The delightfully abominable Yeti Crab

Kicking off my favorite fauna list at number ten is the Yeti Crab. Ghostly white and bristling with fine hairs, this species dwells in one of the most interesting habitats on the planet – the hydrothermal vent – and it is partly for this reason that I’m so fond of them. Hydrothermal vents are fissures in the earth’s surface that spew superheated water rich with chemicals dredged up from the beneath the ocean floor.

Hydrothermal Vent from ALVIN

Hydrothermal vent from the cockpit of the Alvin submersible.

Heated by magma below and compressed by over a mile of water above, this variety of H2O is supercritical, brought past the point where distinct liquid and gaseous phases exist. Direct contact with a fluid like this would instantly shatter the glass cockpit of even the sturdiest marine submersible, and if that wasn’t enough of a deterrent to life, hydrothermal vents fill the surrounding waters with toxic chemicals like hydrogen sulfide.

Amazingly, exotic bacteria are able to use these deadly chemicals to produce food through a process known as chemosynthesis. Such bacteria form the basis of entire ecosystems completely unreliant on the Sun, and extreme isolation means that each vent community is unique, like islands in the pitch-black ocean depth. Perhaps the most scintillating thing about them for me is they thought to exist elsewhere in the solar system on Jupiter’s moon Europa. It’s possible that these hydrothermal vents represent glimpses into nearby alien ecosystems. Some scientists have even suggested that life itself began around them.

Alvin

The Alvin submersible, commissioned in 1964 and still operational in 2012.

Solid evidence for such chemosynthetic ecosystems was first found in 1977 and mankind came to have a firsthand look five years later using the Alvin submersible. Owned by the U.S. Navy and operated by the Woods Hole Oceanographic Institution, Alvin is a landmark scientific instrument. Almost 30 years after its first voyage, Alvin shepherded scientists to the discovery of the first species of Yeti Crab (Kiwa hirsuta) in 2005. The find turned out to be so unusual that an entire new family (two levels above species) had to be created to classify it: Kiwaidae, named for the Polynesian deity Kiwa.

Yeti Crabs consume sulfur through the bacteria they eat, so only those made of pears and grated cheese are of gastronomic use..

Yeti Crabs belong to a group of crustaceans known as squat lobsters, but are neither lobster nor true crab. They fall under the infraorder Anomura, which is the sister group to Brachyura, the “true” crabs. The subtle distinction is that all true crabs developed the modern form we know today through a shared evolutionary ancestry. Anomurans (squat lobsters, king crabs, hermit crabs, coconut crabs, etc.) represent separate branches of the crustacean family tree that also independently developed crab-like forms. The jargon for this is carcinisation, which was described by the person who coined the term as “one of the many attempts of Nature to evolve a crab”.

Yeti Crab Appendages

Close-ups of Yeti Crab appendages from the specimen recovered by Alvin.

The namesake characteristics of the Yeti Crab are obviously its lack of color and hair-like filaments, known as setae. The former is easy to explain; there’s no need for skin color if there’s no light to see it by. A life in perpetual darkness has also led the Yeti Crab to near, if not full, blindess. These are characteristics often shared by creatures of the dark, like the cave dwellers known as troglobites.

Left: Microscope image of bacteria fixed to the crabs hairs (setae). Right: Scanning electron microscope image of seta tip. Scale bars: 20 and 100 µm.

The hair (setae) of the Yeti Crab is more interesting. Microscopic examinations have shown that each seta is covered in the same sorts of sulfide-loving bacteria that form the base of hydrothermal vent communities. I’ve heard of a number of species that are able cultivate bacteria or algae for their own purposes, often in symbiotic relationships like the jellyfish of Palau, but none “farm” in quite the same way as the Yeti Crab. When the second species (Kiwa puravida) was discovered in 2006, it was observed to perform a curious dance. The dance, as it were, churns up water surrounding the bacteria, ensuring a fresh supply of oxygen and chemicals to help the harvest grow. Other crustaceans have also figured out how to grow their own crops of bacteria, and it is this type of adaptability that has allowed them to thrive in such extreme environments. More recently, a third, yet-unnamed  species of Yeti Crab was discovered alongside a whole host others around vents off the coast of Antarctica.

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