Growth and recruitment of the deep-sea urchin Echinus affinis

Large samples of the sea urchin Echinus affinis Mortensen were obtained from a time-series of Agassiz trawlings from a 2 200 m-deep permanent station (Station M), and at neighbouring positions, in the Rockall Trough (North-east Atlantic Ocean) over a period of 7 yr (1978 to 1985). Counts of growth zones visible in the skeletal elements of the test were used to age individuals. Various growth functions were fitted to counts from a full range of the sizes available. Functions giving a sigmoidal growth curve fit the early phase of growth better than the von Bertalanffy model, although the latter provided better fit amongst larger sizes. The fit of a robust and flexible model recently developed by Preece and Baines to describe the human growth curve overcame this limitation. Skeletal banding is thought to result from seasonally varying growth as a result of annually pulsed fallout of phyto-detrital food to the deep-sea floor. Early stages were found in only a few of a time series of samples obtained with a fine-meshed epibenthic sledge, suggesting that recruitment to the population from its annual breeding may only occasionally be successful. Postlarval growth was estimated from samples taken soon after presumed settlement and later in the year. The fitted growth curve showed good agreement to that obtained from annual banding, and corroborates an initially exponentially increasing growth rate. Postlarval survivorship was estimated, by means of computer simulation, from a sample that included postlarvae along with a range in juvenile size, to be about 10% amongst postlarvae after settlement and thereafter about 90% yr^-1. Adults are inferred to be up to about 28 yr old. A markedly uneven representation of ages in a large subsample of aged adults of even frequency in size is interpreted by means of simulations as probably reflecting multi-year cycles in recruitment success. The possible causes of a varying size structure amongst large samples of adults, which showed some spatial segregation in relation to the track of the trawl on the bottom at Station M, are discussed.