Signal Value of the Genital Tassel in the Male Tilapia macrochir Blgr. (Pisces: Cichlidae)

Abstract

IN some of the mouth brooding Tilapia species, during breeding season the males develop highly SolSsjdi appendages on their genital papillae. These are long bifid filaments which are either fringed or festooned with colourful blobs of tissue. The biological significance of these ‘genital tassels’ is still obscure, although field-studies have indicated that they play a part during spawning activities^1,2. Careful observations of T. macrochir in our laboratory tanks have revealed some interesting details. The genital tassel of the male in this species consists of two bright white threads fringed with a few smaller threads of the same colour (Fig. 1). In a fully grown male of 30–40 cm the appendage reaches a length of 5 cm. It acts as a special signal which is presented to the ‘ripe’ female during courtship and spawning, and assures fertilization of the eggs in the following way: (1) When a female is ready to spawn she enters a male's spawning pit. After some courtship movements, the male starts to drag his tassel over the bottom of the pit and up the surrounding wall. On top of the wall, he presses his genital papilla to the ground and then rises and swims tail-first back over the female across the spawning pit (Fig. 2). Simultaneously he ejects from his genital opening a long, 0.5-mm-thick whitish thread which sticks by its free end?for a very short time only—to the spot where he stopped dragging his tassel, and a few seconds later the threads drift away in the water. (2) The female follows the male and grasps the spermatophore-like thread between her lips and inhales it. Very often she not only takes the thread but also the whole genital tassel, or parts of it (Fig. 1), into her mouth. She does this even before she has started to lay eggs and later fairly regularly following each single spawning act. She repeatedly drops some ten eggs, often from a rather peculiar position, namely, standing obliquely head-down several centimetres over the pit. She does not, however, immediately snap up the eggs into her mouth, but will usually follow the tassel-dragging male once more. (3) The sperm-thread which the male ejects does not normally meet the eggs immediately, since it drifts away from the pit while the eggs are still laying there. The female, therefore, has to snap up the thread to bring it into contact with the eggs, which she starts to gather into her mouth immediately afterwards. The solid sperm mass of this species seems rather favourable in this situation since it is not blown out through the female's gill-openings as readily as would be a loose cloud of milt-fluid found in most of the other Tilapia and cichlid-species. (4) The tassel directs the female's attention to the male's genital opening just before the sperm is ejected, and hence minimizes the danger of the ‘spermatophore’ being lost. This seems to be the main function of the tassel, which in T. macrochir resembles a large number of whitish sperm-threads and, indeed, is treated by the female as such. I hope to investigate this further with dummy experiments.