How hermit crabs get new shells


This 25 February 2019 video says about itself:

Hermit crabs are drawn to smell of flesh torn from their kin | Science News

Within three minutes on a beach at Osa Peninsula, Costa Rica, land hermit crabs (Coenobita compressus) crowd a tube containing flesh bits of their own kind. Researchers say the smell signals that an empty shell may be available for the taking.

By Yao-Hua Law, 8:00am, February 25, 2019:

Hermit crabs are drawn to the smell of their own dead

Competition for abandoned shells turns into a lively gathering

The death of a millionaire with no heir draws an opportunistic crowd. So, too, does the demise of a land-dwelling hermit crab. Researchers working in Costa Rica found that the curious crabs are drawn to the smell of flesh torn from one of their own.

Dartmouth College biologist Mark Laidre, along with undergraduate student Leah Valdes, set out 20 plastic tubes on a beach, each holding bits of hermit crab flesh. Within five minutes, almost 50 hermit crabs (Coenobita compressus) swarmed around each sample, the pair reports online February 10 in Ecology and Evolution. “It’s almost like they were celebrating a funeral,” Laidre says.

The reality, however, is more macabre. That scent of flesh is a signal that a fellow land hermit crab has been eaten, and that its empty shell is available for the taking, Laidre says. The crabs “are all in an incredible frenzy to try to move into that leftover shell.”

None of the roughly 850 known hermit crab species, most of which live in the sea and some on land, can grow their own shells. Instead, the crabs occupy shells originally left behind by dead snails. A hermit crab grows to the size of its shell, but to grow further, the creature must find and occupy a larger shell.

For the roughly 20 or so species of land hermit crabs, finding a suitable shell can be especially challenging. Big shells with lots of extra room to grow may be too heavy in the short term for a hermit crab to tote around on land without the buoyancy of water to help lighten the load, and lighter shells may be too small.

Land hermit crabs can remodel their shells, making them bigger, Laidre reported in 2012. The animals use corrosive secretions and scraping to widen a shell’s opening, remove the internal spiral and reduce wall thickness. Remodeling can double the available space while trimming one-third the weight. But remodeling is taxing and slow. It’s much faster to take over an already remodeled shell of another land hermit crab, alive or dead. Hence the strong attraction of land hermit crabs toward smells that suggest another is dead, Laidre says.

The researchers also found that land hermit crabs approached bits of snail flesh, though the scent appears to be far less alluring than that of their own species. Sea hermit crabs, however, didn’t find the smell of another hermit crab’s corpse more attractive than those of snails.

That makes sense to Laidre. For sea hermit crabs, upsizing to bigger and heavier shells is relatively easy, thanks to water’s buoyancy that helps the crabs support a shell that’s a little too big at first. That, combined with the fact that there are also many more empty shells in the sea than on land, means that sea hermit crabs face less competition when looking for a home, he says.

By highlighting that shell availability is limited for land hermit crabs, the study makes an important point for conservation, says ecologist Chia-Hsuan Hsu, who studies hermit crabs at National Taiwan University in Taipei and wasn’t involved in the research. “We can tell the public: ‘Don’t take shells from the beach’, ” he says.

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Sand fiddler crabs’ burrows, new research


This video from the USA says about itself:

The Atlantic Sand Fiddler Crab (Uca pugilator) performing a very entertaining claw waving display. They do this to attract mates and establish their territory.

Filmed at Little Neck Bay, Bayside, New York on 6/17/2010.

From ScienceDaily:

Sand fiddler crabs have home advantage in competition for breeding burrows

February 6, 2019

Sand fiddler crabs that reside in a burrow usually prevail if challenged by another, intruding crab, provided their claw pinching strength is similar to that of the competing crab, a study suggests.

The features of sand fiddler crabs that determine the outcomes of competition between intruders and residents of breeding burrows are identified in a paper published in the Springer journal Behavioral Ecology and Sociobiology. Dr Denson McLain and colleagues at Georgia Southern University, USA, found that when a resident sand fiddler crab was challenged by an intruder, it took refuge inside its burrow, forcing the intruder into a prolonged fight that was twice as long as other contests. These lengthy contests require the intruding crab to display stamina alongside pinching strength, while the resident crab only needs strength. The mismatch provides the resident crab with a competitive advantage, according to the new study.

Dr McLain, corresponding author of the study said: “Strength and stamina have long been associated with victory in contests between males for breeding territories. However, territory owners may utilize features of their territories to gain an advantage over rivals who possess greater fighting ability. We found that greater claw pinching force leads to victory for burrow owners but that among intruders it only leads to an additional requirement for victory, the display of stamina.”

The researchers observed contests between resident and intruder sand fiddler crabs competing for breeding burrows on a beach in Florida. They analysed competitions between 159 pairs of crabs and measured their claw pinching strength, stamina (measured by the number of pinches delivered at a high level of force), and resilience (the ability to return to former strength and stamina after being pinched). Despite being stronger, intruder crabs only won around 40% of contests.

Dr McLain said: “An intruder crab can only win if it pinches with a high force and also has the energy to endure a long, physically taxing contest. The difficulty of evicting another crab from a burrow may be the reason why residents guard burrow entrances very diligently and why they are quick to retreat when challenged by a strong intruder.”

The researchers explained that when the resident crab retreats into the burrow, the intruder cannot fully open his claw, rendering any advantage in strength held by the intruder ineffective. The intruder may enlarge the burrow tunnel to enable greater access to the retreated resident, but this approach also requires stamina.

The authors found that resilience did not play a role in contest outcome. However, higher resilience enabled intruders who lost contests to challenge more residents, which increased their odds of winning a burrow. Being resilient was also found to be favourable for resident crabs, as it enabled them to engage in multiple contests in short periods of time.

Cave crayfish in Florida, USA


This 2 December 2018 video from the USA says about itself:

In this exciting cave diving episode, Jonathan, Zach and Todd venture into Little River Spring in Florida on a cave diving search for the rare endemic Pallid Cave Crayfish. If you like Blue World cave diving episodes, you are going to love this one!

JONATHAN BIRD’S BLUE WORLD is an Emmy Award-winning underwater science/adventure series featuring underwater cinematographer/naturalist Jonathan Bird.

Zombie shrimp playing dead


This video from the Monterey Bay Aquarium Research Institute (MBARI) in California in the USA says about itself:

Zombie shrimp play dead to avoid being eaten

28 November 2018

While exploring the depths of the Gulf of California with the remotely operated vehicle Doc Ricketts, MBARI researchers saw an eerie sight: the small shrimp, Hymenopenaeus doris, hanging upside down, motionless in the water. At first, the shrimp appeared dead, but a closer look revealed that the animal was making tiny adjustments of its antennae and legs to maintain a head-down position while very slowly sinking. When the submersible got too close, the shrimp sprang back to life and quickly swam away.

While performing this “zombie-like“ behavior, the shrimp looked a lot like a discarded exoskeleton sinking slowly through the dark midwater. The researchers speculate that the shrimp might reduce their chances of being eaten by mimicking a sinking molt.

This odd behavior might also be an adaptation to conserve energy, since the shrimp live at depths where the seawater contains very little oxygen. Animals found in low-oxygen environments have a harder time moving rapidly or for long distances.

The researchers observed three zombie shrimp hanging right underneath large mucous webs or nets. Many deep-sea animals use mucous webs to gather marine snow (small particles of debris drifting down from the surface) for food. The biologists were unable to confirm a connection between the shrimp and the webs, leaving this mystery to be solved on a future expedition.

For more information see here.

Publication: Burford BP, Schlining KL, Reisenbichler KR, Robison BH (2018) Pelagic shrimp play dead in deep oxygen minima. PLoS ONE 13(11): e0207249.

Video editor: Kyra Schlining
Music: Stranger Danger (YouTube audio library)

How water fleas detect predators


This 2017 video says about itself:

Daphnia magna under the microscope

Daphnia magna is a small crustacean that is related to crabs, shrimp and lobsters. They live in many aquatic environments and are very important to the food chain. Daphnia are plankton that feed on algae and other microbes and are themselves food for small fish and other aquatic animals. They are popular as fish food and studied in biology labs that look at evolution or ecology.

While there are both male and female daphnia that can mate and reproduce, females can actually clone themselves through a process called parthenogenesis.

The anatomy of the daphnia is labeled in certain shots. The magnification of each shot is shown in the bottom right corner.

From Ruhr-University Bochum in Germany:

How water fleas detect predators

November 27, 2018

Water fleas of the genus Daphnia detect via chemical substances if their predators, namely Chaoborus larvae, are hunting in their vicinity. If so, they generate defences that make them more difficult to consume. The signalling molecules that enable detection have been identified by biologists and chemists from Ruhr-Universität Bochum, the University of Duisburg-Essen and the University of Birmingham. It is a cocktail of substances that occurs during digestive processes of Chaoborus larvae. The researchers discuss their findings in the journal Nature Chemical Biology from 14 November 2018.

For the study, the team headed by Dr. Linda Weiss and Professor Ralph Tollrian from the Institute for Animal Ecology, Evolution and Biodiversity in Bochum collaborated with Professor Nils-Metzler-Nolte’s Chair of Inorganic Chemistry I in Bochum, Professor Oliver Schmitz’ Chair of Applied Analytical Chemistry in Duisburg, and Dr. Ulf Sommer from the University of Birmingham.

Neckteeth and spines against predators

Daphnia are able to grow neckteeth, i.e. thorns in their neck region, or spines on their exoskeleton, that make it more difficult for Chaoborus larvae to consume them. However, Daphnia grow these defences only if the predator is actually in their vicinity. “It had long remained a mystery why the predators alert to their presence via chemical substances even though it clearly is a disadvantage for them,” says Linda Weiss.

“As far back as 40 years ago, researcher tried to identify the substances with which predators betray their presence,” describes Ralph Tollrian, Head of the Institute for Animal Ecology, Evolution and Biodiversity, who has been studying this subject ever since his PhD thesis. But it took modern methods such as high-res mass spectrometry to unravel the mystery.

Chemical signals caused by digestive processes

Together with their colleagues, Weiss and Tollrian found out that Chaoborus larvae secrete at least five different substances into the water that Daphnia can detect. The substances play a crucial role in the larvae’s digestion processes, as they are released when the predator spits the indigestible components back out. “This explains why the animals cannot stop the release of chemical signals,” says Tollrian. “The advantage for the larvae when the substances are part of their digestive processes is greater than the disadvantage of betraying their presence to prey.”

Substances manufactured artificially

The researchers, moreover, developed the substances in the lab, in order to verify their effect. They added them to culture-bred Daphnia and analysed the defences grown in consequence. The Daphnia reacted to the artificially manufactured substances in the same way as to the presence of Chaoborus larvae in their vicinity.

In follow-up studies, the researchers intend to identify the receptors used by Daphnia to detect the signalling molecules and decode the precise pathway of signal transmission.

Shrimp healing injured fish


This July 2018 video says about itself:

These Shrimp Are a Clean-Up Crew For Dirty Fish | Nat Geo Wild

Along this Caribbean reef, Pederson cleaner shrimp wait to feast on the parasites of these fish.

From James Cook University in Australia:

Shrimp heal injured fish

August 23, 2018

James Cook University scientists in Australia have discovered that shrimp help heal injured fish.

PhD student David Vaughan is working on a project led by Dr Kate Hutson at JCU’s Centre for Sustainable Tropical Fisheries and Aquaculture.

He said it was important to know how the shrimp interact with fish, as the team is in the process of identifying the best shrimp species to use to clean parasites from farmed and ornamental fish.

“Between 30 — 50% of farmed fish in Southeast Asia, the largest fish producing region in the world, are lost to parasites.

“We know that shrimp clean parasites from fish and if we can identify a species that does it efficiently, and does no harm, it offers a ‘greener’ alternative to chemicals”, he said.

Mr Vaughan said scientists knew injured fish visited shrimp ‘cleaning stations’ to have parasites removed — but the question was whether shrimp then took advantage of the injured fish and fed on their wounds. He said the relationship between cleaner shrimp and their client fish was complicated, with the shrimp known to eat the mucus of the fish and the fish occasionally eating the shrimp.

The scientists used high-definition cameras to record the details of the interaction between the species. “We found that shrimp did not aggravate existing injuries or further injure the fish”, said Mr Vaughan.

He said image analyses showed the cleaner shrimp actually reduced the redness of the injury. “Injuries in fishes are susceptible to invasion by secondary pathogens like viruses and bacteria, and the reduction in redness by shrimp indicates that cleaner shrimp could reduce infections.”

Mr Vaughan said cleaner shrimp are also known to indirectly influence the health of client fishes by reducing stress levels as a function of cleaning — which also increased the ability of the fish to heal.