Sexually mature European eels ( Anguilla anguilla L.) stimulate gonadal development of neighbouring males: Possible involvement of chemical communication

Abstract

This study was aimed to investigate whether sexual maturation of immature male eels could be stimulated indirectly by placing them in contact with either male (M inj) or female (F inj) eels in which sexual maturation had been stimulated directly by weekly injections of human chorionic gonadotropin (hCG) or salmon pituitary extract (SPE), respectively. Untreated males were placed either in the same tank or in a separate tank that was linked to the injected fish via a recirculation system. The hormonal treatments stimulated spermatogenesis and spermiation in M inj, and ovulation in F inj as well as an increase of the ocular ( I o) and gonadosomatic (GSI) indices in both sexes. Plasma levels of testosterone (T) and 11-ketotestosterone (11-KT) increased in M inj and T and 17β-estradiol (E 2) in F inj. A small peak of plasma 17,20β-dihydroxypregn-4-en-3-one (17,20βP) occurred during ovulation, while the plasma levels of 17α-hydroxypregn-4-ene-3,20-dione (17P) were undetectable in both males and females. The water conditioned by M inj and F inj induced significant, though relatively minor, increases in I o and GSI in uninjected males. In addition, uninjected fish showed small changes in plasma T and 11-KT levels, apparently related to the timing of spermiation and ovulation of M inj and F inj, respectively, as well as an activation of spermatogenesis (but not spermiation). Injected fish released free and conjugated T, 11-KT and E 2 into the water, although immature eels were unable to smell (by electro-olfactogram) any of these steroids or prostaglandin F 2α. However, immature males were highly sensitive to water extracts conditioned by spermiating M inj and pre-ovulatory and ovulated F inj. These preliminary results suggest the existence of chemical communication between maturing eels and immature males that stimulates gonad development, although the putative pheromone(s) involved has/have not yet been identified.