN AND ABYSSAL SIMILARITIES

The deep sea abyssal region and subterranean aquatic environments have both similarities and differences.  The similarities usually include absence of light, stability of temperature, no green plants, low input of food, low predator density, cool temperatures.

In response to those features of the environment there is no selection eliminating loss of vision, pigmentation, and defensive weapons.  Slender, elongated appendages provide some adaptation for loss of vision,  They may also enable walking on soft, unstable sediment accumulations and sensory input for feeding.

SUBTERRANEAN AND ABYSSAL DIFFERENCES

The abyss
Soft sediments dominate abyssal environments and currents probably cause less erosion and relocation of sediment.  Temperature away from thermal vents are slightly above the freezing temperature of water.  Invertebrate species present are from major groups first found in sediments from older sediments that were in near shore environments (Jablonski et al., 1983;  see references at end of the June 22, 2013 post of this blog).

 This trend to migrate to deeper water and newer related groups evolve in shallower sea areas was not documented for abyssal species but seems to be a reasonable projection.

Temperate Zone Caves
Numerous instances of cave dwelling crustaceans were originally lumped into one or a few species with what were thought to be their closest relatives in other caves.  Later research showed their closest relatives were in surface waters adjacent to the caves; the rapid adaptation to cave dwelling led to similar loss of pigmentation and vision of less value in showing their evolutionary origin in post-glacial times.  This is one example of how rapidly evolution can occur to bring about the loss of characteristics of little use but essential to survival in a different environment.

EXTREME PRESSURE
https://evolutioninsights.blogspot.com/2013/06/evolution-in-deep-sea.html
notes the factors thought to be responsible for slow metabolism and increased longevity in abyssal animals.  I speculate that the slight compression of water at great depths decreases molecular mobility and so called Brownian movement noted with oil immersion microscopic viewing of cells and related particles; this could be a major factor in the slow pace of life and evolution in the abyss.  I have not searched the literature for related research for many years,  One suggestion in an older article about pressure interference with cell activities was the variable rate of compression of organic compounds could stop or reverse the direction of action toward the product most greatly compressed.  If so I would think alternative metabolic pathways might evolve, but oh so slowly under abyssal conditions.

It seems to me that it would be simple for a physicist or chemist with access to equipment with extreme pressure to compare the rate of diffusion from a grain of dye, or the rate of sedimentation of fine particles, or both to a range of pressures. ( see blog noted  above for reference to Yayanos, Dietz, and Boxtel 1979 study of pressure effects on growth)

My nebulous account of factors enabling understanding of the pogonophoran story is, unfortunately, not easily told in a concise way.  The scattered threads that lead to understanding of the story is perhaps inaccessible to many viewing one of the disconnected bits I provide.  It took a lifetime for me to see the things I present.  I hope others do it more easily and find the trip interesting.  If you see what I am attempting to say, I encourage you to point it out to Wikipedia.  You will need to include a few scientific journal references noted elsewhere in this blogsite to have any hope of others resisting the urge to expunge the entry as not being in sync with existing views.  It is normally good, but preserving the status quo can be a major stumbling block to creativity.

Joseph Engemann     Kalamazoo, Michigan      March 4, 2019