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