Individuals of Lysmata wurdemanni first mature in a male phase (MP) and then change to the external phenotype of females (female-phase=FP). The FPs retain the male gonadal system and function both as male and female (simultaneous hermaphrodites). The cost of maleness to FPs on fecundity was estimated with an experiment in which the social environment (FP opportunity to function as a male) was varied. The effect of maleness on brood size (number of embryos brood−1) and frequency of spawning (interspawn interval in days) was measured in FPs maintained in different social environments (treatments) over two complete spawning cycles. In treatment 1 (isolated FP) and 2 (1 FP, 1 MP), FPs could only function as females. In treatments 3 (2 FPs) and 4 (5 FPs, 5 MPs), FPs could breed both as male and female.

Baseline brood size was described for FPs with a regression equation of brood size on FP body size (carapace length). Brood size is positively and isometrically related to FP size. Brood sizes, adjusted for FP body size using the baseline regression equation, were compared among treatments 2–4 (isolated FPs in treatment 1 did not produce fertile broods). Brood sizes were significantly greater in FPs in treatment 2 (only female breeding) than in treatments 3 and 4 (both male and female sexual function possible). However, spawning was less frequent (longer interspawn intervals) in treatments 1 and 2 than in treatments 3 and 4. Total fecundity over a four-month breeding period was estimated with a model using mean treatment brood sizes and interspawn intervals. Estimates of total fecundity in FPs functioning both as male and female were much lower, in spite of somewhat more frequent spawning, than those in which male function was not allowed. The ‘price’ (cost) of maleness on fecundity appears to be compensated by the high selective advantage of male mating ability in the simultaneous hermaphrodite FPs of Lysmata species.