THE LIFE-HISTORIES OF THE OFFSHORE OEDICEROTIDS WESTWOODILLA-CAECULA AND MONOCULODES-PACKARDI (CRUSTACEA, AMPHIPODA) FROM LOCH FYNE, SCOTLAND

Results are presented of a comparative study on seasonal variations in population density, population structure and reproductive bionomics o f two species of offshore benthic oedicerotid amphipod, Westwoodilla c aecula (from 35 m depth) and Monoculodes packardi (from 190 m) in Loch Fyne (Scotland). Observations on live coloration and behaviour of the se species in laboratory aquaria are also included. Male W. caecula we re, on average, smaller than females. In M. packardi (atypically among Oedicerotidae) the reverse was true. Population densities of both spe cies, calculated from sledge net tows (l-mm mesh), fluctuated erratica lly over one year (1992). Ovigerous W. caecula were recorded virtually year-round, but were commonest from midsummer to early autumn. Oviger ous M. packardi were noted in spring and early autumn, Monoculodes pac kardi had a semi-annual life cycle, but the pattern for W. caecula was less clear. The two generations of M. packardi grew at broadly simila r rates, presumably associated with the damped seasonal fluctuations i n physical environmental factors at 190 m. Female W. caecula possibly moult eight or nine times, and may produce three successive broods in a Lifetime (M, packardi may only produce two). A positive correlation was found between fecundity (stage I eggs) and body length in W. caecu la but, although a similar trend was apparent in the data for M. packa rdi, relevant data were too few for significance to be achieved. Ovoid eggs increased in volume by x2 from stage I-IV in W. caecula, or x1.5 from stage I-III in M. packardi. Brood sizes were much smaller in W. caecula than in M. packardi, although apparent brood mortality was hig her in the latter. There was no significant relationship between the p roportion of ovigerous females in the populations of either species an d seasonal variation in physical environmental factors considered (tem perature, salinity, daylength).