ScienceWeek

ZOOLOGY: ON ECHINODERMS

The following points are made by C.T. Amemiya et al (Current Biology 2005 15:R944):

1) Echinoderms -- from the Greek for "spiny skin" -- are invertebrates that exclusively inhabit marine environments. Most species have a five-fold radial-symmetry body plan. Echinoderms are monophyletic and comprise a sister-group to the hemichordates (acorn worms). There are approximately 7000 extant echinoderm species that fall into five well-defined taxonomic classes: Crinoidea (sea lilies and feather stars); Asteroidea (starfishes); Ophiuroidea (basket stars and brittle stars); Holothuroidea (sea cucumbers); and Echinoidea (sea urchins, sand dollars, and sea biscuits). This group has left an exceptional fossil record dating back to at least the Cambrian; this well defined fossil record allows comparative molecular studies to be made over a wide range of relatively well documented divergence times.

2) Echinoderms have several shared features that distinguish them from other animals, including a water vascular system and a characteristic form of calcium carbonate endoskeleton called "stereom". Most echinoderms begin life as larvae and undergo complex metamorphosis to form an adult. As a group they display a diverse array of life history traits. Most have a sexual mode of reproduction, although some can reproduce asexually. Their larvae are primitively free-living and planktonic, and show a considerable diversity in morphology and function, some of the traits being clearly shared with the larvae of certain hemichordates.

3) Nearly all echinoderm adults adopt a benthic adult form following metamorphosis, though a few deep-water holothuroids are pelagic or epibenthic swimmers. Adult echinoderms are radially symmetrical, usually pentameric, with intricate internal skeletons of calcium carbonate ossicles, supported by a characteristic array of collagenous ligaments. When present in larvae, skeletons take the form of elaborate rods which are independently derived in Ophiuroidea and Echinoidea (and absent in larvae of the other three echinoderm classes).

4) Echinoderm embryos and larvae have been used as experimental model systems for more than a century. Research on echinoderms has led to significant advances in the areas of developmental biology, cell biology, and immunology, several specific lines of research being recognized with Nobel Prizes. Echinoderm larvae were central to classic studies that resulted in fundamental biological concepts, including Hans Driesch's demonstration of nuclear equivalence in development, Theodor Boveri's characterization of the chromosomal basis of inheritance, and Elie Metchnikoff's exploration of cellular immunity.

5) Echinoderm species exhibit a variety of developmental strategies, from maximally indirect development, where adults emerge from metamorphosis of a larva with virtually no similarity to the adult, to direct developmental transition from a fertilized egg into an adult. The bilaterally symmetrical larvae of indirect developing species are feeding, long-lived, and very simple in structure and cell number (in the most-used sea urchin models the pluteus larva has only about 2000 cells). A variety of intermediate developmental modes exist with, for example, non-feeding larvae or facultative larval feeding. For echinoderms, indirect development is primitive, and a dipleurula-type larva is found in all five living classes, as well as in the sister-phylum Hemichordata.[1-5]

References (abridged):

1. Cameron, C.B., Garey, J.R., and Swalla, B.J. (2000). Evolution of the chordate body plan: New insights from phylogenetic analyses of deuterostome phyla. Proc. Natl. Acad. Sci. USA 97, 4469-4474

2. Davidson, E.H., Rast, J.P., Oliveri, P., Ransick, A., Calestani, C., Yuh, C.H., Minokawa, T., Amore, G., Hinman, V., Arenas-Mena, C. et al. (2002). A genomic regulatory network for development. Science 295, 1669-1678

3. Davidson, E.H. and Cameron, R.A. (2002). Arguments for sequencing the genome of the sea urchin, Strongylocentrotus purpuratus. A white paper submitted to the National Human Genome Research Institute.

4. Duboc, V., Rottinger, E., Lapraz, F., Besnardeau, L., and Lepage, T. (2005). Left-right asymmetry in the sea urchin embryo is regulated by nodal signaling on the right side. Dev. Cell 9, 147-158

5. Ettensohn, C.A., Wessel, G.M., and Wray, G.A. eds. (2004). Development of sea urchins, ascidians and other invertebrate deuterostomes: Experimental Approaches, Volume 74 (London: Elsevier Academic Press)

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