Age structure in populations of the deep-sea brittle star Ophiomusium lymani: a regional comparison

Abstract

Size frequencies of samples of brittle stars from previous studies are re-examined in the light of a model of survivorship and asymptotic growth developed for the population at Sta. M in the Rockall Trough. An age structure was inferred consisting of discrete year classes resulting from seasonally pulsed recruitment. Polymodality amongst juvenile frequencies in samples from both sides of the North Atlantic and from the eastern North Pacific (San Diego Trough) reflect this, but the samples differ in the fraction of the population made up by adults. Those from the eastern North Atlantic are characteristically dominated by adults making up a large right-hand mode in the size-frequency distribution that is less well developed in samples from the western North Atlantic and the San Diego Trough. Disk-size frequencies observed were simulated from mixtures of normal distributions conforming to frequencies resulting from specified growth and mortality schedules. The large mode characteristic of eastern North Atlantic populations probably results from a ‘stacking’ of adult size classes approaching a growth asymptote determined by a decrease in growth rate on attaining reproductive maturity, that observed in Rockall resulting from a low (⩽10%) loss of each adult year class. Simulated frequencies resulting from higher mortalities ( ca. 22 to 50% y −1) fitted western North Atlantic and Pacific samples closely. The low, or absent, adult frequencies in certain samples from off New England are interpreted as resulting from decreased survivorship in marginal populations. Lower adult survivorship and possibly lower fecundity in the western North Atlantic and San Diego Trough seem to be related to higher population densities than in the eastern North Atlantic. Possible mechanisms for this are discussed along with possible causes of differences evident both in degree of overlapping of year classes and in modal disk size of adults.