Evolution of abbreviated development in the ophiuroid Ophiarachnella gorgonia involves heterochronies and deletions

Abstract

The ophiuroid Ophiarachnella gorgonia Müller and Troschel, 1842 has the Type II mode of ophiuroid development through a short-lived (lecithotrophic) vitellaria larva that settles 3 days after fertilization. Development in O. gor gonia is characterized by a reduction in larval structures and settlement of a precocious juvenile. In comparison with other ophiuroid vitellariae, the larva of O. gorgonia has reduced ciliated bands that lack prominent epithelial ridges. Ab breviated development in O. gorgonia is achieved through prompt formation of a radial hydrocoel and radial juvenile skeleton. Newly settled juvenile O. gorgonia also lack the feeding and locomotory structures found in the settlement stage of species with ophioplutei. Juveniles of O. gorgonia settle with one pair of buccal podia, and the mouth is not functional. The first arm segment forms before the mouth opens. In comparison, in species with ophioplutei the first two pairs of buccal podia and the first two arm segments develop before the mouth opens and before settlement occurs. Comparison of development of O. gorgonia and that of other species with vitellariae suggests that hetero chron ies and deletions in the metamorphic and settlement programs may be involved in the evolution of abbreviated development.