Depth-related gradients in size structure and the bathymetric zonation of deep-sea brittle stars

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Brittle stars show rather diffuse zonation patterns with depth in the deep ocean compared to other fauna. Measurements of disk diameters of samples taken from 1973 to 1980 from populations of two species in the Rockall Trough (N.E. Atlantic Ocean) also show marked differences in frequencies of juvenile and adult sizes present at different depths. The breeding population of one of these species, Ophiocten gracilis G. O. Sars, is concentrated at about 1 000 m depth, from where samples show a polymodality in disk-size frequencies indicative of marked seasonal recruitment and von Bertalanffy growth of year classes identified from Gaussian mixtures fitted to the frequencies. A general trend of an increasing proportion of postlarval and juvenile sizes with increasing depth was evident in samples from various depths down to 2 900 m, where a marked annual settlement survives at best only a few months. However, active gametogenesis was detected in specimens as small as 1.4 mm in disk diameter and from as deep as 2 076 m. Models of age structure fitted to the size frequencies indicate a roughly depth-related gradient in decreasing survivorship and possibly also in growth rate of individuals. The population of the other species, Ophiura ljungmani Lyman, is best developed at 2 500 to 3 000 m. O. ljungmani also shows annual recruitment, and models of probable age structure fitted to samples from around 2 900 up to 1 632 m depth indicate a similar depth-related gradient in survivorship, but inconclusive evidence for depth-related differences in growth rate. With both species, certain samples from intermediate depths deviated markedly from the expectation from models of simple bathymetric gradients. This indicates that factors controlling size and age structure in these populations are complex size and age structure in these populations are complex and only indirectly related to depth. Factors controlling the patterns observed may reflect local differences in predation pressure and trophic resources as well as more direct controls such as the effect of hydrostatic pressure on physiological functions.