Ecology and distribution of two sympatric, closely related sponge species, Halichondria panicea (Pallas, 1766) and H. bowerbanki Burton, 1930 (Porifera, Demospongiae), with remarks on their speciation

North Atlantic coastal waters harbour two closely related sympatric sponges belonging to the genus Halichondria, viz. H. panicea and H. bowerbanki. From the study of their littoral and sublittoral microdistribution and ecology in the Oosterschelde area (= Eastern Scheldt, S.W. part of The Netherlands) it is concluded that both occupy largely similar niches, although it was found that H. panicea is better equipped to endure exposure to air since it occurs up to a higher intertidal level than H. bowerbanki, while the latter is better equipped to withstand siltation. Other environmental factors as depth, light, current velocity and salinity affect the distribution and behaviour of both species similarly. From these observations it can be inferred, that speciation from a parent species cannot have been sympatrically through ecological segregation of individuals. The possibilities of geographical isolation of populations of a parent species on both sides of the Atlantic Ocean during past glacial periods resulting in an originally American species ( H. bowerbanki) and an originally European species ( H. panicea) are discussed.