Crabs, Whelks, and Oysters: Life in Tomales Bay’s Food Chain

By Joe Eaton
Thursday July 30, 2009 - 11:21:00 AM
The European green crab, a recent invader on the West Coast.
Hans Hillewaert
The European green crab, a recent invader on the West Coast.

We’re surrounded by non-native plants and animals, most of which would qualify as what biologists and resource managers call invasive exotics. The thistles in your garden, the possum in your garage, the house sparrows nesting under your eaves, the Argentine ants in your kitchen, the blue gum eucalyptus up the hill—all are invasives. San Francisco Bay has been called the world’s most invaded estuary, the adopted home of aquatic creatures native to the East Coast, Europe, Asia, and elsewhere. Many have displaced native species that filled a similar ecological niche. 

You might wonder what difference that process makes in terms of overall ecosystem function. Are exotic and native species interchangeable parts in the great machine? What happens when one kind of ant or clam or shrub replaces another? 

It has become pretty clear that such substitutions can have far-reaching consequences. Exotic species can change what ecologists call the trophic flow—the way nutrients travel from primary producer to secondary consumer to predator to scavenger-within natural systems. They can increase the frequency and intensity of wildfires. They can affect human health and livelihood. 

In a recent study at Tomales Bay, a team of marine biologists led by David L. Kimbro at UC Davis looked at the interactions of natives and exotics in a triple-decker trophic system. At the bottom rung, there’s the native Olympia oyster (Ostreola conchophila), a sedentary filter-feeder. At the study site, its historic primary predator was a whelk (Acanthinucella spirata)—and if you’re feeling an impulse to snicker, blame that Monty Python routine. 

Some references call it the angular unicorn snail or spotted thorn drupe. The whelk in turn was preyed on by the spot-bellied rock crab (Cancer antennarius), which peels open the shells of its victims. 

Kimbro and his co-authors posit that the crabs are indirect benefactors of the oysters. On one hand, a large enough crab population will keep the whelks in check and reduce predation pressure on the tasty bivalves. On the other, the whelks have evolved anti-crab defensive behavior, namely avoiding areas-including oyster beds—where the rock crabs are likely to be.  

The crabs, specialized whelk-crackers, leave the oysters alone. 

Enter the invasives. Years ago, when the native oyster population had been depleted, growers introduced eastern oysters to supplement them. With the eastern oysters came another predatory whelk, the oyster drill (Urosalpinx cinerea). The founding oyster drills at Tomales Bay originated in Long Island Sound. The European green crab (Carcinus maenas) arrived in San Francisco Bay in 1989 and spread north to Tomales. Both the drill and the green crab can tolerate fresher water than their native counterparts and are more common in the inner portion of the bay, near creek outflows. 

To the native whelk Acanthinucella, a crab is a crab. They avoid the exotic green crab as they do the native rock crab. When the green crabs are in the oyster beds, the whelks opt to feed on barnacles instead. But the oyster drills had evolved in a habitat free of competently predatory crabs. They were, as the researchers put it, naïve; they had no idea how to react to a crab.  

That would make the drills easy pickings for the rock crabs. But the green crabs were another story. They’re more generalist feeders, augmenting their mollusk diet with seaworms and algae. Instead of peeling off the shells of their prey, they use their claws to crush them. This works for juvenile whelks of both species, but not for adults. Once a whelk grows large enough, green crabs are no longer a threat. 

The naïve oyster drills, then, are not deterred from preying on oysters by crab avoidance. The rock crabs could potentially control them, but where the green crabs have replaced the native rock crabs, the top-down pressure is off and the oysters bear the brunt. The effect of removing a top tidal-zone predator in Tomales Bay is similar to what happens on land when, say, coyotes are killed off and foxes, freed from their own predator, wipe out ground-nesting birds. 

So it appears that in this ecosystem at least, the pieces are not interchangeable. The players in an ecosystem have a shared history. Spot-bellied rock crabs, Acanthinucella whelks, and Olympia oysters have had a long time to work things out. The crabs have evolved an effective predatory strategy; the whelks have evolved a well-founded fear of crabs. 

Switch whelks and the oyster beds are no longer a no-go zone. Switch crabs and you get a top predator that’s incapable of controlling the lesser predator; to paraphrase Marx, the big bully is no longer picking on the little bully. (Groucho in Night at the Opera, that is.) Either of those changes is bad news for the oysters, and for anyone who’s trying to raise them.