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Spatial and seasonal changes of brackish peracaridan assemblages and their relation to some environmental variables in two tidal channels of the Ria de Aveiro (NW Portugal)'~e p a r t a m e n t o de Biologia, Universidade de Aveiro, 3810 Aveiro. Portugal '~aboratoire dVOceanographie Biologique, UMR 5805 (CNRS-UBl). 2 rue Jolyet, 33120 Arcachon. France ABSTRACT: The shallow-water fauna of 2 rnain channels in the Ria de Aveiro (Canal de Ovar and Canal de Mira) was sarnpled between June 1995 and June 1996 using a suprabenthic sled. Sarnples were collected monthly at 5 stations and every third month at 11 other stations, covering both channels from the lower to the upper reaches. The channels differ in their hydrodynamics. in the distribution of the sediment environments and in the salinity gradient, thus providing an invaluable exarnple for field studies. The results presented herein concern peracaridan crustaceans. Multivariate analysis (MDS, ANOSIiM and UPGMA) of the faunal data showed significant differences between the 2 channels. Spatial and seasonal variations within the channels were also significant. The distinctive structural features were the higher number of species and overall dorninance of mysids in the Canal de Mira as opposed to the dorninance of arnphipods and higher densities in the Canal de Ovar. In both channels there was a spatial pattern of high densities in areas with a rich sediment environrnent (fine sediments. high organic content, more diversified and stable food sources). Specific life cycle features, unfavourable ternperature and salinity conditions, and probably also increased predation pressure induced a general impoverishment of the peracaridan assemblage during winter. Amphipods (rnainly epibenthic and tube dwellers) and mysids (characteristic suprabenthic) showed defined spatial and seasonal patterns. Amphipods dominated the areas with rnore stable food sources, while the highly rnotile mysids succeeded in colonising and dominated in rnore dynarnic and unpredictable environrnents. Density fluctuations of amphipods and rnysids occurred seasonauy in non-coincident cycles. On the whole, rnysids were dominant during winter and spring and arnphipods dunng surnmer and auturnn.

The population of Corophium multisetosum Stock, 1952 in AreaÄ o displayed a semiannual, iteroparous life history. Mean longevity was 6 mo, with the estimated life span longer for overwintering individuals born in autumn than for individuals born in spring. Length-frequency data indicated that the length increment per moult is probably higher in males than females; however females moulted more frequently and achieved a larger body size. Preliminary growth rates were 100 lm d )1 for juveniles and 19 to 29 lm d )1 for mature females, with the lower values occurring during the winter. It was estimated that under favourable conditions females may attain reproductive size and mature within 1 mo. Although incubating females were present all year round, recruitment occurred in spring, almost ceased during the summer, peaked in autumn, and decreased again during the winter. Extreme temperatures and very low salinities during winter and summer may have deterred breeding, while moderate temperatures (15 to 20°C) and salinities >1 psu in spring and autumn were apparently favourable for reproduction. The unfavourable summer conditions constrained breeding and synchronised the timing of reproduction. In late-autumn and during the winter, as temperature decreased and brooding time increased, synchrony was progressively lost. Brood size varied as a function of embryonic developmental stage, size of incubating females, and season. The life-history pattern and reproductive features of C. multisetosum in AreaÄ o are closely related to temperature and salinity; other environmental conditions such as oxygen content of the water and food availability may also be relevant.