Tale of two oysters

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Shellfish of any species ranks among my favorite foods, and oysters on the half-shell rank among my favorite shellfish. I recently enjoyed two surprisingly different kinds of local oysters, and the differences between them underscored the various roles these remarkable animals can play on the Vineyard.

Aquaculture oysters from Katama Bay vary somewhat; average ones are excellent, and the best are surely among the finest shellfish available anywhere. The meats are large, succulent, and delicate in flavor; the shells are thin, pale, and uniform in size and shape. Grown suspended from rafts among brisk currents, these oysters hit the market clean and ready to open, and a half-dozen at the Alehouse on Saturday made an exquisite lunch.

Appealing in a different way were wild-grown Tisbury Great Pond oysters, on sale for a song at Net Result last week. Similar in size and shape to the Katamas, these shellfish had an earthier and, surprisingly, a brinier taste. But the most striking difference was the composition of the shells, thick, layered, and slightly crumbly. Bits of weed clung to the outsides, and some sort of worm had formed tunnels in the robust shells. As I shucked them, I found the oyster shells proved to be hosting tiny clam worms hiding in crevices. These were real oysters — probably not of truly wild stock, but wild animals nonetheless.

The presence of worms on the outside of an oyster is not what most people would call a marketing point, so I hope you won’t think me too odd when I say I was pleased to see them. If you’re munching up entire mollusks, it’s a bit hypocritical to whine about a little worm. I’m sure clam worms of any size are edible — heaven knows gulls and striped bass love them — and it’s just a cultural foible that keeps us from viewing them as food. But mainly, the bonus wildlife on the shells reminded me that these oysters had grown on the bottom of a productive pond, living as part of an aquatic ecosystem.

The bottom of a great pond can be a boring place, flat and featureless. From the perspective of pond plants and animals, there is nothing to hide in or under, nothing to attach to, and unless you’re a species that actually lives in the sediment like a clam, life on such a surface exposes you to predators of all kinds. Oysters help solve this problem.

Oysters themselves need some sort of hard surface to get established: Rocks, the shells of past generations, or “cultch,” shells added by humans to encourage oysters. But once they are growing in numbers on the pond bottom, convoluted oyster shells offer countless hiding places for tiny animals, and the complex surface of the oyster bed itself offers shelter and attachment sites for fish, plants, and invertebrates of all kinds.

By filtering water near the pond bottom, oysters promote clear water that allows more light to reach plants attached to the bottom. Even the bodily waste of oysters — “pseudofeces” made up of indigestible organic bits — transfers nutrients from the water, where not much can make use of them, down to the sediment, where they support a pond-bottom food chain of bacteria, algae, and invertebrates. Oysters are what biologists call a “keystone species” — an animal that forms the foundation of an entire productive ecosystem.

Shellfish of many kinds can be seeded into a body of water, and town shellfish departments on the Vineyard are active in adding immature clams and bay scallops to heavily fished beds. But it is the species that spend their adults lives attached to objects — oysters and mussels — that offer the most promising subject for true aquaculture. And while all shellfish tend to improve water quality by virtue of their filter-feeding, it is oysters and mussels that produce the greatest habitat benefits for other wildlife.

My recent oyster-eating, then, sampled the results of two basic models for oyster cultivation, both requiring the availability of seed oysters but diverging in the methods used and the advantages yielded. The intensive aquaculture model, with oysters carefully tended and suspended in the water column, produces a reliable product with high commercial value. But being less fully integrated with the natural system they’re in, farmed oysters don’t provide the habitat benefits that wild-grown oysters, coarse and less valuable in the marketplace, do.

The aboriginal oysters of the Vineyard are effectively gone, depleted by many decades of heavy fishing, disease, and other threats. But the economic and ecological benefits those oysters produced are still within our reach, and the creativity with which the Island shellfish community encourages oysters is a positive thing. There is room in our expansive ponds and bays for both high-value farming and free-living oysters, harvestable as a lower-value resource but also energizing an entire aquatic ecosystem.