Emergence rhythms and tidal migrations in the brown shrimp Crangon crangon (L.)

The brown shrimp Crangon crangon (L.) is a burrowing caridean decapod, inhabiting sandy, sandy-mud and muddy substrata down to 90 m, but occurring mainly in shallow water and between tidemarks (Lloyd & Yonge, 1947; Allen, 1966b). The species also exhibits seasonal inshore and offshore migrations, reportedly in relation to salinity, temperature and availability of food (Ehrenbaum, 1890; Havinga, 1929, 1930; Meyer, 1935; Broekema, 1941, Academy Thesis, Groningen; Lloyd & Yonge, 1947; Allen, 1966a, b; Tiews, 1970). In addition Hartsuyker (1966) reported up and downshore migrations on each tidal rise and fall in the Wadden Sea. In contrast, Havinga (1929) stated that the species rests quietly buried in the substratum by day and feeds at night, and nocturnal emergence was confirmed by Lloyd & Yonge (1947) in aquarium observations. However the latter authors found no difference between field catches during day and night tides in the Bristol Channel and Severn Estuary. Tiews (1970) reported greatest activity at dusk and that C. crangon continued to swim during the night even after feeding. So far there is therefore evidence of nocturnal and tidal rhythms of emergence in C. crangon which deserves further experimental investigation.