Population structure and reproduction ofCalanus helgolandicus (Copepoda, Calanoida) along the iberian and Moroccan slope

Abstract

off the Iberian Peninsula and Morocco, the abundant calanoid copepod Calanus helgolandicus (Claus) was collected from a depth of 1000 m to the surface. Differences in depth preference were correlated with the life stage and geographically differing vertical salinity structures. In autumn and winter, only stage V copepodids (CV) and adults were found, in spring also younger copepodid stages. Within the range of the Mediterranean outflow water (MOW), a sharp decline of abundances of all stages was evident during all cruises. In autumn 1991, the bulk of the population was recorded south of the MOW; in winter 1992 north of it. During winter, numbers had declined by 70 %, supporting the idea that winter individuals represent the same generation as was encountered in autumn, and that they had been transported northwards. CV stages preferred the depth layer 400-800 m, in autumn and winter. Adults were found in autumn at the same depth south of the MOW, while they preferred the 0-400 m layers north of it. In winter, the abundance of adults increased, males preferred the 400-600 m depth layer, while females stayed at 200-400 m. In spring 1991, stages younger than CV were found in high densities, all stages concentrating in the upper 200 m. During the crossslope survey in spring off Portugal, an absolute abundance maximum of females was found. In contrast, offshore densities were very low. On the basis of these findings, the hypothesis of a Mediterranean centre of distribution and dispersal into the Atlantic is questioned. It is suggested that a separate, reproductively active population of C. helgolandicus exists off NW. Africa.

Barriers in the pelagic: population structuring of Calanus helgolandicus and C. euxinus in ­European waters

“…(1) The NE Atlantic and Tyrrhenian Calanus helgolandicus populations are presently connected by high levels of dispersal. For instance, there is some evidence of transport of diapausing copepods across the Gibraltar Strait/ AOF (Stöhr et al 1996). (2) The populations are not or hardly connected, but our genetic data failed to reject homogeneity.…”

On the surprising lack of differences between two congeneric calanoid copepod species, Calanus finmarchicus and C. helgolandicus

“…Populations exist in regions with sea bed depths ranging from 1000 to 4000 metres (Bonnet et al, 2005;Stohr et al, 1996;John et al, 1998;Andersen et al, 2001). It has been observed living predominantly at depths of 2000 m in June in the Levantine Sea (Bonnet et al, 2005) which indicates a seasonal vertical migration may occur.…”

The boundary circulation along the European continental slope as transport vehicle for two calanid copepods in the Bay of Biscay

“…The following information is mostly summarised from Conover [6], Williams and Conway [33] and Stiihr et al [28,291 and the respective literature cited therein:…”

“…Diapausing copepodids may be recirculated by slope undercurrents to the habitat of the adults 141. From many additional studies worldwide on the biology of C. carinatus as well as our own data from off Morocco and Portugal, Stohr et al [28] selectively discussed information on the along-slope, cross-slope and vertical distribution of resting CSs in relation to slope currents and the resulting large-scale distribution of this species. [29] presumed that resting C5s also cross the Bay of Biscay by means of the above-mentioned currents.…”

“…Here we combine previously unpublished data obtained from 13 plankton stations by German RV Heincke in the Bay of Biscay with a review of the recent data obtained by Stijhr et al off the Iberian Peninsula ( [28] for Calanus helgolandicus; [29] for C. carinatus) as well as some of those accumulated during the Continuous Plankton Recorder (CPR) surveys [33]. We then analyse the biological information with respect to the spreading of water masses in the Bay of Biscay in order to exemplify the likely drift patterns of two copepod species in relation to their seasonal vertical distribution.…”