Spatio-temporal variability in the gonad growth of Ophiothrix fragilis (Echinodermata : Ophiuroidea) in the English Channel and estimation of carbon and nitrogen outputs towards the pelagic system

Abstract

Ophiothrix fragilis forms dense aggregations (up to 2000 individuals. m -2 ) and represents more than 60% of the biomass on the pebbles of the sessile epifauna community in the Dover Strait (eastern English Channel). This population shows a relative stability (in terms of density and magnitude of benthic recruitment) which contrasts with the variability of influx and of local hydrodynamical conditions. Gonadal change was studied in the Dover Strait during 1995 and 1996 and for another population offshore Roscoff (western English Channel) in 1996. Gonadal growth appeared to be cyclical, with higher gonad indices in June and July and a resting phase between September and February for the population of the English Channel. This change showed little inter-annual variability for the population of the Dover Strait, but a high spatial variability between populations of the English Channel. The population offshore Roscoff had lower gonad indices; this difference could be linked to lower temperature and lower food availability (phytoplanktonic production). However, whatever the gonadal growth pattern was, the main spawning of July was synchronous for females and males in both areas. Carbon and nitrogen production towards the pelagic system during the spawning period was very important (up to 20 g C. m -2 and 5 g N. m -2 ) and was up to 16% and 13% of the annual carbon and nitrogen production of the population in the Dover Strait. After the spawning period, larvae are brought under strong hydrodynamic conditions during ca. four weeks before metamorphosis and settlement. This is probably the most important period for the understanding of the stability of the population.