Development of the brittle star Ophiothrix exigua Lyman, 1874 a species that bypasses early unique and typical planktotrophic ophiopluteus stages

Evolution of Maternal Provisioning and Development in the Ophiuroidea: Egg Size, Larval Form, and Parental Care

The Ophiuroidea is the most speciose class of echinoderms and has the greatest diversity of larval forms, but we know less about the evolution of development (evo-devo) in this group than for the other echinoderm classes. As is typical of echinoderms, evo-devo in the Ophiuroidea resulted in the switch from production of small eggs and feeding (planktotrophic) larvae to large eggs and non-feeding (lecithotrophic) larvae. Parental care (ovoviviparity or viviparity/matrotrophy) is the most derived life history. Analysis of egg data for 140 species (excluding viviparity and facultative planktotrophy) indicated a bimodal distribution in egg volume corresponding to planktotrophy and lecithotrophy + ovoviviparity, with three significant egg size groups due to the very large eggs of the ovoviviparous species. The marked reduction in fecundity in species with extremely large eggs is exemplified by the ovoviviparous species. Egg size in the two species with facultative planktotrophy was intermediate with respect to the two modes. Identifying the ancestral larval life history pattern and the pathways in the switch from feeding to non-feeding larvae is complicated by the two patterns of metamorphosis seen in species with planktotrophic development: Type I (ophiopluteus only) and Type II (ophiopluteus + vitellaria larva). The variability in arm resorption at metamorphosis across ophiuroid families indicates that the Type I and II patterns may be two ends of a morphological continuum. This variability indicates ancestral morphological plasticity at metamorphosis, followed by canalization in some taxa to the vitellaria as the metamorphic larva. Vestigial ophiopluteal traits in lecithotrophic ophioplutei and vitellaria indicate evolution from the ancestral (feeding larva) state. Parental care has evolved many times from an ancestor that had a planktonic ophiopluteus or vitellaria and is often associated with hermaphroditism and paedomorphosis. A secondary reduction in egg size occurred in the viviparous species.

Evo-Devo in Ophiuroids: The Switch from Planktotrophy to Lecithotrophy in Ophionereis

AbstractUnderstanding the evolution of development (evo-devo) in the Ophiuroidea and the pathways in the switch from a feeding to a nonfeeding larva is complicated by the variability in the phenotype of the metamorphic larva, being a reduced yolky ophiopluteus in some species (type I development) and a vitellaria larva in others (type II development). We investigated evo-devo in the family Ophionereididae, a group dominated by lecithotrophic development through a vitellaria larva. We reared the planktotrophic larvae of Ophionereis fasciata to settlement to determine the metamorphic phenotype. Counter to expectations, O. fasciata did not exhibit type II metamorphosis through a vitellaria, although it did exhibit transient vitellaria-like features. Resorption of the larval arms in the same interradial positions where the ciliary bands form in vitellariae gave them a fleeting vitellaria-like appearance. Development of O. fasciata exhibits heterochronic features in early formation of the skeletal primordium of the third pair (postoral) of larval arms and in the presettlement juvenile early appearance of the juvenile terminal arm plates on external view in parallel with larval arm resorption. Development of the fourth pair (posterodorsal) of larval arms, the last pair to be formed, is plastic, with 44% of larvae exhibiting partial arm growth. Heterochronic traits in development, as seen in O. fasciata, may have facilitated evolution of a lecithotrophic mode of development in Ophionereis. Comparison of the ophiopluteus of O. fasciata and the vestigial pluteus of O. schayeri provided insights into the simplification of larval form from the ancestral (feeding larva) state in Ophionereis. The diverse metamorphic phenotypes in ophiuroids indicate that type I and type II development may not be completely divergent lines of evo-devo and point to selective pressure in the pelagic-benthic transition in the evolution of ophiuroid development.

Initiation of Posterior Coeloms of an Ophiuroid (Brittle Star) and Plasticity in Their Development

AbstractIn the ophioplutei of brittle stars, the posterior coeloms are commonly assumed to be produced by a transverse fission of the initially formed coeloms; but in ophioplutei of Ophiopholis aculeata, the posterior coeloms first appear separately as aggregations of mesenchyme-like cells near the base of the posterolateral arms. Initiation of posterior coeloms was similar in ophioplutei of another family and may be similar in diverse ophiuroids. Initiation is easily missed without frequent observations. Early interpretations that diagrammed a fission of the first-formed coeloms appear to have influenced later authors for more than a century. Growth of posterior coeloms from a small initial size facilitated observations of developmental plasticity in growth of coeloms relative to that of larval arms. This plasticity, as observed in echinoplutei of echinoids, is relatively greater growth of a ciliary band for food capture when food is scarce and relatively greater growth of juvenile structures that will function after metamorphosis when food is abundant; however, juvenile structures develop extensively as a rudiment within the echinopluteus prior to settlement and metamorphosis, whereas in ophioplutei there is little development of juvenile structures until metamorphosis. In ophioplutei there is, therefore, less scope for shifting growth to structures that gain function after metamorphosis. Nevertheless, we found that when ophioplutei were at higher concentrations of food, the growth of the posterior coeloms was greater relative to the growth of the larval arms. Developmental plasticity in allocation of growth to larval and postlarval equipment can occur despite disparate patterns of development.