Bi-penta-bi-decaradial symmetry: A review of evolutionary and developmental trends in holothuroidea (echinodermata)

Holothuroidea, comprising the sea cucumbers, is the least studied class of extant echinoderms, yet this group possesses a wealth of features of potential interest to developmental and evolutionary biologists. Holothuroids include the most morphologically derived echinoderms, including pelagic species and spheroid, plated taxa with mouth and anus adjacent at the end of a long, flexible stalk. To begin investigating this diversity of body form, we first estimated evolutionary relationships in the class Holothuroidea based on maximum parsimony analyses of 1,075 nt of the nuclear small subunit rDNA (for six species in four orders) and on 52 informative morphological characters (for the 25 extant families). Both the morphological and molecular evidence suggests almost an inversion of the prevailing higher level classification. Character-state optimizations indicated that pronounced adult bilateral symmetry evolved three times. In one group even a regain of secondary radial symmetry is found. Respiratory trees, structures unique to holothuroids, are a relatively late innovation, are ectodermally derived, and are bilaterally symmetric, supporting the possibility that the secondary gain of bilateral symmetry in holothuroids is ectodermally derived analogous to, say, the derivation of vertebrate limb dorso-ventral axis. The test of imbricating plates found in 10% of living holothuroids is apparently not homologous with that of other heavily armored echinoderms, evolving much later and at least twice. Indirectly developing larvae, auriculariae, occur in two evolutionarily disparate clades and unlike echinoids comprise a minority of clades. We suggest that this implies the parallel convergent evolution of this larval type or, more speculatively, some form of retention of developmental constraints. J. Exp. Zool. (Mol. Dev. Evol.) 285:93–103, 1999. © 1999 Wiley-Liss, Inc.