Tag Archives: hermit crab

A, an individual in an aquarium, carrying the coral; B, an individual removed from its host coral. Scale bar: 1 mm. Credit: PNAS.

Newly-described hermit crab species makes permanent homes out of coral — a first

Hermit crabs are famous for constantly changing ‘homes’ as they outgrow the discarded shells of various other species like snails. A newly discovered species that lives in the Amami Islands, a subtropical island chain north of Okinawa, Japan, is no squatter. According to Japanese biologists, this hermit crab is the first to take residence in a living, growing coral home. This hermit crab has finally found a permanent home.

A, an individual in an aquarium, carrying the coral; B, an individual removed from its host coral. Scale bar: 1 mm. Credit: PNAS.

A, an individual in an aquarium, carrying the coral; B, an individual removed from its host coral. Scale bar: 1 mm. Credit: PNAS.

The clever Diogenes heteropsammicola found a lucrative arrangement with two kind landlords, Heteropsammia and Heterocyanthus. The two coral species, however, typically have a different tenant, a symbiotic marine worm called sipunculan.

The peanut shell-shaped sipunculan usually lives in crevices in rocks or in empty shells. Some lucky fellows, though, take on the coral up for the ride — a rather sweet deal for both parties. The worm gains both shelter and protection, thanks to the coral’s stinging tentacles, while the coral itself is offered transportation and a guarantee that it won’t get stuck in seafloor sediment.

A hermit crab with a home for life. Kudos!

Everybody wins in this relationship. Too bad a home wrecker like D. heteropsammicola had to barge in and ruin it for the peanut worm. According to the biologists at Kyoto University who discovered the new hermit crab species, after D. heteropsammicola evicts the worm (or finds an unoccupied coral), it permanently turns the two coral species into its home.

There are many reasons to believe this has been going on for some time, judging from several bodily features. The crab has a long and thin body with red-white claws that are significantly longer than its relatives’. Its telson — the posterior-most division of the body of an arthropod — is symmetrical, whereas other crabs have a lower abdomen shaped like a corkscrew that twists towards the right-hand side. Crabs that go through various shell homes need this abdomen configuration to match the spiraling direction of the snail shell. D. heteropsammicola, on the other hand, is adapted to coral cavities that twist both left and right.

“A shift in symbiont acquistion strategy by the corals has allowed the hermit crab to take over the transportation role of the usual sipunculan partners. Our data suggest that the corals are obligately symbiotic with sipunculans and this hermit crab, both of which are also obligately symbiotic with the two genera of the corals,” the authors wrote in PNAS.

A–C, sequence of behaviors to recover from an overturned to upright position in which the hermit crab leans out of the overturned coral (A), grasps the bottom with its ambulatory legs and left cheliped (B), and turns the coral upright using the pleon (C); D–F, sequence of behaviors to overcome burial in sediment, whereby the buried hermit crab (D) pushes away the sediment using its chelipeds and ambulatory legs (E), and then crawls away (F). Credit: PNAS.

A–C, sequence of behaviors to recover from an overturned to upright position in which the hermit crab leans out of the overturned coral (A), grasps the bottom with its ambulatory legs and left cheliped (B), and turns the coral upright using the pleon (C); D–F, sequence of behaviors to overcome burial in sediment, whereby the buried hermit crab (D) pushes away the sediment using its chelipeds and ambulatory legs (E), and then crawls away (F). Credit: PNAS.

When the Japanese researchers studied the coral and crab in a controlled environment, i.e. an aquarium, they witnessed how the crustaceans would brush debris and sediments off their gracious coral hosts. This crab obviously knows what it’s doing. It better takes care of the coral, too, since it grows in sync with the anthropod. A hermit no more!

“By becoming symbiotic with the corals, the hermit crab has probably gained extra security through protection by coral nematocysts and its more permanent lodging means that it no longer needs to change shells as it grows in size,” the scientists concluded.

Henderson Island was once regarded as one of the most pristine landscapes in the world. This makes this desolate sight from Henderson even more hurtful. Every day, some 27 new pieces of junk wash up on the island according to a new study. Credit: Jennifer Lavers/University of Tasmania.

Remote Island in the middle of the Pacific is the most plastic-littered place on Earth

Henderson Island was once regarded as one of the most pristine landscapes in the world. This makes this desolate sight from Henderson even more hurtful. Every day, some 27 new pieces of junk wash up on the island according to a new study. Credit: Jennifer Lavers/University of Tasmania.

Henderson Island was once regarded as one of the most pristine landscapes in the world. This makes this desolate sight from Henderson even more hurtful. Every day, some 27 new pieces of junk wash up on the island according to a new study. Credit: Jennifer Lavers/University of Tasmania.

Thousands of miles away from the nearest city, smack in the middle of the South Pacific Ocean, lies the Henderson Island. This island, part of the Pitcairn Islands and a UNESCO World Heritage Site, has never been permanently inhabited and only a few humans have ever made the trip by ship to set foot on it, usually scientists on research expeditions. Knowing this, Henderson ought to be one of the most pristine places on this planet. The sad reality, however, is that Henderson Island has the highest density of plastic trash ever reported in nature.

No escaping plastic pollution

“What’s happened on Henderson Island shows there’s no escaping plastic pollution even in the most distant parts of our oceans,” lead author Jennifer Lavers from the University of Tasmania said in a statement. “Far from being the pristine ‘deserted island’ that people might imagine of such a remote place, Henderson Island is a shocking but typical example of how plastic debris is affecting the environment on a global scale.”

According to the University of Tasmania team, huge quantities of plastic waste in all shapes and sizes have washed up along the years on Henderson. Every day, thousands of new tiny bits of plastic are thrown by tides on the white-sand beaches of the island.

Henderson Island is polluted with the greatest density of plastic in the world. The situation is so bad that hermit crabs use plastic debris for homes. Credit: Jennifer Lavers/University of Tasmania.

Henderson Island is polluted with the greatest density of plastic in the world. The situation is so bad that hermit crabs use plastic debris for homes. Credit: Jennifer Lavers/University of Tasmania.

For three months since May 2015, Lavers and colleagues lived on Henderson Island and documented all of the man-made trash there. Their estimates suggest there are over 37,661,395 pieces of anthropogenic trash which amount to 17.6 tons of plastic waste. This mass, however, represents only “1.98 seconds’ worth of the annual global production of plastic,” researchers wrote in the Proceedings of the National Academy of Science.

These stats are extremely worrisome and scary for a number of reasons. The report gives to show there’s no escaping human influence, i.e. trash, even in the remotest places on Earth. The oceans, like the land and atmosphere, is a unified environment which is why trash from San Francisco can wind up in the ocean and, eventually guided by currents, on a remote island such as Henderson. By some estimates, there may be some 5 trillion pieces of plastics and microplastics swirling around the world’s oceans. These wash up on shores across the planet where they litter and pollute the environment. Animals, both marine and land-based, eat these plastics causing them to suffer or die. Ultimately, humans can end up ingesting these plastics too since we’re at the top of the food chain.

Most of the items found on Henderson were largely disposable or single-use; things like razors, cigarette lighters, toothbrushes, etc. All of this trash doesn’t bode well with the island’s unique ecology, which includes ten endemic plant and four bird species. The image above with the hermit crab that uses a plastic container for a shell is most telling in this respect.

So, who’s to blame? Everyone basically. The paper doesn’t explicitly say this, but the millions of plastic debris on Henderson come from all over the world.

Wrapped in a free for all tug of war, the hermit crabs stack as each is looking to displace its shell for a more spacious one.

Hermit crabs socialize in order to back stab their neighbor and steal their ‘homes’

There are around 800 hermit crab species living in the ocean. These crustaceans like to mind their own business and are rarely seen alongside one another, hence their species’ name. The dozen or so terrestrial hermit crab species, however, are forced to engage socially in order to survive. Their social pattern reveals a selfish agenda, though, after a recent study by biologists at Berkeley University found a vicious behavior – the land hermit crabs gather and kick other crabs out of their shell, in order to move in it.

Wrapped in a free for all tug of war, the hermit crabs stack as each is looking to displace its shell for a more spacious one.

Wrapped in a free for all tug of war, the hermit crabs stack as each is looking to displace its shell for a more spacious one.

These hustling hermit crabs have been forced to adapt this way, though one might suggest that evolution could have brought them down a more altruistic path. Typically, ocean hermit crabs use empty snail shells that are abundant through out as shelter and egg deposits. On land however, the only empty snail shells available are the few that happen to wash ashore. Pushed by scarcity, the terrestrial hermit crab adapted and developed a unique trait, distinct from their ocean dwelling cousins – they’re the only ones that hollow out and remodel their shells, sometimes doubling the internal volume.

Even with this impressive skill, however, the hermit crab still winds up in trouble since sooner or later it will outgrow its shell. In order to survive, the hermit crabs developed a sort of sacrificial social gathering. Thus, as three or more crabs gather around, others flock by the dozens as well, eager to trade up. Curiously, they first line up in a sort of conga line, smallest to largest by the shell, each holding on to the crab next to him. As they trade shells, most of the time the largest of the group gets more than he bargained for when he first joined, as he gets wrenched from its shell .

“The one that gets yanked out of its shell is often left with the smallest shell, which it can’t really protect itself with,” said Mark Laidre, a UC Berkeley Miller Post-Doctoral Fellow, who is in the Department of Integrative Biology. “Then it’s liable to be eaten by anything. For hermit crabs, it’s really their sociality that drives predation.”

Hermit crab shell

A marine snail shell newly vacated by its gastropod owner (left) and a shell that has been remodeled by a hermit crab.

If they can modify shells themselves, why trick their brethren in such a despicable manner? Well, in an experiment in which Laidre and colleagues pulled crabs from their homes and instead offered them newly vacated snail shells, the researchers found that only the smallest hermit crabs took advantage of new shells, as they’re the only ones to fit in and thus begin the remodeling process. The rest perish.

Laidre believes this behavior to be an excellent example of how niche construction leads to unusual traits, like this out of the ordinary socialization in an otherwise solitary animal.

“No matter how exactly the hermit tenants modify their shellters, they exemplify an important, if obvious, evolutionary truth: living things have been altering and remodeling their surroundings throughout the history of life,” wrote UC Davis evolutionary biologist Geerat J. Vermeij in a commentary in the same journal.

Findings were documented in the journal Current Biology.

source: Berkeley Newsroom