Littorina obtusata

(Linné, 1758)

Description (shell):
Shell solid, with blunt apex and very much reduced spire, last whorl tumid, comprising >90 % of shell height; sutures shallow, lying well above periphery of preceding whorl so as to obscure much of it from view. Shell appears smooth but has very fine reticulation of numerous spiral striae and prosocline growth lines. Aperture broadly oval, drawn out adapically to an anal sinus, giving a tear-drop shape. Inner lip reflected over columellar region, outer lip thickens rapidly toward the throat; no umbilicus or umbilical groove. Juvenile aperture more elongate, its base drawn out into a slight spout, with a more pronounced anal sinus, throat not thickened.

Up to 15 x 17 mm.

Colour extremely variable and a number of colour morphs have been recognised: olivacea (uniform light to dark green), citrina (uniform yellow to orange, with white throat), and reticulata (green, brown, or black, with reticulate pattern) are most common.

Body similar to Littorina littorea , but colour tends to follow that of shell; cephalic tentacles with two longitudinal lines and no transverse stripes. Examination of the penis (flat and with short tip), penial glands (16-53 glands in two to three rows) and ovipositor (pigmented) may be necessary to confirm separation of this species from Littorina fabalis (L. obtusata-drawing).

On weeds, especially Fucus vesiculosus and Ascophyllum nodosum, from MHWN to MLWS; occasionally in shallow sublittoral. Tolerant of low salinity and a wide range of exposure. Colour morphs appear to have separate, though overlapping habitat preferences.

Distributed from western Mediterranean to northern Norway; on all British coasts, Holland and Helgoland (Distr. L. obtusata).