Leaf oysters: The unsung heroes of estuaries are disappearing

Camouflaged by a layer of silty mud, most people probably wouldn’t notice the large flat oysters lurking beneath shallow water in Australia’s coastal estuaries. These are remarkable “leaf oysters,” and they can form reefs, produce mauve pearls, and reach the size of a dinner plate.

Of the 14 species of reef-forming oysters and mussels in Australia, leaf oysters (Isognomon ephippium) are the least well known. Our review, published last year, found only 30 publications globally that mention leaf oysters. Half of those were only incidental recordings.

This is a huge problem because there is widespread evidenceof significant declines in the number and condition of shellfish reefs. In Australia, 99% of shellfish reefs have been described as “functionally extinct,” meaning the habitat these reefs previously provided has now been lost. 

This has led to serious efforts in shellfish reef restoration. Leaf oysters are crucial members of these ecosystems, and we need substantially more information about them to ensure they’re not left out of these programs. Let’s delve into what we do know.

Meet the leaf oyster

Oysters are often associated with summer feasts and intensive aquaculture. While leaf oysters are edible, they have a large shell to meat ratio and so aren’t particularly attractive as a source of food for humans.

But like the iconic pearl oysters, leaf oysters are members of the Pteridine family of bivalve molluscs and have an inner nacre layer. This means they can produce pearls mainly mauve in color, or sometimes purple, bronze, cream or silver.

Although not much is known about the life history of leaf oysters, we do know they reproduce by spawning. Thousands of eggs and sperm are released into the water and develop into swimming larvae after fertilization. Only a fraction of these survive and settle onto the substrate, where they develop into juvenile oysters. 

Like other reef-forming oysters such as the Sydney rock oysterand the Pacific oyster, leaf oyster larvae appear to be attracted to the shells of the adult oysters. They attach to the surface via “byssus”—a matt of strong hair-like threads. This enables shell clusters to form, which can develop into leaf oyster reefs.

Leaf oysters are ecosystem engineers. When they live in dense clumps, they support an entire ecosystem of fish and other invertebrates. 

The flat, plate-like shape of the leaf oysters provides a complex three-dimensional habitat, with many nooks and crannies for species to seek shelter from drying out at low tide, and to hide from predators at high tide. Their shells provide a hard surface for other invertebrates to attach to, and form biofilms grazed by snails and fish.

Our preliminary studies on leaf oyster beds have detected a high diversity of fish species. Using underwater videos, we recorded a number of important fishing species, including yellowfin bream, dusky flathead, sand whiting, sand mullet, leatherjacket and black spotted snapper.

FOR MORE INFORMATION:https://phys.org/news/2022-01-leaf-oysters-unsung-heroes-estuaries.html

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