Hydrobiologia 414: 25–34, 1999.
J.-C. Dauvin (ed.), European Colloquium on Marine Science.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
25
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
A. Lefebvre
1,∗
, D. Davoult
1
, F. Gentil
2
& M. A. Janquin
1
1
Station Marinede Wimereux, Universit´e des Sciences et Technologiesde Lille, B.P.80, F-62930Wimereux, France
2
Station Biologique de Roscoff, Universit´e P. & M. Curie, Paris VI, B.P. 74, F-29682 Roscoff Cedex, France
Key words: Echinodermata, ophiuroidea, reproduction, gonad index, carbon and nitrogen production
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 pop-
ulation 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
spawningofJulywassynchronousforfemalesand males in both areas. Carbon and nitrogenproductiontowardsthe
pelagic system during the spawning period was very important (up to 20 g C. m
−2
and5gN.m
−2
)andwasupto
16% and 13% of the annual carbon and nitrogen production of the population in the Dover Strait. After the spawn-
ing 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.
Introduction
The ophiuroid Ophiothrix fragilis (Abildgaard) is a
very common echinoderm all along the coasts of the
EnglishChannel.It is widelydistributed in the western
part of the English channel (Cabioch, 1968; Retière,
1979), in the Bay of Seine (Gentil, 1976), in the east-
ern part of the English Channel (Cabioch & Glaçon,
1975, 1977) and in the Dover Strait (Holme, 1966;
Davoult, 1988). In the Dover Strait, the density of
the population of Ophiothrix fragilis is about 1500–
2000 ind. m
−2
(Davoult, 1989) and it accounts for 62
± 12% of the total biomass (ash free dry weight) of
the pebbles with sessile epifauna community (total
biomass: 281 ± 86gm
−2
; Migné & Davoult, 1997).
∗
Author for correspondence
E-mail: lefebvra@pop.univ-lille1.fr
The English Channel has a megatidal regime. Be-
cause of the tidal residual and the dominant south-
westerly winds, the residual circulation drifts north-
eastward, from the English Channel to the North
Sea.
Because of its density and its suspension feed-
ing, Ophiothrix fragilis takes an important role in the
coupling between the pelagic and the benthic systems
(Migné, 1996). This high density area can be con-
sidered as a source of carbon dioxide (Davoult et al.,
1998). This species can be then considered as a reg-
ulator of eutrophication as its food appeared to be
mainly constituted by allochthonous sources.
In this context, the study of Ophiothrix fragilis re-
production could allow us to answer two questions:
1. qualitatively, in the particular hydrodynamical area
of the Dover Strait (variability of food supplies and