Sexual dimorphism and intraspecific aggression, and their relationship to sex ratios in Caprella gorgonia Laubitz & Lewbel (Crustacea: Amphipoda: Caprellidae)

Abstract

The amphipod Caprella gorgonia Laubitz & Lewbel is an obligate commensal on gorgonian octocorals. Its primary host is Lophogorgia chilensis (Verrill), found below 20 m. C. gorgonia breeds throughout the year, with wide fluctuations in abundance. Mating and oviposition follow molting. Sex reversal does not occur; two distinct sexes are present from the first instar after emergence from the brood pouch. Young males and females grow at approximately the same rate, but males are larger by a relatively constant increment. Males continue to grow at their original rate to a maximum size (about twice that of females). The growth rate of females is not limited by the onset of reproduction and brooding, but rather by an approach to maximum size when the rate is greatly reduced. Fecundity of females is not affected by size. The population sex ratio is about 1:3 (males:females), and about 1:4 among adults. The secondary sex ratio is 1:1. The post-emergence sex ratio bias is a result of heavier mortality among males. Sex ratios drop from 50% at emergence to 25% as females approach maximum size, then rise to 100% in larger size classes. Differential predation on males did not appear to be a source of any sex ratio bias. Adult males possess a “poison spine”, a puncturing weapon on the large second gnathopod, which functions in mating-related intraspecific combat with other males. Intraspecific male aggression during mating is a major cause of sex ratio bias. In the laboratory, increased density in breeding groups may affect mortality due to male aggression. In nature, adult sex ratios are negatively correlated with population density. The reproductive capacity of the population is not limited by a shortage of adult males, despite the low adult sex ratio.