A preliminary study on genetic differentiation in Littorina saxatilis fromGalway Bay, Ireland: Littorina tenebrosa Montagu - a valid species or ecotype?

Littorina tenebrosa is a small fragile-shelled periwinkle which lives on pe rmanently submerged algae in coastal lagoons and non-tidal brackish pools. This periwinkle is a member of the rough periwinkle group which also compri ses Littorina saxatilis, L. arcana, L. compressa and L. neglecta and is mos t closely related to L. saxatilis although its exact systematic status is i n some doubt. Based on its unique ecology many believe L. tenebrosa to be a valid species. However, shell morphometric and allozyme analyses on Scotti sh and Swedish populations of L. tenebrosa and L. saxatilis have indicated that the two periwinkles are virtually identical. Preliminary results on fi ve allozyme loci (AAT-1, GPI, PGM-1, PGM-2 and PNP) in samples of L. tenebr osa and L. saxatilis from Golam Head, Lettermullen, and other locations on the west coast of ireland show L. tenebrosa to be genetically differentiate d from L. saxatilis. At Golam Head, where opportunities for gene flow occur between the two taxa, L. tenebrosa is as genetically differentiated from l ocal L. saxatilis as it is from L. saxatilis from more geographically dista nt locations.