Two new porcelain crab species discovery


This April 2018 video says about itself:

Species Corner – Porcelain crab in a symbiotic relationship with a bubble-tip anemone

The Porcelain Crab is common throughout the tropical oceans of the world, and has a flat, round body with two large front claws. They are brown and orange in coloration with bright blue spots. These crabs have a pair of front arms called maxillipeds, which have ends that are feather-like in appearance. They use these appendages to filter the water for any passing food. They are peaceful and interesting invertebrates for the marine aquarium..

From the Smithsonian Tropical Research Institute:

Two new porcelain crab species discovered

October 15, 2019

Summary: Two new symbiotic porcelain crab species have been described. One of them, from the South China Sea of Vietnam, inhabits the compact tube-like shelters built by the polychaete worm with other organisms. The other inhabits the intertidal vermetid snail formations in the Colombian Caribbean.

Two new porcelain crab species have been described in the ZooKeys journal by scientists from the Smithsonian Tropical Research Institute (STRI) and the Institut fur Tierokologie und Spezielle Zoologie der Justus-Liebig-Universitat Giessen. One of the new species, Polyonyx socialis, was discovered in the South China Sea of Vietnam. The other, Petrolisthes virgilius, has a new identity, after initially being taken for a similar-looking species — Petrolisthes tonsorius — four decades ago in the Colombian Caribbean.

Porcelain crabs belong to a highly diverse family of marine crustaceans, distributed in the shallow waters of oceans worldwide. They also are known as “false crabs”, because they evolved a crab-like form independently of true crabs. A relatively large number of porcelain crab species are symbiotic with other organisms, allowing scientists to tell a story of a long-time relationship between species from distantly related taxa.

“Most porcelain crabs live on the hard substrates of shallow waters like the surface of corals or rocks overgrown by algae, microbes and decaying material,” said STRI research associate Alexandra Hiller, co-author of the papers. “Others live as symbionts of invertebrates like sponges, anemones, sea urchins, polychaete worms and other crustaceans.”

The two recently described species are examples of these symbiotic porcelain crabs. P. socialis derives its name from the Latin word for “social” because it was found living with other organisms — including the larger porcelain crab species, Polyonyx heok — in the compact tube-like shelters built by the polychaete worm Chaetopterus sp. Its broad, flat walking legs and claw-bearing extremities appear to have been adapted for living tightly-attached to the worm tube walls and avoid being perceived as an obstacle for the other organisms.

Although initially mistaken for P. tonsorius in the 1970s, the uncommon colors and atypical habitat of P. virgilius — intertidal vermetid snail formations in the Colombian Caribbean — led the scientists to corroborate through genetic analyses that it was a new species. As a symbiont, P. virgilius has evolved in tight association to its distinctive surroundings: a reef-like microhabitat exposed to wave action and consisting of snail shells cemented to each other and to a hard substrate.

Despite the relatively high number of known symbiotic porcelain crab species, such as P. socialis and P. virgilius, the researchers believe this aspect of their ecology could hinder their long-term survival, particularly in the shallow-water ecosystems where they typically occur. These habitats are often more vulnerable to climate change, ocean acidification and contamination.

“Symbiotic species are thought to be more vulnerable to environmental challenges than free-living organisms,” said Prof. Dr. Bernd Werding, from the Institut fur Tierokologie und Spezielle Zoologie der Justus-Liebig-Universitat Giessen, and co-author of the studies. “Their fate depends on the fate of their host, which may also be affected by local and global conditions and abrupt changes.”

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Crab changes it shell, video


This 10 September 2019 video is about a shore crab changing its shell in the Grevelingen lake in the Netherlands.

Diver Bram van der Sanden made this video.

Angular crabs on Terschelling island


This video from England says about itself:

Angular or Square crab in Devon

Goneplax rhomboides, filmed at Sprey Point, Teignmouth January 2018. These odd-looking crabs can sometimes be found alive washed up on the sandy beaches of Torbay, particularly after rough seas.

They usually live in burrows on muddy/sandy bottoms from the shallow sublittoral to 100m depth.

Translated from wildlife warden Joeri Lamers on Terschelling island in the Netherlands, 28 August 2019:

In recent months I have seen more and more reports of discoveries of angular crabs on the beaches. Usually they are dead animals, but also some live ones. The Wadden Sea Society even had one in their aquarium this summer. The angular crab is a newcomer to our coast, it used to be found in somewhat more southern waters that stay warmer on average, and therefore it benefits from the warmer water.

Beach finds

This weekend I found a dead specimen on the beach at beach pole 8. Earlier, I had already found loose carapaces, especially after the hard January storm in 2017. Actually since that time you see the discoveries increase, including through waarneming.nl. This summer, I think it suddenly became a lot, and also spread over a large part of the Terschelling beach. Most finds are between beach pole 3 and pole 7, but they are also found at pole 16. I like to hear new notifications, put them in the comments under this blog post.

Warmer seawater

This crab lives on average in reasonably warm waters, especially south of the English Channel and in the Mediterranean Sea. The North Sea was always too cold for them because this is a shallow sea that cools quickly in the winters. Deeper water maintains a more constant temperature. The water in the North Sea gets warmer on average, with opportunities for these and other species.

In 2016, this species was found for the first time on a Dutch beach; on Ameland island, east of Terschelling.

Fiddler crabs’ eyes help against predators


This 2016 video is called Fiddler crab dance.

From the University of Bristol in England:

Separate polarization and brightness channels give crabs the edge over predators

August 21, 2019

Fiddler crabs see the polarisation of light and this gives them the edge when it comes to spotting potentials threats, such as a rival crab or a predator. Now researchers at the University of Bristol have begun to unravel how this information is processed within the crab’s brain. The study, published in Science Advances today [Wednesday 21 August], has discovered that when detecting approaching objects, fiddler crabs separate polarisation and brightness information.

The key advantage of this method is that the separate visual channels provide a greater range of information for the crab. The research also suggests that when it comes to detecting predators’, polarisation can provide a more reliable source of information than brightness.

The researchers from the Ecology of Vision Group in the School of Biological Sciences tested how crabs responded to visual stimuli presented on a special computer monitor developed by the lab. By changing the polarisation and brightness of the stimuli the researchers were able to test whether certain combinations of polarisation and brightness appeared to cancel each other out.

Sam Smithers, PhD student in the School of Biological Sciences and one of the authors, said: “If you look through Polaroid sunglasses at the sky, you will notice that the brightness changes when you tilt your head. This is because the light from the sky is polarised and your sunglasses allow you to see differences in polarisation as differences in brightness.”

“For our experiments we tested whether fiddler crabs see the same effect. However, we discovered that the crabs detect polarisation in a completely different way to this and polarisation has no effect on how the crabs detect the brightness of a scene.”

The next step for the research team is to find out what happens to the brightness and polarisation information deeper in the brain. This will help them to understand how and when polarisation and colour information provide a visual advantage to the viewer.

The new study is about the fiddler crab species Afruca tangeri.

Land crab visits Bermuda petrel chick


This video from Bermuda says about itself:

Land Crab Pays Visit To Cahow ChickBermuda petrel parents visit their chick – May 29, 2019

Watch a land crab slowly creep into the Bermuda Petrel nesting burrow and make its way around the sleeping chick. The crab doesn’t stay for long; it scuttles away as soon as the large nestling is awakened.

The CahowCam is a collaboration between the Cornell Lab of Ornithology and the Nonsuch Expeditions. You can watch the cam live at http://allaboutbirds.org/cahow.