This is the Sea Potato (Echinocardium cordatum), a heart-shaped sea urchin (a spatangoid echinoid) clothed in dense, fine spines. It lives buried in sandy or sandy-mud seabeds and is often encountered as empty tests washed ashore after storms.

Location:
Widespread in temperate seas of the North-east Atlantic, including around the British Isles, the North Sea, and the Mediterranean. It occurs from the lower shore and shallow subtidal down to deeper offshore waters, typically up to about 200 metres. The species has a broad temperate distribution, though local populations may vary.

Average Size:
Typically 6–9 cm in test (body) length. Juveniles are smaller, and maximum sizes vary with population, habitat, and age. Adult tests are usually about 25–30 mm wide.

Family:
Loveniidae (heart urchins and related spatangoids)

Species:
Echinocardium cordatum

Physical Description:
The test (hard outer covering) is heart-shaped and slightly flattened, covered in a dense mat of short, hair-like spines that give a furry appearance when the animal is alive. The upper surface is slightly depressed near the front, with ambulacral furrows forming a subtle star pattern. Live animals are yellowish-brown or fawn in colour; dead tests found ashore are often bleached white.

Habitat:
Lives buried in soft sediments such as clean sand, muddy sand, or muddy gravel, usually at depths from the lower shore and shallow subtidal to around 200 metres. Individuals occupy permanent burrows about 8–15 centimetres deep. Populations prefer well-oxygenated, clean sands and can be sensitive to heavy sediment disturbance.

Feeding:
A deposit feeder: the Sea Potato ingests organic detritus from sediment, including plant and microbial material, using its tube feet to move particles from a shallow conical depression above the burrow into its mouth. This feeding contributes to nutrient cycling and sediment turnover.

Movement and Sensory Features:
Mostly sedentary while buried, but capable of slow movement through sediment using its muscular foot and spines. The dense spines also help trap a layer of water or air, reducing risk of asphyxiation. The small conical depression at the sediment surface marks the animal’s location.

Reproduction:
The species is dioecious (separate males and females). Eggs and sperm are released into the water (broadcast spawning), and fertilised eggs develop into planktonic echinopluteus larvae. After several weeks, larvae settle and metamorphose into juvenile urchins. Spawning season varies with region and environmental conditions.

Ecological Role:
By burrowing and feeding on sedimentary detritus, Sea Potatoes contribute to sediment aeration, turnover, and nutrient cycling. Empty tests provide microhabitats for small marine organisms once exposed on the seabed or strandline.

Conservation Status:
Generally common in suitable habitats and not considered globally threatened. Local populations may be impacted by seabed disturbance from dredging, trawling, or pollution.

Collection Information:
Collected from Broadsands Beach, Devon.