Reproduction and growth of Echinus acutus var. norvegicus Düben & Koren and E. elegans Düben & Koren on the continental slope off Scotland

Abstract

Large samples of two sea urchin species Echinus acutus var. norvegicus Düben & Koren and E. elegans, Düben & Koren were taken from trawl hauls between 220–1075 m depth on an area of the upper continental slope west of Scotland. In both species, biometrical measurements show a direct, linear relationship between test diameter, test height, the size of the peristome and periproct; and, for a limited range of sizes excluding very small and large urchins, between the test diameter and the number of ambulacral and interambulacral plates in each coronal series. Size-frequency histograms for both species show a varying size structure that seems unrelated to bathymetry or time of year. Counts of the number of growth zones, visible as dark rings after charring the plates of the apical system, were used to age individuals. These are thought to be formed annually and to reflect seasonal variation in skeletal growth. Asymptotic growth functions were fitted to growth-zone counts plotted against test diameter. A slightly sigmoidal lower limb of the curve is suggested that is thought to indicate an accelerating phase of growth amongst smaller sizes. Nested-design ANOVA and contingency table analyses of oocyte size-frequencies of females in samples taken at different times of the year indicate a seasonal cycle in oogenesis, with spawn-out, possibly in March, in both species. Comparable data from two congeners, the deep-sea species E. affinis Mortensen and the coastal species E. esculentus L. show a similarly distinct seasonality with the oogenic cycle being initiated in winter followed by vitellogenesis in summer and spawn-out in both species occurring at the same time in late winter to spring of the following year. However, growth and longevity of E. acutus var. norvegicus and E. elegans is intermediate between that of the other two species suggesting that these life-history traits at least may be related to the greatly differing depth range of the species.