Growth, reproduction and possible recruitment variability in the abyssal brittle star Ophiocten hastatum (Ophiuroidea: Echinodermata) in the NE Atlantic
Introduction
Brittle stars are often quantitatively important elements of the benthic community in the deep ocean. Yet the population biology of deep-sea species, particularly those with an abyssal distribution, is poorly understood. Ophiocten hastatum Lyman 1878 (superseded synonym O. latens Koehler 1906, see Paterson et al., 1982) has an apparently cosmopolitan distribution typical of abyssal ophiuroids (Paterson, 1985). In the NE Atlantic O. hastatum is known from the Rockall Trough from 2000 to 2980 m (Gage et al., 1983; Harvey et al., 1988) to the Azores at depths of 1970–4700 m (Paterson, 1985). The brittle star is typically found in small numbers in bottom trawlings. Nothing is known of its population structure, growth and reproduction, although recent work indicates the brittle star, like its bathyal congener O. gracilis (Pearson and Gage, 1984), feeds selectively on phytodetritus (Bett et al., 2001; Iken et al., 2001).
We here provide (1) data on population abundance and size structure, (2) analyses of individual somatic growth from measurements of natural skeletal growth markers present in the vertebral ossicles of the arm from samples from five sites, and (3) data on reproductive biology from histological analysis of oocytes in gonads of individuals from samples taken at different times of the year. This new information is used to provide an interpretation of life history strategy of O. hastatum. Recruitment variability may help to explain how spatial and temporal variability in disc size frequencies of O. hastatum observed in samples at the different sites in the northeastern Atlantic might have occurred.
Section snippets
Samples
Samples came from five areas in the NE Atlantic (Fig. 1): (1) the SAMS Permanent Station, a repeat station in the southern Rockall Trough centred on 54°40′N, 12°16′W that was sampled from 1975 to 1992 by the Scottish Association for Marine Science, SAMS (previously Scottish Marine Biological Association, SMBA); (2) the Feni Ridge in the central Rockall Trough where a single sample had been taken by SAMS from 2000 m depth; (3) the Porcupine Seabight at ca. 3000 m, and (4) the Porcupine Seabight at
Assumptions of growth analysis
We assumed the growth bands reflect an annual cycle in skeletal growth. The grounds for this assumption are summarised in the Discussion. The end of winter growth is interpreted as a band of fine-pored stereom (also recognisable as an often abruptly ending, wave-like ridge in the surface relief) separating it from the beginning of new growth that was recognisable as coarse-pored stereom. This provides a less ambiguous marker than the midpoint of the fine-pored, winter growth zone, and was
Growth bands as annual growth markers
An assumption of a seasonal growth pattern causing the growth-banding pattern visible on the ossicle (Fig. 2) is implicit in the above growth analysis. This remains unproven for O. hastatum, just as it is to date from direct observations of growth of individuals of any deep-sea organisms. But a seasonal growth periodicity now appears very likely since the discovery of seasonally pulsed detrital flux to the seabed (Billett et al., 1983). This has been functionally linked to the discovery of
Acknowledgements
We thank Robin Harvey at Dunstaffnage for assistance in the SEM examinations and photography and Brian Bett and David Billett of SOC, and Myriam Sibuet and Joëlle Galéron at IFREMER, Centre de Brest, France, for generously making available trawl and box-core material collected from the Porcupine Seabight and Porcupine Abyssal Plain, including that collected during the EU MAST III-funded BENGAL project, Contract No. MAS3-CT95-0018 (DG12—ESCY). We wish to thank the referees for their valuable
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