This is the Common Otter Shell (Lutraria lutraria), a large marine bivalve in the family Mactridae. It is a burrowing clam that lives partially buried in sandy or muddy seabeds in subtidal zones.

Location:
Found in the North-east Atlantic, including the British Isles, North Sea, and Atlantic coasts of Europe. It inhabits subtidal zones from low water down to around 50–60 metres.

Average Size:
Adults commonly measure 100–150 mm in length, though some individuals can reach up to 200 mm.

Family:
Mactridae (Trough Clams)

Species:
Lutraria lutraria

Shell Description:
The shell is oblong, smooth, and slightly swollen, with thin but strong valves. The colour is usually pale brown to off-white. The two valves are slightly unequal in size, and the shell has a small, smooth hinge. The long siphons extend to the sediment surface, allowing the clam to feed while buried.

Habitat:
Lives buried in sand or muddy sand, often partially buried with siphons reaching above the sediment. Prefers subtidal zones where it is protected from strong wave action.

Feeding:
The Common Otter Shell is a filter feeder. Water is drawn in through one siphon, filtered for plankton and organic particles, and expelled through the other siphon.

Movement and Behaviour:
Mostly sedentary while buried, moving slowly through sediment using its muscular foot. Burrowing provides protection from predators and environmental stress.

Reproduction:
Male and female clams release eggs and sperm into the water. Fertilised eggs develop into planktonic larvae that drift for several weeks before settling on the seabed and growing into adults.

Ecological Role:
By filtering water and stabilising sediment, the Common Otter Shell contributes to the health of benthic ecosystems. Empty shells and buried bodies provide shelter for other small marine organisms.

Conservation Status:
Generally common in suitable habitats, but populations can be affected by seabed disturbance such as trawling, dredging, and coastal development. Not considered globally threatened.