In praise of the dancing spitting oyster natures great purifier

July 17, 2021 â€" 5.00am

I’ve never liked oysters. Sharp outside and slimy within, they’re too like the worst men in my life. From last Sunday, though, I have a newfound oyster passion. Not to eat. Good lord no. I love them as stalwart little cleaner-upperers that deserve a national heritage listing and could be even more important to our future â€" in ensuring, that is, that we have one. They’re also unexpectedly comic.

Oysters, as you know, are permanently locked down. Even post-mortem, they remain cemented in place, awaiting soft feet to shred. Yet they can perform, as prettily as Naomi Watts ever did for King Kong.

An oyster that’s never seen a plate.Credit:The Nature Conservancy

The Oyster Dance is something I’d not seen before. You have to be slow. Runners and cyclists will pass by, oblivious. And you have to be lucky, since it seems pretty occasional. But if you stroll the Bay Run at three-quarters-low tide, you may have the pleasure. The Hawthorn Canal is little more than a storm-drain but there, under the roaring viaduct where the canal hits the Parramatta River, is a colony of oysters. And their dance is a study in whimsical, water-spitting surprise.

For a good half-hour we watched, astonished, as dozens of little water-jets rose randomly from the surface. Each jet erupted from a different oyster, described a perfect catenary arc some 200mm high and fell back to the sea. It was like one of those random pop-up water-jet fountains that were fashionable as civic art a few years ago.

Unsurprisingly, this antic spitting is part of the oyster’s ablutionary routine.

Like most bivalves, the oyster draws water in and across its gills, directing any food particles into its well-hidden mouth. Out the other end, it ejects tiny faeces, along with much larger masses called “pseudo-faeces”, comprising sand and other inedible particles all wrapped up in oyster-mucus â€" there’s that slime thing again. It achieves this egestion by clapping its shells together, much as kids do that hand-squeeze thing in a swimming pool, and squirts it all out in a curving stream.

At low tide, resisting desiccation, the oyster clams (if you will) up. At high tide, submerged, its jets will be invisible. But at mid-tide, when the oyster has its feet wet but its head dry, you’ll sometimes (but not always) see the squirt-dance. And although the water so ejected contains faeces and pseudo-faeces, these drop to the bottom, providing food for scavengers and leaving the water significantly cleaner than before.

More surprising is the oyster’s extraordinary volumetric capacity. A single oyster, it is believed, can purify up to 100 litres/day.

In thus removing excess nutrients, oysters (and other shellfish) help counteract eutrophication. This is a process whereby excess nitrates and phosphates â€" in particular, from fertiliser-rich farm runoff â€" feed excess algal growth. Deprived of light, the underlayers of algae die and rot, consuming oxygen. The water becomes hypoxic, killing fish, crustaceans and other marine life and becoming toxic also to humans.

(It fascinates me that 95 to 99 per cent of the waterways in pastoral and populated areas of “pristine” Aotearoa (New Zealand) â€" which has legislation giving at least one river “personal” rights â€" are so polluted by farm runoff that many are too toxic for swimming.)

The Australian coastline, including Sydney Harbour, was once rich in huge shellfish reefs â€" the less spectacular subtropical equivalent of coral reefs. These covered many hectares. Accreted over millennia, as new oysters stood on the shoulders of their ancestors, they reached considerable heights and acted as very effective breakwaters, guarding the coastline from erosion and protecting the delicate estuarine ecosystems of seagrass and saltmarsh.

We know, too, that the shores of Sydney Harbour and the Parramatta River, as well as Broken Bay, Botany Bay and much of the coast, were dotted with equally old and almost equally vast Indigenous middens. Up to 100m high and hundreds of metres long, these middens were noted, inter alia, at Cockle Bay, Barangaroo and Bennelong Point, once a tidal island known as Tubowgule.

Now that’s all gone. With white people came exploitation. We set about burning the middens â€" Tubowgule, where the Opera House now stands, became known as Limeburner’s Point â€" reducing the calcium carbonate to mortar. With the middens gone, we started on the reefs. By the 1870s they, too, were 95 per cent destroyed.

Since then, constant dredging and silting have kept any rocky outcrops buried in soft sediment. So, although there’s a thin bathtub-ring of oysters around the intertidal mark, the reefs cannot rebuild. The rich saltmarsh and seagrass ecosystems they protected have been replaced by mangroves. Seagrass is especially critical, not only as a marine nursery but also because it sequesters carbon 40 times faster than tropical rainforest.

Now, remediation projects are under way. With $20 million from the federal government, The Nature Conservancy’s Reef Builder project will provide hard (rock or concrete) substrate for 60 such reefs and seed them with millions of oyster larvae, starting with Port Stephens.

Every hectare of oyster reef, says the Conservancy’s Chris Gillies, will produce 375 kilograms of fish a year, remove 225 kilograms of nitrogen and phosphates and filter 2.7 billion litres of seawater. The cleaner water, and the wave protection, will nurture seagrass.

“There is a massive movement toward co-engineering ecosystems,” says Associate Professor Paul Gribben of UNSW’s Centre for Marine Science and Innovation. “Seagrass, salt marsh, oysters are connected in ways that are subtle.”

Subtle yes, but also fundamental. There’s poetry in returning the calcium carbonate we removed 150 years ago. Perhaps my sweet spitting oysters are overjoyed at the prospect, or perhaps they’re just impatient to have more of their brethren closer, faster, in Port Jackson.

Elizabeth Farrelly is a columnist and author. Her latest book is Killing Sydney.

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