Oxynoe olivacea (Rafinesque, 1819) is an endemic Mediterranean sacoglossan belonging to the family Oxynoidae with a planktotrophic development and a veliger larvae. 20 specimens of O. olivacea were collected in October 1998 on C. taxifolia in the Straits of Messina and maintained in the laboratory. When not used for observations, the animals were kept at ambient light and temperature regimes in 35 l aquaria, with fresh food algae. Copulation was observed through a Wild dissection microscope. Copulatory aggregations are often formed by O. olivacea, but usually only two animals in a group copulate at any one time. Animals form pairs, but other animals remain in close contact, and will attempt to engage in copulation whenever intromission is interrupted. Up to 8 animals have been observed in one aggregation, but 5-6 animals are more common. When two animals meet, facing in opposite direction, they retract the anterior margin of the right parapodium to expose the female genital papilla. This papilla also extends and its aperture flares open. The penis is extruded from the slit-like aperture. Usually the penises of both animals are extruded several times very quickly while the penial apertures are opposing one another. The tip of the penis has a thin, curved extension, which is jabbed against the partner before actual intromission.
Intromission may be bilateral, i.e. both partners have penises inserted simultaneously. However, more often, intromission is unilateral and the two animals take turns functioning as male and female. Occasional unilateral intromission is not reciprocated by the other animal. Individual intromissions are very qiuck, usually lasting between 25 and 70 seconds. In most cases the smallest animal starts out functioning as male, and sometimes the bigger animal does not reciprocate intromission, but it always has its penis extruded and "jabs" at the partner through intromission fails. This may be interpreted as "cheating". If copulation is successful the first egg mass will be produced 1-2 days afterwards forming a regular spiral. The egg capsules are almost spherical (capsule long diameter= 120.0 micron +/- 5.2 (N=12); capsule short diameter=70.0 micron +/- 5.2 (N=12); nucleus diameter=60.0 micron +/- 5.2 (N=12)) and contain an opaque albumen. The number of eggs per egg mass varies considerably from 2000 to 4000. Development time is about 8 days at room temperature.

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