Aspects of the biology of the nonindigenous Foraminifera Amphistegina spp. in Maltese waters

Abstract

Organisms from the genus Amphistegina (Phylum Foraminifera) do not occur natively in the Mediterranean Sea but have been introduced via the Suez Canal. Amphisteginids have invaded most of the eastern basin of the Mediterranean Sea and some central Mediterranean localities, including the Maltese islands. Further western spread of these organisms is expected in the coming years. During the present study the abundance of Amphistegina lobifera in Maltese waters was investigated, by collecting amphisteginid tests from the sediment samples at monthly intervals, extracting amphisteginids and estimating the live: dead ratio monthly. Amphisteginids were abundant in the summer months and declined in the winter months. This species were the dominant foraminiferan species in the sediment samples. An investigation on whether there is a link between the abundance of amphisteginid tests and the alien algae Caulerpa taxifolia var. distichophylla showed that there was no direct relationship between the two. Ex situ experiments on the substratum preferences of amphisteginids showed that these prefer algae such as Halopteris sp., presumably because the frond structure of these minimises high exposure to both light and wave action. Experiments focusing on the temperature, light intensity and salinity tolerances of the amphisteginids were carried out to assess the potential for possible spread of the organisms into the western basin of the Mediterranean Sea in the coming years. The results have indicated that the species is not viable at temperatures below 14°C or higher than 32°C and the optimal temperature range is 20–30°C; the optimal salinity range was found to be between 36–41g/L. This suggests that conditions along North African coasts of Algeria and Morocco are already suitable for this species. Amphistegina lobifera is known to have a mixotrophic diet, relying on both deposit feeding and diatom endosymbionts. Experiments on taxis in this species showed that it is positively phototaxic and negatively geotaxic and is able to emerge from the sediment if it is buried by less than 1cm of sediment, presumably due to their dependence on light energy for nutrition.