Fragum erugatum in Shark Bay (Photo: Lisa Kirkendale)

Fragum erugatum

Shell small to medium, highly variable in form from rounded sub-trapezoidal to obliquely quadrate. Margins serrate and umbos prosogyrate, not central but offset. Sculpture simple of broad, round, flat ribs mean 25 with range 24-28 (Shark Bay population). Largest shell measured 1.74 cm length, 1.88 cm height, 0.90 cm width (Wilson and Stevenson 1977). Shell is white with tinges of yellow or pink in interior, sometimes slightly translucent. Coloration of tissue brown due to presence of photosymbionts entrained in tissue.

Evolution

Placed in the genus Microfragum by some (which includes non-photosymbiotic species), however molecular phylogenetics place it firmly as one of the paraphyletic and wholly photosymbiotic species of the genus Fragum.

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

The species lives in dense populations on sandy bottoms to at least 9 metres in protected embayments.

Distribution

Indian ocean species (Ter Poorten 2009). Type locality is in South Australia, where it is apparently only known as a fossil. Records in WA based on live collected material occur from Rosemary Island in Dampier Archipelago to as far south as the Abrolhos and Geraldton. Extensive populations occur in Shark Bay, Western Australia where the species occurs in huge quantities in hypersaline reaches of Hamelin Pool.

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: erugatum
Name Published Year: 1889
Rank: species
Scientific Name Authorship: Tate
Commercial Impact: 

Fossil information, coquina, links to website, used to make building blocks for houses (e.g. Telegraph in Hamelin Bight). As such, transported around the state so dead shell records are to be treated with suspicion. Found in my chickens feed!

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-erugatum
Accessed 28 Oct 2021

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