In vitro fertilization experiments using sockeye salmon reveal that bigger eggs are more fertilizable under sperm limitation.

Although theory and widespread evidence show that the evolution of egg size is driven primarily by offspring and maternal fitness demands, an additional explanation invokes sperm limitation as a selective force that could also influence egg size optima. Levitan proposed that constraints from gamete encounter in external fertilization environments could select for enlargement of ova to increase the physical size of the fertilization target. We test this theory using in vitro fertilization experiments in an externally fertilizing fish.

Aromatase pathway mediates sex change in each direction.

The enzyme aromatase controls the androgen/oestrogen ratio by catalysing the irreversible conversion of testosterone into oestradiol (E2). Therefore, the regulation of E2 synthesis by aromatase is thought to be critical in sexual development and differentiation. Here, we demonstrate for the first time that experimental manipulation of E2 levels via the aromatase pathway induces adult sex change in each direction in a hermaphroditic fish that naturally exhibits bidirectional sex change.

The role of steroid hormones in protogynous sex change in the Blackeye goby, Coryphopterus nicholsii (Teleostei: Gobiidae).

To detect and describe correlations between steroid concentrations and sexual function in the protogynous Coryphopterus nicholsii, we examined whole-body concentrations of the gonadal steroids testosterone (T), 11-ketotestosterone (11-KT), and 17beta-estradiol (17beta-E2). Our results show that T, 11-KT, and 17beta-E2 are naturally occurring steroids in both female and male C. nicholsii.

PCPA increases aggression in male firemouth cichlids.

We observed the effect of the 5-hydroxytryptamine synthesis inhibitor p-chlorophenylalanine on aggression in male firemouth cichlids (Cichlasoma meeki). p-Chlorophenylalanine-treated firemouths showed a significantly greater number of mirror-directed bites as compared with saline-injected controls. This finding is consistent with the effects of this drug on social aggression in mammals and birds and supplements previous reports that 5-hydroxytryptamine inhibits aggression in fish.

Effect of dominance status on sex hormone levels in laboratory and wild-spawning male trout.

We investigated the relationship between male social status and hormone levels in salmonids spawning under laboratory and field conditions. In small groups of rainbow trout (Onchorhynchus mykiss) spawning in the laboratory, dominant males had higher plasma levels of testosterone (T) and 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P) compared with subordinates. Steroid levels increased in subordinate males that became dominant after dominant males were experimentally removed; higher steroid levels in dominant males appears to be a result rather than a cause of their social status.