Making A Meal Of Things: How Crabs Handle Lunch

There's a lot to marvel at on the rocky shore: from skulking shrimp and shiny sea snails to anemone filled pools and a plethora of tiny plankton. But there's an ongoing battle between these beautiful beasties as they fight to stay safe and fight for their food. Take, for example, the case of the red rock crab (Cancer productus) and the shield limpet (Lottia pelta). The crab is equipped with two heavy-duty chelae, or claws, to crush the shells of lesser invertebrates (like the limpet), while the limpet lays safe beneath a cone-shaped shell, protecting it from predators (like the crab) and preventing it from drying up when the tide is out.

If you've ever tried to pick up a limpet you'll know that it's got an exceptionally strong grip. Beneath that shell is a single muscular foot that pulls the shelly shield down and sucks it to the surface to form an incredibly effective suction cup. If threatened, limpets will clamp down to the rock and resist removal by predators and rock-poolers alike. But like any aerobic animal, it needs to access oxygen, so when it can, it loosens its grip on the surface to trade waste gasses for what it wants from the atmosphere.

Researchers at Florida Museum of Natural History put together platters of limpets for a lucky group of red rock crabs in a bid to find what gave limpets their best defence and how crabs took on the challenge of attacking a limpet lunch.

The study showed that there were three main methods of attack: 1) attempting to lunch on a limpet by crushing the top of its shell, 2) using claws to meddle with a limpet's margins and crush it close to its base and 3) prying a limpet away from the rock.

The first method is adopted by only the odd crab, and, in its rarity, is akin to those acquaintances that use their knife and fork ‘the other way round'. Given this rarity and that you can't do much of a statistical test when you don't have much data, the scientists excluded these top-squeezing oddballs from their study.

The second method, crushing a limpet at its margins, was much more popular among our crabby friends, with a quarter of all crab attacks giving edge crushing a go to get a bite of lunch. However, this marginal crushing method was only successful about 30% of the time, which brings us to method number three...

Using their legs for leverage, and somewhat like an awkward, spikey crowbar, crabs can also pry limpets from their holdfast. Once the limpet tires of resisting the crab's force, or is in desperate need for oxygen, its foot peels back from the rock, releasing a marvellous meal. This little guy is later crushed up as the crab makes for the meat inside. With a 45% success rate, it's no wonder this was the favoured attack strategy!

But it's not all bad for limpets. Crabs can't spend long periods hunting limpets in the open as it puts them at risk - and it's not worth them expending more energy making their meal than they'll get back from it either. Difficult dinners are best left alone. Some limpets have a morphological trick up their metaphorical sleeve too - smooth shelled species are harder to get a grip on, which prevents crabs from gaining purchase on their shells and gives them a higher chance of survival.

Next time you're on a rocky beach, spare a thought for those little limpets that have yet to be made a meal of and keep your eyes peeled for those prying predators!

Reference

Tyler, C. L., Leighton, L. R. and Kowalewski, M. The effects of limpet morphology on predation by adult cancrid crabs. Journal of Experimental Marine Biology and Ecology 451 9-15 (2014)

Images

All crabs and limpets: author's own

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