Control of Gonadal Maturation of the Margined Madtom
Joseph N. Stoeckel and Richard J. Neves
Abstract.—Captive spawning is a strategy to bolster populations of rare madtoms Noturus spp., but very little is known regarding their reproductive development in captivity. The primary goal of this research was to develop methods to stimulate gonadal maturation of captive madtoms. We used the nonimperiled margined madtom N. insignis as a model species to investigate the effects of photothermal regimes on gonadal development and reproductive hormones. We also evaluated testicular development of madtoms injected with common carp Cyprinus carpio pituitary extract (CPE). Changing photoperiod, but not temperature, was required to induce oocyte maturation in a high percentage of captive female margined madtoms. Gonadosomatic index (GSI) values of captive females were similar to those of gravid wild ﬁsh collected during or just prior to the spawning season with the time to maturation of oocytes shortened by as much as 3 months. Many of the captive males developed large, square-shaped heads with swollen cephalic epaxial muscles as spawning conditions approached, but their GSI values were not different from those of ﬁsh sampled at other times of the year. Injections of CPE increased the GSI value and vascularization of testes but not the number of spermatozoa. In general, sperm production in mature male madtoms was enigmatic in captive and wild ﬁsh, inasmuch as motile sperm were observed only once. The heads of margined madtom spermatozoa are slightly ovate (4.3 0.2 μm long and 3.6 0.2 μm wide). The tails are centrally attached to the head and are more than 112.5 μm long. A pronounced, collar-like midpiece encircles the posterior portion of the head and anterior portion of the tail. Plasma testosterone concentrations in males peaked just prior to the spawning season at 6.5 ng/mL, but levels were not correlated with male GSI values. Plasma 17β-estradiol levels in females peaked just prior to the spawning season at 15 ng/mL and were correlated with gonadosomatic values.