Sperm migration prior to fertilization in the hydroid Gonothyrea loveni

The medusiform, gamete‐bearing sporosacs (meconidia) of Gonothyrea are pushed out of the gonangium but remain attached to its distal end. The female meconidium is known to secrete a substance capable of attracting the sperm chemotactically. If sperm approaching the female meconidium pass close enough to one of its tentacles, they stick to the tentacle by their tails and appear to be suspended a few microns above the surface of the tentacle ectoderm cells. They then begin to migrate along the tentacle, eventually arriving at the base where they can enter the meconidium. The direction of movement while on the tentacle surface cannot now be directly related to the source of the chemotactant. The propulsive source is the sperm flagellum, though the beat is markedly suppressed over most of its length. An extracellular surface coat is probably responsible for both the adherence to the tentacle and the suppression of motility of the flagellum. Comparative tests using other genera show that the sperm of Campanularia flexuosa does not stick to the female Gonothyrea meconidium tentacles, but those of C. calceoliferaa do. The specificities are maintained in the reverse case, as well as when active extracts are used against the sperm.