Growth, reproduction and possible recruitment variability in the abyssal brittle star Ophiocten hastatum (Ophiuroidea: Echinodermata) in the NE Atlantic

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Abstract

Growth was studied from skeletal growth markers in the cosmopolitan abyssal brittle star Ophiocten hastatm. Samples for analysis were taken at five sites located in the southern (2900 m) and central (2000 m) Rockall Trough, at ca. 3000 and 4000 m in the Porcupine Seabight, and at 4850 m on the Porcupine Abyssal Plain. Growth bands were assumed to reflect an annual cycle in skeletal growth. Band measurements on arm vertebrae, standardised to disc diameter, were used to provide size-at-age data and size-increment data that took into account overgrowth of early bands in older individuals. The Richards growth function marginally provides best fit to pooled size-at-age data, although the asymptote-less Tanaka function and the Gompertz growth function also provided good fit to size-at-age data which showed a rather linear growth pattern with little indication of a growth asymptote. Loge transformed size-increment data were linearised by applying the Ford–Walford method to approximate Gompertz growth so that growth could be compared at the five sites. Grouped linear regression and analysis of covariance showed no significant differences between growth at the sites and a common fitted regression. However, pairwise comparisons suggest growth differences with increasing bathymetric separation. Oocyte size frequencies measured from histological preparations of the gonad of specimens from the Porcupine Abyssal Plain indicate marked reproductive periodicity, with spawn-out in late winter that is likely followed by planktotrophic early development in spring with benthic settlement in summer. Although usually rare in the trawl and epibenthic sled samples, several years of successful recruitment followed by a period when recruitment was low or absent might explain size structure observed in a single unusually large sample from the Rockall Trough. This is consistent with previous observations during the late 1990s of a large population increase on the Porcupine Abyssal Plain. Analysis of growth bands of these specimens sampled in 1997 suggest the population increase derives from a single or small number of year classes recruited during the early 1990s.

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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