Hamelin Cockle

Domain: Eukarya
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Cardiida
Family: Cardiidae
Genus: Fragum
Species: Fragum erugatum

When I wrote about Shell Beach, Australia, I mentioned the Hamelin cockle, Fragum erugatum. Today, I want to expand on what I wrote.

The Hamelin cockle is a bivalve that belongs to the phylum Mollusca, along with oysters, snails, and squids, to name a few. It’s native to the shallow shores of Western Australia, though it is prevalent in Shark Bay and Shell Beach.

Shark Bay is a hypersaline marine environment. Its seagrass beds restrict tidal movement, and the rate of evaporation is higher than the rate of precipitation, which makes the water really salty. In fact, the water is plankton-deficient because the high salinity makes it hard for plankton to survive.

So what does the cockle do for food? Isn’t it a filter feeder like many of its bivalve brethren?

Hamelin cockles are not strict filter feeders. Instead, they have a partnership with our favorite oceanic BFFs, zooxanthellae. Like coral, the cockle receives leftover food from the zooxanthellae in exchange for protection in well-lit waters. Fragum erugatum will siphon plankton from the water when they can, but it’s never enough to sustain them.

The soft body of the cockle is brown, and the photosynthetic algae live in the soft tissue. The shells are white and appear translucent in the light. Fun fact, zooxanthellae also help to collect calcium carbonate that the cockle uses to make its shell. The entire organism is less than 20 millimeters, which is a little smaller than an inch.

Hamelin cockles are hermaphrodites, meaning they have both male and female sex organs; however, they still need other individuals to reproduce. Between winter and spring, F. erugatum will release their gametes, or eggs, into the water to be fertilized by other Hamelin cockles. The fertilized eggs develop into zooplankton that float around in the water before they settle to the ground and further develop into cockles.

I find these bivalves to be every interesting. They entered Shark Bay over 4000 years ago and really put forth the effort to make the bay and Shell Beach their home. Most living things do not prosper in extreme conditions, especially in areas of high salinity. However, the Hamelin cockle not only adapted to the hypersaline water, but they prospered so beautifully that they left a noticeable mark in the local geology.

Four thousand years’ worth of cockle shells replaced the sandy beach of Shell Beach. Building material was made from the dense accumulation of these shells that, over time, became cemented together. It just blows my mind to think how successful these tiny little organisms are, and that makes them special!

Sources and links:
Ocean the Definitive Visual Guide made by the American Museum of Natural History
https://www.sharkbay.org/publications/fact-sheets-guides/hamelin-cockle/

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