ASPECTS OF THE LIFE-HISTORIES OF PERIOCULODES LONGIMANUS, PONTOCRATESARCTICUS AND SYNCHELIDIUM-MACULATUM (CRUSTACEA, AMPHIPODA) AT MILLPORT, SCOTLAND

Detailed catch and life history data for Perioculodes longimanus and P ontocrates arcticus, together with more Limited comparable information on Synchelidium maculatum, are reported. Samples were taken year-roun d using a 0.5 mm mesh D-net towed over sand in the immediate sublittor al zone (-2 to -6 m Chart Datum) in Kames Bay, Great Cumbrae Island, S cotland. Estimated population densities of Perioculodes longimanus wer e highest in summer. The occurrence of Pontocrates arcticus was more e rratic and S. maculatum occurred only in midsummer in very low numbers . The mean sizes of all population categories of Perioculodes longiman us were bigger in summer than in late autumn and winter. Female Perioc ulodes longimanus parasitized with the copepod Sphaeronella minuta wer e significantly smaller than the whole category 'sexable females'. Adu lt female S. minuta (one per amphipod brood pouch infected) carried si milar to 70-200 eggs per sac. In both Perioculodes longimanus and Pont ocrates arcticus females were larger than males. In Synchelidium macul atum, by contrast, males and females were not different in size. Gravi d females were present virtually year-round in Perioculodes longimanus and Pontocrates arcticus, with particularly high proportions of ovige rous females in midsummer and low proportions in midwinter. Both of th ese species had an annual life cycle with only one generation per year . Longevity was estimated at 15 months. The sex ratio nearly always sh owed substantial female dominance in all three species. Fecundity appe ared to be related to body size but, due to egg losses during sample p rocessing, data were too few for detailed analysis. The eggs of Perioc ulodes longimanus and Pontocrates arcticus were elliptical in shape, w ith winter eggs being smaller than summer eggs in Perioculodes longima nus, but larger in Pontocrates arcticus. The percentage of ovigerous f emales in the population was significantly correlated with a range of environmental factors tested, the highest correlation being with maxim um weekly air temperature (for Perioculodes longimanus) and daylength (Pontocrates arcticus), although causative effects cannot be establish ed since temperature/daylength factors were not independent variables.