Shells of Fragum unedo (Photo: Corey Whisson)

Strawberry Heart Cockle

Fragum unedo

Shell solid and inequilateral, umbonal keel rounded, subquadrate to trapezoidal with strongly digitate posterior margin. No strong keel. Strong broad, flat radial ribs ranging from 23 to 31, mean 27. Lunule narrow, smooth with raised dorsal margin. Escutcheon broad, smooth. Exterior white, ribs with irregular, transverse red scales, sometimes with pale brown blotches; interior white. It is distinct from all other fragines by the prominent red scales on the ribs of the shell. Shell length and width to around 5 cm while depth to above 6 cm.

Morphology

This is the most heavily bodied, heavily shelled species of the subfamily Fraginae. In contrast to Corculum that has a very light, thin and transparent shell (in areas), this species also participates in photosymbiosis but in a very different manner. Soft anatomy is not contained within the shell but is exposed as highly distinct leaf-like extensions of the posterior siphonal area, similar to but unique from the hypertrophied mantle of giant clams. These are spread out on the surface of the sediment and function to greatly increase surface area for photosymbionts that are contained within the exposed tissue.

Evolution

Kirkendale (2009) has shown (and others have confirmed e.g. Herrera et al. 2015) that Fragum is paraphyletic and includes Corculum and Lunulicardia, the two other wholly photosymbiotic lineages of cardiids.The phylogeny of members of the Fraginae subfamily is in a state of flux and the taxonomy of the group is in need of attention

Behaviour

All cockles or cardiids have short siphons. Because of this morphological constraint, infaunal cardiids live close to the sediment water interface in order to filter water. This could be considered a preadaptation for a photosymbiotic lifestyle, as it also facilitates optimal light penetration into the soft tissues in shell interior where photosymbionts are housed.

Method of reproduction

Broadcast spawner.

Habitat

Marine

Intertidal to shallow subtidal in sandy areas. Generally in sheltered areas.

Distribution

QLD, NT and WA in Australia, but widely distributed in the Indo-west Pacific. Wilson and Stevenson 1977 note that placement of red scales appears different than in F. unedo from the Pacific ocean and Philippines.

Life Cycle

Short lived veliger larvae

Taxonomy

Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Subclass: Heterodonta
Superorder: Imparidentia
Order: Cardiida
Superfamily: Cardioidea
Family: Cardiidae
Subfamily: Fraginae
Genus: Fragum
Species: unedo
Name Published Year: 1758
Rank: species
Scientific Name Authorship: Linnaeus
Commercial Impact: 

Sought after by shell collectors, possibly commercially exploitable (WS1977)

Conservation Assessment: Least Concern

Net Conservation Benefits Fund

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Western Australian Museum Collections https://museum.wa.gov.au/online-collections/names/fragum-unedo
Accessed 16 May 2024

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References

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