METAMORPHOSIS OF STICHOPUS CALIFORNICUS (ECHINODERMATA: HOLOTHUROIDEA) AND ITS PHYLOGENETIC IMPLICATIONS

Abstract

Descriptions of holothurian metamorphosis are based on data from the relatively specialized order Apoda. Metamorphosis for a relatively unspecialized aspidochirote, Stichopus californicus, is described here. Metamorphosis in Stichopus is characterized by the following features: the madreporic vesicle is a calcite secreting syncytium, not a coelom. Stichopus has no separate axocoel and no transient axial complex forms during metamorphosis. The buccal podia form from the water vascular ring, not the radii. The axes of bilateral larval symmetry and the pentaradial adult symmetry are congruent; therefore, the secondarily derived symmetry in holothurians is the pentaradial symmetry of the adult, as in the other extant echinoderms. No axial or visceral torsion occurs during metamorphosis. The enclosed ambulacra form in a manner distinct from that of the ophiuroids and echinoids. Perivisceral coelomic pores develop near the end of metamorphosis, and before the definitive anus forms. Assignment of holothurians with echinoids to the subphylum Echinozoa is discussed.

Echinodermata

“…Several sea cucumber species have been the subject of embryological studies (reviewed in Hyman 1955 ;Smiley et al 1991 ), in particular species of the Stichopodidae (e.g., Stichopus , Apostichopus , and Parastichopus species) (Holland 1981 ;Smiley 1986 ;Shoguchi et al 2000 ). A comprehensive gene expression analysis during development in Parastichopus parvimensis is available (McCauley et al 2012 ), rendering this species a reference model for the development of holothurians.…”

“…In euechinoid sea urchins, crinoids, and holothuroids, an ectodermal invagination, the vestibule, forms adjacent to the hydrocoel and the juvenile develops within the vestibule-like invagination (Smiley 1986 ;Burke 1989 ;Holland 1991 ;Smiley et al 1991 ;Ramafofi a et al 2003 ). In other groups, such as the cidaroid sea urchins,…”

“…In holothuroids, a vestibule-like structure forms at the anterior end of the larva in the oral region and the oral-aboral axis of the future adult is positioned on the anterior-posterior axis of the larva (Hyman 1955 ;Smiley 1986 ;Smiley et al 1991 ). In ophiuroids, the juvenile oral-aboral axis develops along the dorsoventral axis of the larvae (Hyman 1955 ;Hendler 1991 ).…”

“…The bilateral larval axis of holothuroid larvae is congruent with the bilateral axis of the juvenile and adult (Smiley 1986 ). These echinoderms have a bilateral symmetry as adults superimposed on pentamery (Hyman 1955 ).…”

A review of echinoderm oogenesis

“…The outer sac is also composed of three tissues: (1) a n internal limiting epithelium that is a myoepithelium in asteroids, a peritoneum in ophiuroids, and incomplete or virtually absent in echinoids, (2) a connective tissue compartment, and ( 3 ) an external limiting peritoneum bordering the perivisceral coelom (Davis, 1971). The outer sac appears to be one result of the torsion that occurs during metamorphosis in asteroids, ophiuroids, and echinoids [Smiley, 1986;Smith, 1940). There are no modern studies that focus on the ontogeny of the outer sac in any echinodem, and, as a result, the comparative histology and ultrastructure of this part of the gonad are confusing and incomplete.…”