Origin of the Class Holothuroidea

• Published in: Paleontological journal Vol. 55; no. 7; pp. 766 - 786
• Main Author: Smirnov, A. V.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01-12-2021; Springer; Springer Nature B.V
• Subjects: Body organs; Body wall; Canals; Canals (anatomy); Developmental stages; Earth and Environmental Science; Earth Sciences; Eleutherozoa; Holothuroidea; Homology; Larvae; Locomotion; Marine invertebrates; Ontogeny; Oral apparatus; Organs; Paleontology; Plates; Reproductive system; Rings (mathematics); Segments; Skeleton; Symmetry; Tentacles; Holothuroidea; ambulacral (water-vascular) system; symmetry; primary tentacles; morphogenetic archetype; paedomorphosis; heterochrony
• ISSN: 0031-0301; 1555-6174
• DOI: 10.1134/S0031030121070108

Among the specific characters of holothurians, the main ones are: the structure of the ambulacral (water vascular) system, the presence of oral (buccal) tentacles, the calcareous ring, and bilateral symmetry of the larval and definitive stages of development. In all modern Eleutherozoa, except Holothuroidea, the five primary outgrowths of the hydrocoel first develop as tentacles and then transform into radial ambulacral canals. In holothurians, five primary outgrowths of the hydrocoel give rise to five primary tentacles arranged around the oral opening, and the radial ambulacral canals develop on the hydrocoel between them later and end aborally. I propose that Echinodermata descended from ancestors which, like modern Pterobranchia, had tentacles, with coeloms which were derived from the mesocoel. I previously proposed a hypothesis of a two-stage development of the ambulacral system of Eleutherozoa. The first stage is the development of tentacle-shaped outgrowths of the hydrocoel. Then there is a switch to the next stage, i.e., the development of the ambulacral canals incorporated in Eleutherozoa into the body wall. I suggest that the ancestors of holothurians switched to locomotion on the ventral side of the settled larva, as is observed in modern holothurians, and fed using the primary tentacles. Obviously, since the primary tentacles of holothurians remained as tentacles, they could not have been transformed into ambulacral canals. As a result, the ontogeny of holothurians changed. It is logical to assume that these changes in the structure of the ambulacral system of holothurians were associated with changes in the mechanisms for realization the program of transformation of primary tentacles into ambulacral canals. It is possible that the mechanism of regulation of the development of the ambulacral system of holothurians occurred by splitting the initial program and / or duplication of its initial stages. Radial ambulacral canals began to develop later between the primary tentacles. Changes have also taken place in the development of other organ complexes. Some of them, as a result of equifinality of development, have a structure similar to homologous organs in Echinoidea. Other organ systems: the skeleton, the reproductive system and, possibly, the axial complex developed through paedomorphosis. As a result of these processes, the morphogenetic holothurian archetype was formed. It is probable that the segments of the calcareous ring of Holothuroidea are modified first ambulacral plates and are homologous to some parts of the oral apparatus of other Eleutherozoa. It is less likely, though hypothetically possible, that the radial segments of the calcareous ring of holothurians are homologous to the terminal plates of Eleutherozoa. Bilateral symmetry in holothurians is not a novelty but arose on the basis of the initial bilateral symmetry of the echinoderm ancestor. The pentaradial symmetry developed in the course of evolution did not disappear, but was coordinated with bilateral symmetry, and the symmetry of holothurians can be characterized as pentaradial-bilateral-asymmetrical.