This past July, the National Oceanic and Atmospheric Administration started a campaign encouraging people to eat more fish. Lionfish, that is.
In the past 20 years, lionfish have become one of the Atlantic Ocean’s worst invasive species. Lionfish are voracious eaters, devouring a variety of native fish but lacking many predators. After lionfish were spotted off the coast of St. Croix in 2008, scientists became concerned that the fish would destroy the biodiversity that characterizes the coral reef.
In the past few years, a few other invasive marine species have joined lionfish as a species of concern in US waters. Two of them are the veined rapa whelk (a nonnative snail) and the European green crab. When combined with native mollusk populations of whelks, crabs and oysters, the invasive whelks and crabs cause a shift in the “trophic cascade,” a food web in which the top predator preys upon a secondary predator, indirectly releasing the next level species from predation.
In Tomales Bay in California, half of the native Olympia oyster population has perished in the last 10 years. While he was at the University of California-Davis, marine biologist David Kimbro (now at the University of Florida) studied the interaction of native crabs and whelks, invasive crabs and whelks, and native oysters to see if the introduction of the invasive mollusks was related to the decline of the oysters. Whelks, or snails, are direct predators of oysters.
In native mollusk populations, native snails, which prey upon the oysters, kept in check by two types of crabs: king rock crabs that crush or peel open the snails and consume them, and non-predatory crabs that cause the snails to go into hiding rather than eating the oysters.
The invasive crabs, on the other hand, are not as aggressive about killing snails as the native crabs. Invasive crabs rely on crushing only and crush only the tip of the snail shell. Invasive snails did not hide from either invasive crabs or native crabs and were aggressive at consuming oysters.
Kimbro and his colleagues hypothesized that the invasive snails might be causing the high oyster mortality; however, they did not rule out other causes, such as temperature. To test both possibilities, Kimbro’s team placed several cages with oysters in two sites: one in the inner bay and one in the middle bay. Some cages also had invasive snails, while others did not.
At the end of the first experiment, all of the cages contained invasive snails, so Kimbro still could not conclude the main cause of oyster mortality. So the team repeated the experiment, this time with stronger cages, wrapped in mesh with smaller openings so the snails could not enter. Aside from a shorter observation time, all other factors were identical to the first experiment. The team tested both water salinity and temperature from May to August and from November to February.
The team also did food-web experiments, which combined oysters with either native or invasive snails and native lethal and non-lethal crabs or invasive crabs.
Results revealed that oysters were more plentiful in the middle bay, not because the environmental conditions were more conducive to their survival, but because fewer invasive snails survive lethal crab interactions in the middle bay. Invasive snail and crab populations are higher in the inner bay, where the salinity is too low for native crabs and snails.
Although the rapa whelk and the European green crab are not yet crowding out native crabs and snails from the middle Tomales Bay, they are still interrupting the food chain balance. Native crabs consuming rapa whelks leave native snails to consume oysters. Rapa whelks in the company of non-lethal native crabs continue to consume oysters. So like the lionfish, invasive crabs and whelks will change the biodiversity of the inner Tomales Bay, consuming the Olympia oyster and threatening the species that depend on it.
“A fellow researcher likened the native crabs and snails to a long-married couple who have learned to coexist, leaving oyster populations intact,” Kimbro said, quoted in Science Daily. “But the lack of experience the invasive crabs and snails have with each other has led to the destruction of parts of the oyster population, much as an incompatible couple might destroy an entire family.”