A comparison between the hydroida fauna of the eastern Mediterranean and the Red Sea

Abstract

        This comparison is based on 480 plankton samples from the Red Sea and 133 from the Eastern Mediterranean, originating from various expeditions and from the author’s collections. Relating total number of hydromedusae species to the number of samples taken either in various regions of the Red Sea or in the coastal or oceanic waters of the Mediterranean shows that there are twice as many species of hydromedusae on the average in the oceanic waters of the Eastern Mediterranean than in coastal waters. Only samples from Elat show comparably low numbers of species, the average for the Red Sea being five to eight times higher. The Eastern Mediterranean is, therefore, conspi­cuously poorer in species than the Red Sea, even on the level of orders and suborders within the hydromedusae.

       Three hydromedusae, which were known previously from the Atlantic and Mediterranean waters only, are recorded from the Red Sea. A transport through the Suez Canal seems to be sure. On the other hand, two hydrome­dusae have been transported in the opposite direction. Little is known about faunistics and ecology of hydroids from the Eastern Mediterranean. Until now 49 species are known from the Red Sea, the majority from hard subtrates, making a comparison with the Eastern Mediterranean difficult where the hard substrates are relatively scarce. Five species of hydroids of Atlanto-Mediterranean origin could be found in the Red Sea. A transport through the Suez Canal is to be assumed. So far only two species seem to have been transported in the opposite direction.