Population structure of the hadal amphipod Hirondellea gigas (Amphipoda: Lysianassoidea) from the Izu-Bonin Trench

The population structure of Hirondellea gigas (Birstein and Vinogradov, 1955), collected by baited trap from 8172 and 9316 m in the Izu-Bonin Trench (NW Pacific) was examined. Specimens were categorized according to sex and life stage. At 8172 m, juveniles comprised the overwhelming majority of the population, whilst at 9316 m the male: female: juvenile ratio was more evenly distributed, suggesting vertical ontogenetic structuring. Furthermore, juveniles from 8172 m were significantly smaller than those from 9316 m with an average body length of 11.1 mm (+/- 4.6 S.D.) compared to 19.8 mm (+/- 3.1 S.D.). Females and males showed the opposite trend to juveniles, with both the largest individuals and the greatest proportion of males and females occurring at 9316 m, no female 6 nor brooding females were captured. Female reproductive strategies and the environmental drivers of ontogenetic structuring of H. gigas populations are discussed. We conclude that pressure per se does not drive the observed trends but rather an interaction between depth (pressure) and topography-influenced distribution of resources in terms of both quality and quantity.