Clayton Kunio Fukakusa

Abstract

All fishes in the tribe Glandulocaudini have hypertrophied tissue with club cells in the caudal fin (the caudal organ). Because this structure is present only in adult males, it is hypothesized that these cells secrete a reproduction-related pheromone. The hypothesis that the caudal organ releases chemicals that attract females is tested in Mimagoniates inequalis. In a Y-maze and an aquarium, females were attracted to a caudal organ extract and to water that was conditioned with caudal organ-bearing males, respectively, but not to caudal-fin lobe extract or water conditioned with males from which the caudal organs were removed (control stimuli). In tests with male-female pairs, there were no differences in the responses to caudal organ extract and male caudal organ-conditioned water, but the responses to both stimuli differed in relation to the controls. Male-female pairs engaged in fewer courtship events and more agonistic interactions than they did without chemical stimuli and with control stimuli. These results provide evidence for a possible pheromonal system in M. inequalis. The caudal organ is a specialized secretory structure that produces a chemical signal that attracts females and increases the aggressiveness of males.

Link: https://www.pfeil-verlag.de/wp-content/uploads/2020/09/IEF-1127-www.pdf

Fig. 1. Caudal fin of Mimagoniates inequalis: a, UFRGS 19190, adult male, 34 mm SL, arrow points to caudal organ.

Fig. 1. Caudal fin of Mimagoniates inequalis:  b, osteology of caudal skeleton and caudal-fin squamation (lower left inset), side view. Highlighted are the regions removed for extract production. Caudal rays region (CL, ). Caudal organ region (CO, ). Modified from Menezes & Weitzman (2009).