As littorina mariae:
Diagnostic characters
Closely similar to L. obtusata with a shorter, even totally flat spire, the outer lip arising higher on the last whorl and nearly normal to the shell axis. In apical view (in young shells especially) there is no notch where outer lip and last whorl meet. Throat constricted. Penis with long tapering tip held under glandular part, which has about twelve glands in one row. Ovipositor not pigmented. Oviparous, hence female duct glandular.
Other characters
The same colour morphs occur in mariae as in obtusata, yellow and dark reticulated shells being the commonest. Up to 11mm high, 12 mm broad; last whorl occupies 95-100% of shell height, aperture 85-90%
L. mariae has the same habits as L. obtusata but is often found at a slightly lower level on the shore, with Fucus serratus; it also prefers more exposed situations (Goodwin & Fish, 1977). Its full distribution within the British Isles is not certain since its recognition as a distinct species is relatively recent. It has, however, been found in Kent (Shellness), and on most southern and western shores as well as in Jersey, and on northerly east coast ones to Orkney and Shetland (McMillan, 1981).
The spawn of this species is, in general, like that of L. obtusata. It is almost always laid on Fucus serratus and is more commonly kidney-shaped rather than oval in outline and is not circular (Goodwin, 1975; Goodwin &Fish, 1977).