Co-author: Clive Craik, Aug. 2012
Up to 80mm long [image23] and 19g mass. Posterior inside thin flexible oval shell which cannot accommodate whole animal. Cephalic disc on dorsum has curved antero-lateral expansions forming white rimmed ear-like lobes  behind inconspicuous eyes. When not swimming, foot extends with no visible junction into parapodial lobes reaching up flanks and overlapping to cover part of the dorsum and shell. All surfaces smooth. No digitiform tentacles on cephalic disc, but sometimes slightly bilobed  at front, especially at early post-settlement stages. Contracts into spheroid when alarmed . Spreads parapodial lobes when swimming . Epithelium grey or brownish overlaid with light and dark blotches and, on cephalic disc, dark streaks . Undersides of parapodia lighter than rest of animal . Very small, recently metamorphosed specimens (up to about 50 mg mass) lack pigment so are translucent ; dark pigment appears rapidly and those larger than this are usual dark colour. Gizzard contains triangular horny tubercles.
Usual maximum height (longest dimension) 30mm, occasionally 40mm. Very thin, glossy, translucent, variably ovate ‘bubble’  with fine prosocline growth lines  and microscopic spiral striae.
Fresh shells have brownish periostracum ; when it is worn off dead specimens, whitish to pale brown shell exposed .
Truncated spire protrudes a little beyond final whorl in some populations , or hardly at all in others . Suture deep. Spiral slit  near suture follows it around all whorls; sealed by membrane apart from third of final whorl, which remains separated from next whorl. Slit sometimes on top of slight keel . Slit allows shell to flex. Membrane dries and cracks on dead shells  causing open slit to widen and lengthen.
Aperture long , sometimes almost as long as shell’s height . Columellar part of inner lip pale, slightly folded and projecting . Internally, all whorls open to and spiral round wide central cavity instead of solid columella .
Various named shell varieties probably growth forms of little taxonomic significance.
Haminea hydatis & H. navicula
- Parapodial lobes do not meet and overlap over body.
- Eyes more dorsally postioned than on Akera, and lack ear-like lobes behind them.
- Wide flat pallial lobe extends from posterior.
- Aperture extends beyond spire of shell.
On mud and muddy sand, especially on Zostera beds and filamentous green algae , in sheltered bays, lagoons, and man-made ponds and tanks containing seawater. Near low tide mark and sublittorally. Diet herbivorous or saprozoic; larger ones occur on sediment or on green algae, and their faeces consist of sand, silt or part-digested green algae; smaller ones are almost always found on filamentous green algae (presumably grazing); and huge concentrations of larger ones form on decomposing organic matter. Eaten by flat fish, ducks and Aurelia aurita (jellyfish). When threatened, some can discharge defensive "ink". Those in Fleet lagoon feed on Zostera and can not discharge ink, unless fed on red algae in laboratory (pers. comm. H.C. Marshall). Simultaneous hermaphrodite, mates in curved chains of individuals, often partially concealed below surface of sediment . Some crawl out of water and into wet algae at the water’s edge to spawn . Long white or yellow-white spawn-string  tangled around algae, with 1 000 to 100 000 ova per spawn batch, depending on body size. In captivity, some lay repeated batches over several weeks in summer before dying. Thousands of dead shells may litter the strandline  after spawning. Spawns in Isefjord, Demark, when temperature 10ºC and over, May or April to October. Hatching time 20 to 30 days, depending on temperature. Veliger larva before settling and metamorphosing.
Bobbing swimming motion caused by parapodial lobes opening and shutting like a parasol . Shell and viscera hang down like clapper of a bell . On downward power stroke, edges of parasol are firm  and animal is carried upwards. At end of downstroke, parasol is wrapped tightly around animal . During upward  stroke the animal sinks , and limp edges  of parasol are reflected upwards  by relative movement of water. Swimming not reported from some populations, e.g. the Fleet lagoon. Small specimens can sustain swimming for about half an hour, but largest specimens have difficulty. Morton & Holme (1955) observed swimming in April at Plymouth and associated it with breeding. Swimming has been observed in April at Oban (J.C.Craik, in litt.), and in Orkney in August (pers. obs.), September and October (P. Martin, in litt.).