Ovarian development and spawning in relation to the moult cycle in Northern krill, Meganyctiphanes norvegica (Crustacea: Euphausiacea), along a climatic gradient |
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Authors: | J Cuzin-Roudy F Buchholz |
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Institution: | (1) Observatoire Océanologique – UPMC-CNRS-INSU, LOBEPM – Océanographie Biochimique et Ecologie, BP 28, F-06230 Villefranche sur Mer, France, FR;(2) Biologische Anstalt Helgoland – AWI, Marine Station, Marine Zoology, D-27483 Helgoland, Germany, DE |
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Abstract: | Adaptive processes linked to reproduction were studied comparatively for three populations of Northern krill, Meganyctiphanes norvegica (M. Sars, 1857), sampled during winter and summer cruises in the Clyde Sea (W Scotland), the Kattegat (E Denmark), and the
Ligurian Sea (NW Mediterranean). The aim was to investigate the functional relationship between egg production and moulting
under contrasted climatic and environmental conditions. A staging system for female sexual development established for live
krill was complemented by a histological study of the ovary at various developmental steps. During the reproductive season,
all adult female krill were engaged in cyclical egg production. During experiments, female krill released one batch of mature
oocytes in one or two spawning events. The ovary of postspawn female krill still contained developing oocytes for another
egg batch. In the non-reproductive period, all female krill had a resting ovary. Ovarian structure and pattern of egg production
were identical in the three populations, but seasonal timing of egg production was different. The model proposed for the Ligurian
population of the annual cycle of ovarian development can be extended to the other two populations, taking into account the
seasonal characteristics of each site. Random field samples were staged simultaneously for moult cycle and for sexual development.
Moult stages and the seasonal variation of the intermoult period were studied for the Kattegat population using multi-year
data and compared to data obtained during summer/winter cruises in the Clyde and the Ligurian Sea. At the three sites, intermoult
period was shorter and temperature-dependent during the reproductive period, concurrent with the season of greatest food availability.
During most of the year and the period of sexual rest, moulting activity was reduced. The relationship between spawning and
the moult cycle was studied comparatively for the three populations. Eggs were released during the premoult phase of a “spawning
moult cycle”, in one or two spawnings associated with apolysis and Moult Stage D1, respectively. Yolk accumulation for the
next egg batch was completed during an alternating “vitellogenic moult cycle”. A model for the timing of cyclical egg production
in relation to moulting, as proposed for the Kattegat, can be extended to the other populations, taking into account intermoult
period variation with temperature. Temperature appeared to be the principal environmental factor controlling growth (through
moulting) and egg production during the reproductive season, in connection with favourable trophic conditions.
Received: 22 December 1997 / Accepted: 29 August 1998 |
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