Feeding preferences of Oxynoe olivacea (Opisthobranchia: Sacoglossa) among three Caulerpa species

Abstract

The opisthobranch order Sacoglossa (Ascoglossa) is a relatively widespread group of stenophagous sea slugs comprising about 250 species. Sacoglossan are deemed as marine specialist herbivores and are often among the rarest components of inshore communities being strongly entrained upon the life histories of their algae. 1 The close association between these molluscs and their food algae has invoked terms such as co-evolution 2 and provides a fascinating biological puzzle, distinct from other photosymbiotic associations in several ways. Moreover, physiological adaptations as functional kleptoplasty (� chloroplast symbiosis) and the capability of taking up and concentrating secondary metabolites from the algal food have been described. 3,4 The majority of species feed upon siphonalean green algae, 5,6 and the stenophagy is plesiomorphic, feeding being restricted to the green, siphonalean algal genus Caulerpa 7,8 for the entire suborder Oxynoacea. The most representative species in the Mediterranean is the endemic Oxynoe olivacea (Rafinesque, 1819) characterized by small size (20 mm long), an extreme host-specificity, a cryptic coloration and a planktotrophic veliger larvae. 9 Over the last century the algal genus Caulerpa has been undergoing prolific growth in the coastline of Mediterranean Sea. 10 In particular, the tropical algae Caulerpa taxifolia (Vahl) C. Agardh and the Lessepsian migrant Caulerpa racemosa (Forsk.) Agardh are rapidly colonizing the coastal water of the Mediterranean Sea. 11,12 The rapid expansion of these algae could be attributable to their functional plasticity to different environments, 13,14 and to anthropogenic and climatic causes: probably unusual endogenous or exogenous factors are encouraging their spread in the Mediterranean sea, 15 which is becoming a new favourable environment for tropical species. 16,17 These invasions may cause serious ecological problems, such as habitat alterations and changes in the dynamics of macrobenthic assemblages. 18 The marked stenophagy of O. olivacea could make this sacoglossan a potentially important biological control agent of allochthonous Caulerpa populations. 9 The siphonalean, stoloniferous C. taxifolia and C. racemosa represent, in fact, potential host species to the suctorial O. olivacea, a species so far specialized on the autochthonous Caulerpa prolifera (Forskal) Lamouroux. The aim of this study is to investigate food preferences of the native O. olivacea using C. prolifera, C. taxifolia, and C. racemosa as food items. Thirty specimens of adult Oxynoe olivacea were collected with Caulerpa taxifolia from Punta Faro, in the Straits of Messina, on November 2000, and brought back to the laboratory. In the same months, samples of Caulerpa prolifera and Caulerpa racemosa were collected at Santa Panagia Bay (south east Sicily) and maintained in aquaria. Prior to experimentation, all specimens were acclimated for two weeks: during this period, they were kept in aquaria with circulated natural seawater, at ambient light and temperature regimes. Furthermore all O. olivacea were maintained on a diet exclusively of C. prolifera and C. racemosa, because they came from a C. taxifolia meadow and probably needed to acclimatize to the other two algae. After two weeks, a series of choice experiments were conducted to assess whether or not slugs preferred to consume either native or host algae. Before starting experiments all specimens were starved for 2 h. Then, one slug was placed in the centre of a 20-cm dish with filtered seawater and containing equal masses of the three species of Caulerpa (~6 g wet weight) set randomly at an angle of 120°. All 30 O. olivacea were tested independently, thus ensuring the independence of data. The number of animals reaching and staying on an alga was recorded until no more changes occurred, as this can be considered an index of feeding preferences recorded. 19 The results of all experiments were combined and analysed by non-parametric statistics using chi-square analysis: the observed and the expected frequencies of animals on each algal species were calculated. All slugs chose one food and thus were included from the computation of chi square analysis. Most animals used in the food preference experiments headed towards the foods and started feeding immediately; a smaller number were engaged in crawling or orientating behaviours before feeding. The experiments lasted from 30 to 70 min. The observed frequencies of animals on each algal species showed a clear tendency for Oxynoe olivacea to prefer the native Caulerpa prolifera to the introduced Caulerpa taxifolia and Caulerpa racemosa. This resulted in a highly significant chi-square test (Table 1, P