Looking for friends, lobsters may stumble into an ecological trap

Gathering for mutual defense puts young spiny lobsters at risk of predators.

Lobsters are generally notable for their large claws, which can serve as a deterrent to any predators. But there’s a whole family of spiny lobsters that lack these claws. They tend to ward off predators by forming large groups that collectively can present a lot of pointy bits towards anything attempting to eat them. In fact, studies found that the lobsters can sense the presence of other species-members using molecules emitted into the water, and use that to find peers to congregate with.

A new study, however, finds that this same signal may lure young lobsters to their doom, causing them to try to congregate with older lobsters that are too big to be eaten by nearby predators. The smaller lobsters thus fall victim to a phenomenon called an “ecological trap,” which has rarely been seen to occur without human intervention.

The study was performed in the waters off Florida, where the seafloor is dotted by what are called “solution holes.” These features are the product of lower sea levels such as those that occur during periods of expanded glaciers and ice caps. During these times, much of the area off Florida was above sea level, and water dissolved the limestone rocks unevenly. This created an irregular array of small shallow pits and crevices, many of which have been reshaped by sea life since the area was submerged again.

The crevices offer a great gathering place for spiny lobsters, which can congregate in a way that ensures their spiky bits are all arranged to effectively block an attack from the limited directions of approach. Unfortunately, they are also home to groupers, large fish that are capable of gulping down the smaller lobsters. But groupers don’t have the jaws to break down the shells of larger ones, rendering more mature spiny lobsters immune from their predation.

The authors, Mark Butler, Donald Behringer, and Jason Schratwieser, hypothesized that these solution holes represent an ecological trap. The older lobsters that find shelter in a solution hole would emit the chemicals that draw younger ones to congregate with them. But the youngsters would then fall prey to any groupers that inhabit the same solution hole. In other words, what is normally a cue for safety—the signal that there are lots of lobsters present—could lure smaller lobsters into what the authors call a “predatory death trap.”

Testing the hypothesis involved a lot of underwater surveys. First, the authors identified solution holes with a resident red grouper. They then found a series of sites that had equivalent amounts of shelter, but lacked the solution hole and attendant grouper. (The study lacked a control with a solution hole but no grouper, for what it’s worth.) At each site, the researchers started daily surveys of the lobsters present, registering how large they were and tagging any that hadn’t been found in any earlier surveys. This let them track the lobster population over time, as some lobsters may migrate in and out of sites.

To check predation, they linked lobsters (both large and small) via tethers that let them occupy sheltered places on the sea floor, but not leave a given site. And, after the lobster population dynamics were sorted, the researchers caught some of the groupers and checked their stomach contents. In a few cases, this revealed the presence of lobsters that had been previously tagged, allowing them to directly associate predation with the size of the lobster.

So, what did they find? In sites where groupers were present, the average lobster was 32 percent larger than the control sites. That’s likely to be because over two-thirds of the small lobsters that were tethered to sites with a grouper were dead within 48 hours. At control sites, the mortality rate was about 40 percent. That’s similar to the mortality rates for larger lobsters at the same sites (44 percent) or at sites with groupers (48 percent).

Separately, the researchers tested whether lobsters can sense the presence of a red grouper by putting a lobster in a tank with water flowing in from pools, one with red groupers and one without. The lobsters appeared unable to tell the difference. That’s a bit of a surprise, given that earlier studies had shown that they are attracted to a healthy population of lobsters, but avoid populations that are injured or suffering disease. They’re also able to sense and avoid a different predator, the octopus.

The finding is relatively distinct. While there’s a long literature on ecological traps, most of them have developed due to changes humans have caused in the ecosystem, either accidentally or intentionally. A past review found that only about seven percent of the ecological traps described in the literature came about without human influence. That makes a degree of sense, given that human-driven ecological changes are likely to be relatively recent, and so haven’t given the animal being trapped the time to evolve a way of avoiding it.

For the handful of naturally occurring ecological traps that have been described, it’s possible that the benefits that get the animal caught in the trap are large enough to offset the risk of getting trapped. And that might be the case here, with the losses of juveniles being balanced by better survival of the adults, allowing greater production of juveniles to offset the losses. In either case, it would seem that evolving a way of detecting their presence could eliminate the trap, so it’s unclear why that hasn’t happened yet. It’s possible that the red groupers are relatively recent arrivals to this ecosystem, and so there hasn’t been time to evolve in response to them.