Post-larval development in deep-sea echinoderms.
University of Southampton, Faculty of Science, School of Ocean and Earth Science,
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 postmetamorphic
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 postmetamorphic
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 in
different 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 shield
spine 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"1, with the population of the Hebridean Slope being able to produce probably up to 16 million eggs m"2. Post-larvae start settling in May and numbers settling reached over 3,200 post-larvae m"2. The settling
speed of post-larvae in the water column is estimated to be around 500 m day"1, 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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