Trivia monacha

(da Costa, 1778)

Description (shell):
Adult shell solid, convolute (spire overgrown by last whorl); egg-shaped, with flattened apertural side. Sculpture of strong, often branching ridges and grooves, those at ends of shell more curved, almost U-shaped. Juveniles (up to 5 mm) with low spire of two or three tumid whorls. smooth at first, developing spiral striae in older shells. Aperture narrow, running whole length of shell; turned to left (i.e. towards swollen parietal region of body whorl) at ends, and ridged on both sides. Juvenile aperture widely open, narrowing adapically, not ridged (T. monacha-drawing).

Up to 12 x 8 mm.

Juveniles white, adults pinkish or reddish brown, with three dark spots; apertural side white.

Cephalic tentacles long, slender; mantle drawn out into long siphon anteriorly, extending laterally over all or most of shell in life. Foot with double-edged anterior margin and posterior pedal gland on sole; females with additional ventral pedal gland. Males with slender, cylindrical penis (T. monacha-animal).

Fairly common on rocky shores, under stones and ledges at LWST; associated with compound ascidians on which it feeds.

Distributed from Mediterranean to British Isles, more common in south (Distr. T. monacha).