Post-larval development in deep-sea echinoderms

Abstract

The post-larval phase is an essential period in the life history of marine invertebrates; vulnerable to high mortality, it ultimately influences the distribution and abundance of adult populations. The post metamorphic ontogenesis of thirty species of deep-sea echinoderms, belonging to three classes (Ophiuroidea, Asteroidea and Echinoidea), is described using scanning electron microscopy. The life history of Ophiocten gracilis is also examined as a case study for future research on post-larval organisms. The analysis of development in ophiuroids reveals that species can be identified from a very early post metamorphic stage, even in congeneric species, contrary to the findings of other authors. The ontogeny of homologous structures is similar within related groups, but may give rise to different adult structures indifferent taxa. The mouth papillae within the ophiurids are serially homologous, originating from the jaw, but the fourth mouth papilla may have a different origin. In the families Ophiactidae, Ophiacanthidae and Amphilepididae examined, the mouth papillae have different origins, as, for instance, the adoral shieldspine or tentacle scale. Data on the post-larval development of Ophiura affinis suggest that this species is more closely related to the genus Ophiocten and a change in the generic status is proposed. Ophiocten gracilis is a bathyal brittle star occurring on both sides of the North Atlantic and its life history is studied in the eastern side of the North Atlantic. In this area, O. gracilis spawns in February/March of each year producing a large number of eggs. Fecundity is estimated to be around 40,000 eggs/ind, with the population of the Hebridean Slope being able to produce probably up to 16 million eggs/m². Post-larvae start settling in May and numbers settling reached over 3,200 post-larvae/m². The settling speed of post-larvae in the water column is estimated to be around 500 m/day, settling faster in warmer than colder water. Settling speeds appear to be similar for post-larvae ranging from 0.6 to 0.9 mm in disk diameter. Size at settlement is around 0.6 mm in disk diameter and 5-6 arm segments. The settlement of post-larval O. gracilis on the bottom of the Hebridean Slope also represented a considerable fraction of the particulate organic carbon (POC) flux in the area, reaching over 7% of the total daily flux. This is likely to have a considerable impact in the benthic community as competition and predation and as an additional food source for demersal and benthic organisms. The occurrence of post-larvae of O. gracilis in sediment traps also represented a large problem for POC flux measurements, with ophiuroids consuming part of the flux. In future works with sediment traps, such errors must be taken into account and ophiuroids must be included in the total POC flux. The deep-sea juvenile asteroids of the NE Atlantic could be distinguished to species level from a very early stage of development. The ontogenesis of Porcellanaster ceruleus shows that this species is likely to undergo a shift in habitat and diet during the juvenile phase. This is evidenced by the appearance of the epiproctal cone, the changing of the furrow and apical spines, the early development of the cribriform organ adjacent to the madreporite and the appearance of sediment in the stomach. P. ceruleus is probably a predator on meiofauna and small macrofaunal organisms during the early stages of life, changing to a burrowed life style ingesting sediment particles. Most juvenile sea stars analysed during the present study showed wider bathymetric distribution than their adult counterparts, suggesting that events occurring during the early stages of life are important for the maintenance of the local population structure and diversity in the deep NE Atlantic.The post-metamorphic development of three deep-sea spatangoid echinoids is very similar, but the morphology and formation of fascioles facilitate the distinction of the species examined. Whereas in Hemiaster expergitus and Spatangus raschi the fascioles present in the post-larvae develop to form the adult fascioles, in Brissopsis lyrifera post-larvae there is a juvenile fasciole, which disappears during ontogenesis giving way to the adult fascioles. The function of the juvenile fasciole is unknown in B.lyrifera. The development of the periproct in all spatangoids examined is similar to that described by other authors, with the periproct being initially endocyclic and migrating towards the rear of the animal as development progresses. Post-larvae of the genus Echinus could not be separated into different species, which may be linked to the recent diversification of the genus in the North Atlantic. The widespread settlement of echinoderm post-larvae reported in the present thesis and in other works is thought to have been very important for the colonization of the deep-sea through the supply of stages to deeper areas and selection of pressure adapted animals and subsequent speciation.

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