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(Pennant, 1777)

Description
A small snail. Shell an elongate cone, length/width ratio 2:1, slightly transparent, glossy, with six to seven flat-sided or slightly tumid whorls, apex rounded; last whorl comprising >50 % of shell height, slightly angulate at periphery, especially in younger shells. [H.ulvae-s]. Superficially smooth but with microsculpture of fine prosocline growth lines and finer spiral striae. Aperture slightly pear-shaped (more angulate in young shells), outer lip arising tangentially to last whorl, forming a slight spout basally where it meets columella; inner lip reflected over last whorl, leaving a small umbilical chink [H.ulvae-s].
Body with long bifid snout, often with transverse band of dark pigment anteriorly and a triangular or shield-shaped pigmented area between cephalic tentacles; a single, small pallial tentacle projects from mantle edge near junction of last whorl and outer lip. Cephalic tentacles long, slender, setose, left slightly thicker than the right; eyes at base. A dark pigmented band placed at a distance greater than its own length from the tentacle tips. Penis large, a smooth sickle-shaped curve, with pointed tip [Hydrobia ulvae].
Juveniles with little more than one whorl at time of hatching. No sculpture of shell beak. Velum bilobed; lobes small, unpigmented or with a semilunar, purplish-black on each. Preoral cilia weak and can scarcely lift the larva; no food groove. Foot: propodial region well developed when larva hatches and conspicuously ciliated. Mesoposium triangular, bluntly pointed posteriorly. Statocysts conspicuous.

Colour
In the veliger larval stage transparent and colourless, becoming horn-coloured. Body of the snail is a clear grey frequently with various pigment spots. The shell is brown to yellow. Periostracum horn-coloured, underlying shell white; viscera showing through apical whorls gives living shells a darker appearance.

Size
The shell is up to 6 mm high but usually smaller, around 4 mm. Body 0.5-3 mm.
Parasite infestation is believed to cause increased growth rates, gigantism and altered morphology in this species. Parasitised snails may reach up to 9 mm in height.

Depth range
Upper and lower littoral fringe, also common in the eulittoral and sublittoral zones. The species can frequently spend its afloat upside-down on the surface film by means of mucous raft. By performing this behaviour the species is observed at high water in the inshore plankton of estuaries and in water over mudflats.

Remarks
Hydrobia ulvae may live as high as the high level strand line in a dried blanket of green algae. Hydrobia ulvae does not undertake any true migration but considerable dispersal is possible through floating upside-down on the surface film.
Females lay a maximum number of eggs of 50 in one egg mass. Snails producing planktotrophic larvae have several smaller eggs that hatch into veliger larvae at around 150 microns. Snails producing lecithotrophic larvae lay considerably fewer and larger eggs.
Egg hatching time: 5-8 days. Larval developmental time: for the planktonic stage up to 4 weeks, for a nonfeeding benthic larva metamorphoses probably lasts just two days.
The timing of the breeding season varies with latitude. In the northern parts of the North Sea there is a short spawning period in Spring. In southern populations the spawning period is more protracted and occurs in spring and autumn. The eggs are laid preferentially on the shells of live individuals of this species but also on empty shells and grains of sand. They are protected by a layer of sand grains.

Distribution in the North Sea
Very common snail of the seashore. Found on all North Sea coasts. Specimens may migrate up to 10 km.

World distribution
NE Atlantic from Senegal to N Norway, English Channel, North Sea and Baltic Sea.

Hydrobia ulvae