Relationships between size, wet weight, dry weight, carbon and nitrogen content were determined in Ophiothrix fragilis (Echinodermata: Ophiuroidea). Such relationships appeared very useful when studying dynamic processes such as nutrition, growth, excretion or reproduction, considered as fluxes of carbon and nitrogen in the ecosystem.

Relationships between size, wet weight, dry weight, ash free dry weight, carbon and nitrogen content of organisms are of fundamental interest in ecological studies of ecosystems. They allow us to understand and quantify the role of a species as it stores, consumes or produces organic matter during its life cycle. Determination of conversion factors are time-consuming but very useful for estimating flows in the ecosystem (Brey et al., 1988). Some compilations of conversion factors have been published (Båmstedt, 1981; Rumohr et al., 1987).

Ophiothrix fragilis (Abildgaard) is the dominant species of the ‘pebbles with sessile epifaunal community’ (Davoult, 1990), located in the Dover Strait (eastern Channel), where tidal currents are very strong. It is considered as an efficient suspension feeder (Roushdy & Hansen, 1960; Warner, 1971) and lives in dense populations (1000–2000 individuals m-2; Davoult, 1989) which are assumed to have a significant effect on the fluxes of organic matter from the pelagic to the benthic system and on the fluxes of ammonium to the water column (Davoult et al., 1991).

Individuals were sampled in May and June 1991 off the Cape Gris-Nez (50°55′N 1°35′E). Two-hundred individuals were measured (diameter ±01 mm, after Guille, 1964), dried at 60°C for 48 h, weighed (±0·1 mg), burnt at 520°C for 6 h (Anonymous, 1986), and the ash weighed (±0·1 mg). Total organic carbon and nitrogen content were determined with a CHN element analyser (Carlo Erba 1106): after a flash combustion in a helium stream temporarily enriched with pure oxygen, quantitative combustion was achieved by passing gases over Cr2O3 after a transfer through a reduction reactor to eliminate the excess of oxygen; components were separated in a chromato-graphic column, eluted, then measured by a thermal conductivity detector. Thirty-eight dried ophiuroids were ground with a micro-grinder, then two sub-samples (except for individuals <2 mm) were weighed at ±1 μg (76 measurements).