Fragum fragum in situ (Photo: Lisa Kirkendale)

Saw-toothed Heart Cockle

Fragum fragum

Shell medium, inequilateral, shell notably higher than long, sharply pointed ventrally with strong keel. Posterior flattened and narrow, steep. Sculpture of strong radial ribs circa 32 range 27-37 (WAM material), surfaces broad and flat. Ribs flattened with close set transverse curved scales, becoming coarser anteriorly. Lateral teeth extremely inequidistant from cardinals. Shell cream, rugae or nodules on ribs often yellowish. Interior color white, but with often yellow-orange flares radiating from umbonal cavity.

Morphology

Fragum fragum is a photosymbiotic species intermediate between Corculum and Fragum unedo. Semi-transparent posterior areas (pigment and windows) exist to allow light penetration. As well, siphonal tentacles are exposed to light on posterior shell surface and contain photosymbionts.

Evolution

This is the type species of the genus Fragum. 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 insandy areas. Generally in sheltered areas. In reefal habitats can be found from sandy areas through to seagrass bed perimeters and rubble.

Distribution

QLD, NT and WA in Australia, but widely distributed in the Indo-west Pacific.

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: fragum
Name Published Year: 1758
Rank: species
Scientific Name Authorship: Linnaeus
Commercial Impact: 

None

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-fragum
Accessed 30 Aug 2023

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References

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Kirkendale, L. (2009). Their Day in the Sun: molecular phylogenetics and origin of photosymbiosis in the ‘other’ group of photosymbiotic marine bivalves (Cardiidae: Fraginae). Biological Journal of the Linnean Society of London, 97, 448–465.

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