Littorina fabalis.
Aperture size is affected by the physical and biological environment and by growth stage. The shell walls thicken with maturity in both species, constricting the aperture internally, generally more markedly in L. fabalis, but varying in degree with environment. The most useful measure of shell thickness is that of the lip at the base of the columella (C) divided by the length of the aperture (LA). In a sample of 59 shells, including less frequent extreme forms, C/LA was usually less than 0.29 on L. obtusata and greater than 0.29 on L. fabalis, but only with a 75% accuracy (Reid, 1996). Accuracy was greater if extreme forms were excluded.
On this specimen of L. fabalis C/LA = 0.30
Juveniles of the two species are often very similar with thin shell walls, sharp aperture rim and unconstricted opening, and often the anterior (base of aperture) is drawn out into a moderate spout.
Menai Strait, Wales. August 2010.
Full account at: 02 Differentiation L. obtusata/L. fabalis
41 Differentiation L. obtusata/ L. fabalis
