The mysid fauna of the River Tamar Estuary was sampled monthly between January and December 1989. The following species of mysid were found In the estuary. MosopodOPSIS slabborl, Noomysis Intogor, Praunus fioxuosus and SchistomysIs omata. The latter two species, present In low densities throughout tho year, occupied the lower, more salino (>30%o) parts of the estuary, whereas M. slabborl and N. Intogor were abundant In the upper estuary (0.5-30%o). Mosopodopsis slabbori produced between two and three generations per year, and showed a pattern of alternating early and lato-brooding generations. A maximum growth rate for this species was 0.24 mm d" for thd fastestgrowing summer cohort and annual production estimates ranged between a moan value of 50.6 mg dry weight m*3 y'and a maximum (based on the highest density measured on each sampling date) of 241.4 mg dry weight m*3 y'. This Is the first time that growth and production have been estimated for M. slabbort. There were marked seasonal and spatIal changes In abundance and distribution of M. slabborl. Abundance Increased from a low of <50 m-3 In winter to a maximum of 354 m3 (100 mg dry weight m3 ) In summer. Salinity appeared to be the major variable Influencing spatial distribution and the different agoclasses of M. slabborl had clearly Identifiable and distinct salinity distributions. There was evidence of a seasonal (winter) down-ostuary movement of the M. slabborl population. The ostuadno distribution of N. Intogorovorlappod with that of M. slabborl, but N. Intogor had a wider salinity distribution and colonisod lower salinity regions of the estuary (<5%*.) than M. slabborl. Largo Immature and adult N. Intogorworo not sampled successfully In those surveys, suggesting that those stages exhibited a different behaviour from other lifehistory stages. The physiology undorlying the wide salinity tolerance of A Intogor was examined. NoomysIsintogormaintainod Its blood hyporosmotictosoawatorln the salinity range 0.5 to 20%*. and the haomolymph responded rapidly to acute changes In salinity. This high osmorogulatory ability was correlated with changes In amino acid concontratlons, and glutamino, glycino, taudno and alanino were Identified as important osmoolloctors In this species.

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