Common Otter Shell (Juvenile), also known as the Great Otter Shell or Gaper.
Phylum: Mollusca | Class: Bivalvia | Order: Adapedonta | Family: Mactridae | Genus: Lutraria | Species: Lutraria lutraria (Linnaeus, 1758). · Mactridae (Trough Shell family). · Bivalve (part of a two-shelled mollusk). This specimen is a single valve (either left or right) with an oval-oblong shape.
Species
Lutraria lutraria (High confidence based on morphology and Scottish beach locality).
Shell Type
Bivalve (part of a two-shelled mollusk). This specimen is a single valve (either left or right) with an oval-oblong shape.
Family
Mactridae (Trough Shell family).
Size
Approximately 12-15mm in width. This is a very young specimen; adults can reach up to 130mm (5 inches) in length.
Color & Pattern
Off-white to pale cream base. It lacks the olive-brown periostracum (outer skin) usually found on live specimens, indicating it has been beach-worn. There are subtle concentric growth lines but no distinct radial patterns.
Rarity
Common. They are frequently found as empty valves on beaches after storms or heavy tides, especially on the west coast of Scotland.
Habitat
Found in the lower intertidal and subtidal zones, burrowed deeply (up to 30cm) in muddy sand or clean sand. Marine environment.
Geographic Range
Common along the coasts of the British Isles, including Troon, Scotland. Its wider range extends from Norway down to the Mediterranean and the coast of West Africa.
Description
An oval, somewhat elongated bivalve shell with an inequilateral shape. The umbo (beak) is positioned slightly towards the anterior end. This specimen features a prominent 'predatory borehole' at the apex, likely from a moon snail. The surface is relatively smooth with fine concentric ridges marking growth stages.
Key Features
Oval-oblong shape that is slightly 'gaping' (the valves don't close completely at the ends); a large, triangular internal ligament pit (chondrophore) inside the hinge (not visible from this side).
Collector Value
Minimal monetary value (less than £1). It is a 'study piece' or a personal souvenir. Specimens with both valves joined (hinged) are more desirable for formal collections.
Condition Notes
Fair condition. While the structure is sound, it is beach-worn and sun-bleached. A significant feature is the neat circular hole at the umbo, which is a 'drill hole' from a gastropod predator like Euspira pulchella.
Interesting Facts
The animal that lives inside has massive, fused siphons that are so large they cannot be fully retracted into the shell. Because they burrow so deep, they are rarely seen alive unless dug up. The name 'Otter Shell' is of Victorian origin, though the exact etymology is debated.
Ecological Role
A suspension feeder that filters plankton from the water. It is a vital food source for bottom-feeding fish and shell-drilling gastropods (as evidenced by the hole in this shell).
Similar Species
Lutraria angustior (narrower and more curved) and Mya truncata (more blunt at one end). This juvenile could also be confused with juvenile Mactra stultorum, which usually has radiating bands.
Beach Finding Tips
Look along the drift line at low tide, especially after a period of strong onshore winds which wash these deep-burrowers out of the substrate.
Notes
Troon Scotland