Deep Sea Research Part I: Oceanographic Research Papers
Depth-related distribution and abundance of seastars (Echinodermata: Asteroidea) in the Porcupine Seabight and Porcupine Abyssal Plain, N.E. Atlantic
Introduction
The zonation of fauna in the deep sea has been documented extensively (for review see Carney et al., 1983). Zones are described as regions of lesser faunal change bounded by regions of greater faunal change. (Menzies et al., 1973; Hecker, 1990; Gage and Tyler, 1991). Previous workers have focused either on general faunal zonation patterns (Le Danois, 1948; Rowe and Menzies, 1969; Haedrich et al., 1975; Ohta, 1983) or on the zonation of specific taxa, e.g. fish (Day and Pearcy, 1968), gastropods (Rex, 1977), echinoderms (Gage, 1986), holothurians (Billett, 1991) and decapod crustaceans (Cartes and Sardà, 1993). These studies have shown, regardless of the taxon examined, that deep-sea fauna undergo a non-repeating sequential change with depth and most species have predictable and restricted depth ranges (Rowe and Menzies, 1969; Carney et al., 1983; Gage and Tyler, 1991).
Previous studies on the zonation of specific taxa have used data on the shallowest and deepest records of a species to recognise critical depths of faunal change. This method is of use in examining major faunal boundaries, but it does not identify more subtle changes that are related to the abundance of species. Use of data on both first and last occurrence and changes in abundance provide a better overall picture of faunal zonation. In terms of species distribution the use of depth of first and last occurrence can reveal only where a species does and does not live. It reveals nothing about how a species occupies the depth range in which it is able to live. Very few studies have examined the depth-related distribution of deep-sea species within their depth ranges in order to elucidate small-scale patterns of zonation (Haedrich et al., 1975; Billett, 1991).
In studies examining general faunal change from the shelf break at 200 m to the abyss, up to seven different faunal zones have been recognised (Menzies et al., 1973; Musick, 1976; Haedrich et al., 1980). Three faunal boundaries have been reported consistently in the literature: (1) the shelf-slope break (200–500 m), (2) a less-pronounced boundary around 1000–1400 m (Day and Pearcy, 1968; Rowe and Menzies, 1969; Sanders and Hessler, 1969; Dayton and Hessler, 1972; Haedrich et al., 1975; Rex, 1977; Carney et al., 1983; Gage et al., 1985; Hecker, 1990), (3) a general boundary at ∼3000 m for megafauna (Vinogradova et al., 1959; Zenkevitch, 1963; Rowe and Menzies, 1969; Haedrich et al., 1980). 3000 m has been proposed as the start of the abyssal fauna (Hansen, 1975; Sibuet, 1979; Billett, 1991). That these depth boundaries occur at many locations worldwide indicates that important controlling variables are present at these depths and that these may occur globally.
There are many logistic problems associated with identifying and measuring factors that may affect zonation and depth related distribution, and as a result, nearly all deep-sea studies have looked at those factors that correlate with changes in the fauna. These factors include temperature (Rowe and Menzies, 1969; Haedrich et al., 1975), pressure (Siebenaller and Somero, 1978; Somero et al., 1983; Young et al., 1996), oxygen minimum (Carney and Carey, 1976; Pearcy et al., 1982; Gage, 1986), sediment type (Day and Pearcy, 1968; Haedrich et al., 1975), water mass structure (Tyler and Zibrowius, 1992), currents, topography and food supply (Rowe and Menzies, 1969; Hecker, 1990; Rice et al., 1990; Cartes and Sardà, 1993), larval dispersal (Rowe and Menzies, 1969; Grassle et al., 1979; Billett, 1991), competition, predation and trophic level (Rex (1976), Rex (1977); Haedrich et al., 1980; Cartes and Sardà, 1993). Identifying specific environmental variables that restrict the depth ranges of deep-sea species and their effects on an organism remains an unresolved problem. However, a more detailed knowledge of the vertical distribution of deep-sea species may help to indicate factors that affect species distribution and large-scale zonation (Young et al., 1996).
This study, based on what are considered quantitative samples and an extensive data set, determines the bathymetric distribution of asteroid species from the shelf break to the abyssal plain and examines the depth-related distribution and abundance of species within their depth ranges. Patterns of asteroid distribution are discussed in terms of what is known of their ecology. Large-scale zonation of the asteroid fauna are also investigated and correlated with available information on the physical environment.
Section snippets
Site description
The Porcupine Seabight and Porcupine Abyssal Plain are located more than 200 km to the southwest of Ireland (Fig. 1). The Porcupine Seabight forms an amphitheatre-shaped embayment in the continental margin, which measures approximately 300 km from north to south and 200 km from east to west. Its sides slope steadily from the edge of the Irish shelf at 200 m down to a depth of ∼3000 m. At the mouth of the Seabight, the seabed slopes away more steeply to a depth of ∼4000 m to join the Porcupine Abyssal
Species distribution
When all the asteroid species are arranged in series based on their depth distribution a non-repeating replacement of species is seen (Fig. 3). Each species shows a discrete depth range with some ranges extending over more than 1000 m. Most species show a patchy distribution through their depth range, often occurring, in any great abundance, only over a very narrow depth range of 200–300 m. The data suggest that although a number of species of asteroid may be present at a particular depth, only 2
Discussion
Many megabenthic animals are known to form aggregations (Pawson, 1976; Nybakken et al., 1998), particularly the echinoderms, such as the ophiuroids (Smith and Hamilton, 1983), holothurians (Billett and Hansen, 1982), and sea urchins (Grassle et al., 1975). Aggregations may be for feeding (Grassle et al., 1975; Billett and Hansen, 1982) or reproduction (Billett and Hansen, 1982; Smith and Hamilton, 1983). They may contain large numbers of individuals. Should any samples encounter such a patch
Acknowledgments
We would like to thank the officers and crew of R.R.V Discovery, Dr. Brian Bett and Dr. Martin Sheader for their statistical advice, Dr. Alex Rogers for advice on the evolution of deep-sea faunas, Ailsa Clark for her expertise in asteroid taxonomy, Dr. Adrian New and Dr. Neil Kenyon for information on the water masses and currents in the N.E. Atlantic and Andrew Whitehouse for his editorial comments. The manuscript benefited from the helpful comments of two anonymous reviews. This project is
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