Shell
Up to 5mm high and 3mm broad. 3 or 4 apical whorls smooth, later whorls smooth, or with costae. White labial varix on adults with groove between varix and edge of aperture [image1]. Juveniles lack varix; their outer lip area may be transparent [3]. Sutures and curve of whorls appear more distinct on costate shells than on smooth ones. Aperture oval to D shape, adapical end pointed, lip everted at base. Ground colour of shell whitish to brown. Apical area may be tinted lilac. Final whorl has transverse dark brown streaks starting at the suture and sometimes ending at a dark brown spiral line [2]. Sometimes, transverse brown streaks on some other whorls, too. On adult shells the streak nearest the aperture bends sharply in a (diagnostic) comma across the varix [1].
Body
Flesh translucent white or yellowish white, sometimes has purple-black streaks. Translucent horn-coloured operculum attached to black opercular lobes [5]. Upper surface of foot has white pigment at anterior, and a broad dark brown to black transverse band [1] next to opercular lobes. Sole white with black/brown and orange-brown marks about a third of way from front [5]. Head tentacles translucent whitish with longitudinal line of white pigment, and a yellow patch basally behind the eye. Short bifid snout [2 ] with a few white pigment marks. Penis, with short filament at tip, behind right cephalic tentacle on males. Single pallial tentacle on right.
- Rissoa guerini
- No brown comma across labial varix.
- Always has costae; no smooth shell form.
- Snout brown.
- Yellow longitudinal line on tentacles.
- Southern species to English Channel and south coast of Ireland.
- Rissoa lilacina
- No brown comma across labial varix.
- Lilac rim on aperture.
- On adults, aperture constricted internally by tuberous growths.
- Snout brown.
- Yellow longitudinal line on tentacles.
- Rissoa rufilabrum
- Resembles R. lilacina but with less constricted aperture and proportionally smaller spire.
- Most records from west Scotland
- Rissoa porifera
- Resembles R. lilacina and R. rufilabrum but no costae or aperture restrictions.
- Most records west Ireland and west Scotland.
Often abundant at LWST on rocky shores. Extend to MTL and 15m sublittorally. On Corallina and other fine weeds, Laminaria holdfasts and under stones, grazing epiphytic growths and detritus. Breed all year, with summer maximum. Lens shape spawn capsules, less than 1mm diameter, containing 6-50 white eggs, laid on weed. Hatch in three weeks into veliger larvae with orange marks on edges of bilobed velum, right lobe larger than left. Settle on fine weed, and metamorphose, when shell 0.5mm.
Fretter, V. and Graham, A. 1962. British prosobranch molluscs. London, Ray Society.
Graham, A. 1988. Prosobranch and pyramidellid gastropods. London.