The breeding cycles of females of seven species of amphipod (Crustacea) from the Clyde Sea area

Abstract

The life cycles of females of seven species of amphipod were determined from fortnightly sampling of sites in the Clyde Sea area. Three species, Lembos websteri, Corophium bonnellii and Bathyporeia pilosa, exhibited semi-annual life cycles with an overwintering generation producing a shorter-lived summer generation. The deep-water species Stegocephaloides christianiensis exhibited continuous breeding, as did the sublittoral species Gammaropsis nitida. The other deep-water species, Ampelisca macrocephala, exhibited a biennial life cycle. The littoral species Echinogammarus pirloti exhibited a reproductive cycle that was closely correlated with the abundance of its food supply. With the exception of E. pirloti and S. christianiensis, the reproductive cycles of the species studied were closely correlated with photoperiod. Environmental temperature was of secondary importance to seasonal variation in breeding in all species. A female bias in the sex ratio was observed throughout the year in four of the seven species. In E. pirloti there was a significant male bias in the sex ratio. No males were discovered in the populations of C. bonnellii and S. christianiensis. Possible factors contributing to the observed seasonal changes in sex ratio are discussed.

Keywords :

• Amphipoda
• life cycles
• breeding periodicity
• sex ratio
• Clyde Sea area