A weakly discharging electric fish, Gnathonemus petersii (Mormyridae, Teleostei), as a model of integrated androgen effects on structure and behavior

This research was designed to examine the plasticity of structural and behavioral male-specific characteristics in a weakly discharging electric fish, Gnathonemus petersii (Günther, 1862) (Mormyridae, Teleostei), and the sexual hormones contributing to the development of these characteristics. Adult males are distinguished by massive bone expansion of the bases of a select number of anal-fin rays, a dorsally directed indentation of the posterior ventral body wall (possibly, a sign of muscular hypertrophy), and a characteristic electric organ discharge (EOD) with a longer duration of phases 2 and 3 and lower associated peak power spectral frequency (PPSF) of the Fourier transform. An anal-fin reflex, a characteristic mating display, was reported in adults of both sexes. In Experiment 1a, juvenile subjects were gonadectomized and implanted with silastic tubing containing either testosterone (T), dihydrotestosterone (DHT), 17β-estradiol (E) or nothing (blanks) for four weeks. DHT was most effective in inducing morphological changes: bone expansion in the bases of the anal-fin rays and alteration of the anal-fin topography. Histological data also suggested DHT-induced hypertrophy of the anal-fin musculature. T was most effective in producing behavioral changes (EOD). Both androgens induced the anal-fin reflex. E was ineffective. Experiment 1b confirmed the ineffectiveness of E: no morphological or behavioral changes were found in gonadectomized animals implanted with E for eight weeks. Experiment 2 tested the reversibility of androgen-induced changes and compared the effects of T and 11-ketotestosterone (11-KT). Gonadectomized subjects were implanted with either T or 11-KT for seven weeks, at which point the original implants were recovered and replaced with blanks, and the fish were observed for an additional six weeks. Both androgens produced male-specific changes in anal-fin morphology, characteristics of the EOD, and resulted in the appearance of the reflex. Following androgen withdrawal, behavioral characteristics reverted to their baseline level and the anal-fin reflex disappeared, whereas masculine morphological characteristics persisted. No significant differences were found between the effects of T and 11-KT. Together, these studies suggest a functional separation of androgen effects in the masculinization process in G. petersii: organizational effects of DHT on anal-fin morphology and activational effects of T on reproductive behavior.