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). Hiatella rugosa may be a separate species, or just a distinct ecotype of H. arctica resulting from boring into rock.
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Size: Usually up to 40mm long. Thick and chalky. Valves: Right and left similar shape and structure. Inequilateral; beaks near anterior. Curved to fit cylindrical boring. Outline irregular, undulating and variable. Outer surface: Coarse concentric ridges, often eroded down. No radiating ribs or spines. 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, substantial; usually part eroded. Shell colour: Chalky white exterior and interior.
Flesh: Yellowish white and yellow, sometimes partly bright orange. Siphons: Only part observable without breaking open rock. Massive, withdraw completely with difficulty. Fully fused. Covered by thin brownish periostracum, most obvious when siphon contracted and periostracum wrinkled. Able to extend to length of shell, or more, to reach burrow opening, or to expand girth greatly to brace the body while boring. Distal ends orange, red or ruby. Two rings of tentacles round each aperture. Mantle: Edge yellow, sometimes bright orange at anterior of young specimens. Foot: Yellowish white, only the tip seen, extended through small gap in fused mantle, on those examined (see two specimens on left of image). No byssus on adults.
** Body description based on dozen specimens from Menai.
Key identification features
1) Bores into solid rock.
2) Shell has curved surface to closely fit boring.
3) Adult lacks byssus.
4) Lacks ribs and spines at all growth stages.
Hiatella arctica (H. rugosa may be ecotype of H. arctica.)
1) Nestles in rock crevices or holdfasts of Laminaria etc.
2) Wedge-shape shell, flattened in parts.
3) Usually has byssus.
4) Two ribs radiate from the beak to the posterior edge; most prominent, and bearing spines, on unworn young specimens. May be obscure on old specimens.
Ecology and behaviour
Rocky substrate, especially limestone, but soft homogeneous rocks also. Lower shore to deep offshore. Can occur at high densities, riddling rock with borings. Outer part of boring narrow as made when young. Impossible to extract without breaking rock. No trace of acid secretion found (Yonge, 1966), but hard to credit that deep precisely bored hole is made solely by movement under hydraulic pressure of soft chalky valves that can be renewed by the mantle on the exterior only at the margin. Large parts of yellow-brown periostracum survive boring process. Siphons circulate water to ctenidium for respiration and filter feeding, control expansion for bracing body during boring, and, when trigger temperature reached, expel milt or ova that pass a veliger stage in plankton before settling on suitable substrate.
Arctic and temperate oceans of Northern Hemisphere, also South Africa, but difficulty of segregating Hiatella spp. makes distribution uncertain. (See gbif map). Widespread and, where suitable substrate, common around Britain and Ireland. But, records of H. rugosa and H. arctica often combined.
References and links
National Museum of Wales Marine bivalve shells of the British Isles. Cardiff
Tebble, N. 1966. British Bivalve Seashells. London.
Yonge, C.M. 1966. The sea shore. London
Yonge, C.M. and Thompson, T.E. 1976. Living marine molluscs. London.