Population Distribution, Sexual Dimorphism, and Reproductive Parameters

Pinnixa valdiviensis Rathbun, 1907 inhabits burrows of the ghost shrimp Callichirus garthi (Retamal, 1975) in the south-eastern Pacific. We described the host-use pattern and sexual dimorphism of P. valdiviensis to test for monogamy considering that the few other studied species of Pinnixa White inhabit their respective hosts as male-female pairs. Against expectations, P. valdiviensis lived either solitarily, or in small groups of up to four individuals; only a few burrows were inhabited by two crabs (either male-female or female-female pairs). The observations above argue against the notion that P. valdiviensis is monogamous. Furthermore, that a high frequency of solitary females were found brooding embryos and that the population sex ratio was skewed toward females suggests that males might roam among hosts in search of receptive females. We argue in favour of additional long-term (seasonal, 1 year) descriptive and experimental studies to reveal the mating tactics used by both males and females of P valdiviensis. We also examined individual-level reproductive parameters in the studied species as little is known about reproduction in symbiotic crabs. Fecundity varied between 383 and 1052 eggs crab(-1) with a mean +/- SD of 774 +/- 160 eggs crab(-1) and increased significantly with female body size. Embryo volume varied between 0.0169 and 0.0443 with a mean SD of 0.0222 +/- 0.0063 mm(3) and did not vary with female body size. Reproductive output (RU) represented a mean +/- SD of 5.74\% +/- 0.86\% of crab body dry weight and increased proportionally with crab body weight. The RU in P. valdiviensis is lower than that reported for other free-living and symbiotic crabs.