Morphological variation in Lacuna parva (Gastropoda: Littorinidae) from different European populations

Abstract

Shells of the littorinid gastropod Lacuna parva were compared from 23 European localities and postglacial deposits in Sweden. The shells from the recent and the postglacial populations are similar with the exception of the recent population from Ellekilde Hage, Øresund, Denmark. Shells from Ellekilde Hage are different in having especially well developed whorls and only one colour morph. Differences in life-cycle and radula morphometrics further distinguish the Ellekilde Hage population from populations from the Isle of Wight, UK, and Roscoff, France. No striking differences in penial morphology were observed between the populations. It is suggested that low salinity and subtidal occurrence might be the causative agents of the conchological differences exhibited by the Øresund population.Keywords Lacuna parva · Littorinidae · Radula · Shell · Variation IntroductionThe family Littorinidae is probably the best-studied prosobranch family (see Reid 1989) and the intertidal genus Littorina has received particular attention (Reid 1996). The subfamily Lacuninae, however, has not been so well studied probably because of the smaller size and generally sublittoral distribution of most of the species. The holarctic genus Lacuna (chink shells) has approximately 22 species classified in the subgenera Lacuna and Epheria. Four of these species are found along European coasts; Lacuna (E.) crassior (Montagu, 1803), Lacuna (E.) vincta (Montagu, 1803), Lacuna (L.) pallidula (da Costa, 1778), and Lacuna (L.) parva (da Costa, 1778) (Reid 1989;Fretter and Graham 1980). L. crassior is an extremely rare and poorly known species. The biology of L. pallidula and L. vincta, which can be extremely abundant on seaweed, has been studied at localities in both North America and Europe (Fralick et al. 1974;Grahame 1977Grahame , 1982Grahame , 1994Martel and Chia 1991a). The European species display considerable variation in shell morphology when comparing specimens from different localities, but studies on this variation are lacking.L. parva is present along the Atlantic coast of North America to Cape Cod and the European coasts from the northern part of Norway to the northern part of Spain (Fretter and Graham 1980;Ockelmann and Nielsen 1981). In the Øresund, Denmark, and the southern Kattegat, on the Swedish coast, it is uncommon. Shells of this species also occur in postglacial deposits in Bohuslän, Sweden. It is more abundant in the English Channel and on most coasts around the British Isles. L. parva has direct development and the eggs are laid in semitransparent hemispherical spawn masses on the seaweeds on which the snails live (Ockelmann and Nielsen 1981).Ockelmann and Nielsen (1981) have pointed out considerable variation in shell characters of L. parva from the northern Øresund and from the Channel: differences in thickness, suture depth, aperture shape, width of umbilicus and colour. In addition, they noted differences in time of breeding and maturation of adults, and in choice of food algae.Ockelmann and Nielsen (1981) argued...