Large-scale patterns of shell variation in Littorina striata, a planktonic developing periwinkle from Macaronesia (Mollusca: Prosobranchia)

Abstract

Littorina striata King and Broderip, 1832 is a strictly Macaronesian, intertidal periwinkle with planktonic development. The species displays a high degree of shell variation involving size and sculpture (nodulose vs smooth shells). The present work provides a preliminary account of some aspects of this shell variation on wave-exposed shores over the entire geographical range of the species. Based on morphological patterns observed among other prosobranchs it was predicted that southern specimens of L. littorina should on the average be larger, heavier, more nodulose, and should show more shell repair marks, than northern specimens. These expectations were confirmed for shell size and weight. In contrast, there was no consistent pattern in nodulosity between archipelagos, even though there were differences at much smaller scales. Shell repair marks were more prevalent in northern populations, but this trend was only due to a significant N–S difference among nodulose shells. This is surprising as nodulose shells displayed significantly fewer shell repair marks than smooth shells. These observations were tentatively interpreted as a function of presumed differential N–S patterns of wave action and ambient temperatures. In this context, wave action in Macaronesia seems to increase in the south (contrary to what current theories predict). This atypical situation may confound the interpretation of morphological patterns in L. striata so that firm conclusions cannot be drawn without further experimental work at different spatial scales. Nevertheless, it seems that generalisations about macrogeographic shell morphology patterns, based on interspecific comparisons, are not directly applicable to intraspecific patterns, and may strongly depend on local conditions which make adequate sampling and data treatment very difficult.