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

ANIMAL KINGDOM EVOLUTION

THE MAJOR GROUPS

The roots of the animal kingdom and other kingdoms are closely intertwined prior to the origin of multi-cellular plants and animals.  We think the earliest organisms are still represented today by the bacteria and other forms lacking a nucleus in their membrane-enclosed selves.  During this stage, perhaps the first billion years of evolution, the basic biochemistry of life evolved.  The RNA, DNA, and much of the basic materials still found in subsequent organisms evolved.

A consequence of the development of photosynthetic organisms in the world, then lacking oxygen in the atmosphere, was the production of oxygen as a toxic waste product that accumulated and changed the biosphere for the remaining time on earth.  Some of the early organisms developed the ability to utilize oxygen to oxidize organic material for their energy.  They could then remain active in the absence of light while extracting more energy from food than was possible by anaerobic process alone.  

Organisms that protected their genetic material from the oxygen with a nuclear membrane could better survive as oxygen reached higher levels.  Some developed a symbiotic relationship with other organisms.  Details of these early steps are discussed by Lynn Margulis (1981, Symbiosis in Cell Evolution, W. H. Freeman and Co., New York).  The evidence that mitochondria of our cells are a result of symbiosis is very strong; perhaps cilia are derived from flagella that also came from a similar symbiotic origin.

At this stage of evolution the Animal Kingdom or its one-celled progenitors, the Protozoa, had representatives so overlapping with plants and fungi that many biologists prefer to put them in a separate kingdom, the Protista.  These early steps were developing during the second billion years of life on earth.

By the beginning of the third billion years on earth a protozoan that could change back and forth from one with a flagellum to one with pseudopodia had evolved.  Sometime the pseudopodia would develop into a collar around the flagellum.  Eventually some of these dual potential cells stuck together and developed small colonies that eventually specialized into sponges.  The single cell with the capacity for diverse structure and a mechanism for controlling it needed a few control changes in a few different cells of the colony to provide the basic material for evolution of many of the features of all animals.

The Porifera were the first phylum of animals to develop.  They diversified into many different sponge types until one group gave rise to coral-like animals as indicated by the similarity to a Middle Devonian anthozoan (Kazmierczak, Jozef. 1984.  Favositid tabulates: evidence for poriferan affinity.  Science, 225:835-837.).  

Recognition of this evidence of anthozoans as the first cnidarians provides a basis for a simple continuity of phyla in the early stem of animals leading to the next phylum, the Platyhelminthes which may be considered the earliest protostomes.  A simple but unconventional view is that anthozoan polyps gave rise to jellyfish ancestral to triclad planarians.  The complexity of the simple process is why I needed to write my manuscript, Evolution Insights, to make it evident.

The sponges have less well-defined tissues than phyla that follow.   But the main mass of sponge is jelly-like with a few amoeboid cells and a tangle of collagen-like fibers and is much like loose connective tissue in our own bodies.  The jelly-like mass is mostly covered with flattened cells and is perforated by many pores leading to canals and or cavities lined with choanocytes.  Choanocytes are cells with a flagellum surrounded at the base with a collar that collects microscopic food items to nourish the sponge.  Water is passed out one or a few large openings.  Most sponges have spicules.  Spicules are mineralized (calcareous and/or siliceous), often needle-like, or three-pointed and other shapes often specific to the class of the sponge.

The protostomes included all the animals above the cnidarians until the deuterostomes evolved.  The seemingly hidden origin of deuterostomes becomes simple and clear when the role of the Pogonophora is known.  The next several blogs are expected to deal with the origin of the deuterostomes.  Then it will be time to clarify the Porifera-Cnidaria-Platyhelminthes links.  Later, the origin of mollusks and arthropods from annelids will be covered.  The foggiest portion of animal evolution, Platyhelminthes to Annelida, is obscure because the intermediate steps left neither a fossil nor living close relative to my knowledge. 

The annelids seem to be the living representatives of the most ancient animals with a true coelom, a body cavity with body wall lined with a cellular layer of flattened cells.  Organs enclosed in the coelom are also covered with a similar cellular layer; the two layers often connect to form a double layered mesentery.  The mesenteries may help keep organs in position, including blood vessels and nerves servicing them.  Of the simple animals, more complex than flatworms, but still lacking both a true coelom and segmentation (or its derivative, metamerism), although having characteristics more in common with advanced animals, we find only the nemerteans.

The protostomes including flatworms, nemerteans, annelids, mollusks, and arthropods get their name from the embryonic origin of the mouth from the blastopore.  The first (proto-) opening becomes the mouth (-stome), thus their name Protostomia.  In deuterostomes a second embryonic opening or region becomes the mouth.  The deuterostomes include hemichordates, chordates, and echinoderms.

Besides mouth origin, major contrasts between major phyla of the two groups (advanced protostomes and deuterostomes) include spiral versus radial cleavage, determinate versus indeterminate cleavage, presence or absence of chitin.  A minor phylum, the Pogonophora, blurs these and other distinctions and gives good reason to be the link between the two branches of higher animals.  To me, the evidence is so good any other proposals lack standing.  

An earlier post (SCIENCE SCREW-UP NO. 1) provides reasons the currently popular view of phylum relationships is incorrect.  Most of my immediately following posts will address various aspects of the origin of deuterostomes.  

Joseph G. Engemann, Emeritus Professor of Biology, WMU, Kalamazoo.  6/20/2013