Why Most Snails Coil to the Right

sn-snailsGUELPH, CANADA—When plucking a snail from the beach you’d be lucky to snag a left-coiling shell. That’s because only 5% of all snails are “lefties,” new research shows. Shell enthusiasts have long marveled at the lack of sinistral (left-coiling) snails among their collections, especially when other shelled mollusks, such as clams and the now-extinct ammonites—nautiluslike creatures that sported dozens of tentacles inside spiraled shells—are just as likely to be left- as right-coiling. Now, in the largest survey of its kind, researchers inspected more than 55,000 snail species—representing two-thirds of all gastropods—to reveal that left-coiling has arisen more than 100 times, and yet few of the species that have made the switch have been particularly successful. In the rare cases where left-coiling took off, it was almost always on land, the team reported here in a presentation last week at the annual meeting of the Canadian Society of Zoologists. The researchers don’t know why sinistrality is so rare underwater, but the most likely explanation, they say, is that unlike land snails that tend to hang around where they hatch out, the microscopic young of sea snails are carried on ocean currents that make the chance of meeting and reproducing with another left-coiling nest-mate slim. Without such a meeting, the left-coiling lineage goes extinct.

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Waving robotic crab arm attracts females

A vigorous wave of the claw can be the key to mating success for male fiddler crabs, report researchers at the 13th Congress of the European Society for Evolutionary Biology.

Male crabs advertise their quality as a potential mate to passing females by waving their large yellow claws.

Using robotic arms, researchers evaluated how the size and speed of the waving claw affected mating success.

The results may help explain why males protect their smaller neighbours.

To the fiddler crab Uca mjoebergi, the Australian mudflats in the north of the country are a heaving dance floor, where a male must rely on his moves to attract a mate.

Males stand outside their burrows and use their enlarged claw to attract females by moving it in circles.

If a female likes the look of a male, she will come closer and disappear down his burrow in the sand, possibly staying to mate.

Wave of waving

When a female wanders through a neighbourhood, “you see part of the mudflat light up” with waving yellow claws, said ecologist Sophie Callander from the Australian National University in Canberra.

Dr Callander and her colleagues used a fully adjustable robotic arm – called Robocrab – to determine what female crabs are looking for in a mate.

Dr Callander set up three robotic arms around a female crab, and sat beneath the unforgiving Australian sun for many hours recording the females’ reactions to different combinations of wave speeds and claw size.

Females approaching from 20cm preferred males with a higher wave rate and larger claws. Intriguingly, this preference increased in strength when the male was flanked by more slowly waving, smaller-clawed crabs.

Fiddler crabs also use these claws in displays of dominance and fighting prowess.

Previous work has shown that larger males sometimes go to the aid of smaller males when an intruder is trying to steal a smaller male’s burrow.

This behaviour is unlikely to be an altruistic form of neighbourhood watch, and Dr Callander thinks that her experiment could offer an explanation.

“If larger males can retain smaller neighbours they might… increase their mating success,” she told BBC News.

For fiddler crabs at least, it pays to keep close to the small and weak.

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Six-Pack Crabs


Hermit crabs have been fighting over housing since long before humans ever got involved in real estate. The crustaceans (Pagurus bernardus) make their homes in marine snail shells, and when these shells becomes too tight, the crabs bang them against the larger shells of other crabs (see video), hoping to evict them. This furious rapping is thought to represent the attacking crab’s stamina, but researchers wondered if it actually represented brawn. And indeed, as the team reports online today in Biology Letters, crabs with bigger muscles relative to their body sizes were more likely to be victorious—both at winning a shell and at resisting eviction—while muscle stamina (as judged by protein concentration) had no impact. The results suggest that muscle strength, and not endurance, is key in this war of attrition.

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Fish Sleep Soundly in Mucous Cocoons

Even the ocean has bedbugs. Tiny blood-sucking crustaceans (inset) roam the seas, nipping at the scales of passing fish. But the parrotfish (Chlorurus sordidus) has evolved an unusual defense. According to a study published online today in the Proceedings of the Royal Society B, the fish spend up to an hour spinning cocoons from their own mucous before they settle down to slumber for the night. These transparent, gelatinous balls of spit are large enough to envelope the fish from head to tail. By gently pushing fish from their cocoons without waking them, researchers showed that those sleeping without protection were 80% more likely to be bitten by the crustaceans than those they left untouched. Producing these mucous membranes costs just over 2% of the fish’s daily energy budget; apparently a worthwhile investment against things that go bite in the night.

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