It has long been uncertain whether there are one or more species of Hiatella in British and Irish waters. They are very variable, “no two specimens being alike” (Tebble). This description is of the nestling, non-boring, species/ecotype.
Shell Size: Usually up to 40mm long. Adult shell thick and chalky. Juveniles thin and silky. Valves: Right and left similar shape and structure. Inequilateral; beaks near anterior. Wedge-form with flattened areas; with age, less distinctly so and more variable, but less cylindrical than H. rugosa. Outline of juvenile trapezoidal; more rounded, undulating and variable as ages, may become modioliform. Subtruncate straight posterior on juveniles, becoming more rounded with age. Anterior of juvenile rounded and much narrower than posterior, broader and blunter with age. Outer surface: Juvenile smooth with fine concentric ridges, adult rough with coarse concentric ridges. Two ribs bearing spines radiate from beak to posterior. Spines very prominent on 5mm long juveniles. Spines and ribs eroded with age, but rib remnants can usually be detected. Valve margin: Unsculptured. Ligament: externally inconspicuous, recessed . Hinge: Small cardinal teeth; one in right valve, two in left, but usually worn away on adults. Interior surface: Pallial line discontinuous, indented by deep pallial sinus. Adductor scars rounded, posterior one slightly larger. Periostracum: Yellow-brown, not much developed on juveniles, but older specimens can have a thick continuous cover that extends onto the siphons and fused mantle, or a thin or eroded cover. Shell colour: Adult, chalky white exterior and interior, juvenile silky white.
Flesh: Translucent whitish. Siphons: Massive, withdraw completely with difficulty, fused. Covered by brownish periostracum, most obvious when siphon contracted, less so when girth expanded. Able to extend to length of shell, or more. Translucent whitish, lacking bright orange, red or ruby. Two rings of tentacles round each opening. Mantle: Edge translucent whitish. Foot: Translucent white. Usually substantial byssus on adults.
** Body description based on dozen specimens from Menai. Please contact author via comments box if you have information that expands or contradicts the above, with photographic evidence if possible. Please view H. rugosa account before writing.
Hiatella rugosa (may be ecotype of H. arctica)
- Bores into solid rock.
- Shell sub-cylindrical from rotating in boring.
- Lacks byssus.
- Lacks ribs and spines at all growth stages.
Rocky substrate, hard and soft rocks, attached by byssus between stones, in rock crevices or holdfasts of Laminaria etc., not boring in solid rock. Lower shore to deep offshore. Siphons circulate water to ctenidium for respiration and filter feeding, and, when trigger temperature reached, expel milt or ova that pass a veliger stage in plankton before settling on suitable substrate.
Polar to sub-tropical oceans of both hemispheres. Difficulty of segregating Hiatella spp. makes it probable that this distribution includes several spp.. (See gbif map). Widespread and common around Britain and Ireland, but, most records, including the map on this page, combine H. rugosa and H. arctica.