Oxynoe olivacea (Rafinesque, 1819) is an endemic Mediterranean sacoglossan belonging to the family Oxynoidae. The species of the genus Oxynoe show a pronounced stenophagy feeding exclusively on the cytoplasm of the green algal genus Caulerpa.. we have investigated the food preferences of we have investigated the food preferences of O. olivacea using C. prolifera, C. taxifolia and C. racemosa as food items.
40 specimens of O. olivacea were collected, November 1998, with C. taxifolia in the Straits of Messina and maintaned in the laboratory. C. prolifera was collected with C. racemosa at Santa Panagia Bay (South East Sicily), October 1998.
When not used for experiments, the animals were kept at ambient light and temperature regimes in 35 l aquaria, with fresh food algae. After a learning time of 5 days on C. prolifera and C. racemosa, food preference was tested by letting animals choose among the three algae. In three series of 20 experiments, individuals of O. olivacea were starved for two hours. Then, 5 specimens for each experiment were placed in the centre of a 20 cm dish containing equal masses of algae. The number of animals on each food alga was counted at 5 min. intervals until either all animals had found food, or no more changes occurred. In the experiments the same animals were tested, but the petri-dish with the food was turned 180° to see what role was played by the incident light. The results from all experiments were analysed by non-parametric statistic using the chi-square analysis.
The experiments lasted from 30 min to 70 min. The observed and the expected frequencies of animals on each algal species were calculated. O. olivacea showed a significant food preference, p< 0,005, when given the choice among C. taxifolia, C. racemosa and C. prolifera it preferred the latter. In the laboratory a marked preference for C. prolifera is exhibited by all animals, while C. taxifolia is consumed in smaller amounts. In the field however, the greatest number of specimens has been found on C. taxifolia.

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