Animal reproduction is a complicated process involving the coordination of many systems that act at different levels to control behavior and gonad function. In teleost fish, steroids produced by the gonads are responsible for sexual behavior and sexual differentiation and also have important roles as pheromones, chemical signals released by an animal that trigger specific physiological processes in the recipient. Many fish species use odors and pheromones for gender recognition, synchronization of gamete maturation and spawning. However, most studies to date have focused on the role of pheromones during spawning and not on their role in pre-spawn events, such as reproductive development. In the present study, Mar Huertas, Joan Cerdàand their team have investigated whether the presence of sexually mature European eels was able to stimulate gonadal development and/or maturation in neighboring immature males.

The team first treated groups of male and female eels with hormones to initiate sexual maturation. Then, as these fish were maturing, immature eels were exposed to the water that had been conditioned by either the maturing males or females. This protocol allowed them to determine if the presence of maturing eels would stimulate reproductive development in the immature males. What the team found was that immature male eels exposed to sexually maturing males or females showed a small but significant increase in the size of their gonads. However, sperm production did not occur in the exposed males, despite their increased gonad size. Histological examination of the testis determined that these exposed males had more highly developed testis compared to control fish that had not been exposed to maturing fish, but they were less developed than mature males.

When the team looked at the plasma concentrations of sex steroids in the maturing males and females that had been treated with hormones, they found that the steroid levels changed significantly at the onset of gonadal maturation, with the plasma levels of 11-ketotestosterone and testosterone becoming elevated in maturing males while maturing females showed an increase in plasma testosterone and 17β-estradiol levels. Immature males in contact with water that had been exposed to maturing males or females showed significant changes in plasma 11-ketotestosterone and testosterone concentrations compared to controls. Together with the changes in gonad size and development, these results suggest that water conditioned by sexually maturing males or females stimulates spermatogenesis in immature males.

But which waterborne compounds are involved in triggering the immature male's gonadal development? The team suspected that testosterone,11-ketotestosterone, 17β-estradiol and other steroids released by the maturing males and females may act as pheromones. However, when the team tested the response of the immature male's olfactory epithelium, which detects pheromones, to these compounds they found no response, ruling them out as potential pheromones. The team went on to assess the olfactory potency of water collected from the maturing males or females and found that it successfully elicited an olfactory response in the immature fish. Moreover,the largest responses were to water collected after the initiation of spermiation or during ovulation, suggesting that a pheromone is released during these times. Whether these odorants are novel compounds, other steroids or steroid mixtures remains to be investigated. However, there appears to be something in the water enhancing maturity in male eels. Now, if only we could bottle that.

Huertas, M., Scott, A. P., Hubbard, P. C., Canário, A. V. M., Cerdà, J. (
). Sexually mature European eels(Anguilla anguilla L.) stimulate gonadal development of neighbouring males: possible involvement of chemical communication.
Gen. Comp. Endocrinol.