Friday, July 19, 2013
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.
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.
499 pp. Macmillan, New York
Kazmierczak, Jozef. 1984. Favositid tabulates: evidence for poriferan affinity. Science, 225:835-837.
Joseph G. Engemann July 19, 2013