Yeti Crabs: Hairy Crustaceans, Hydrothermal Vents, and Cold Seeps
What Are Yeti Crabs?
Yeti crabs are unusual crustaceans that were first discovered in 2005. Their legs or undersurfaces are covered with hair-like structures called setae. The collection of setae sometimes looks like silky fur. Researchers have discovered that yeti crabs have bacteria on their hairs and that the members of at least one of the species known so far “farm” these bacteria and eat them.
Yeti crabs are found in the deep ocean around hydrothermal vents or cold seeps. Hydrothermal vents are openings where superheated water emerges in geysers from beneath the Earth's crust. Cold seeps are areas where fluid at seawater temperature is slowly released from the ocean floor.
The Known Species of Yeti Crabs
The first yeti crabs to be discovered were found around hydrothermal vents in the South Pacific Ocean. These animals have been given the scientific name Kiwa hirsuta. They have the longest hairs of the yeti crab species known so far, especially on their legs and claws. The animals reminded their discoverers of the Yeti, or the Abominable Snowman. The Yeti is a hairy, ape-like creature that some people believe inhabits Nepal and Tibet. The animal in the video screen shown above is Kiwa hirsuta.
In 2006, a species of yeti crab called Kiwa puravida was found around a cold seep in deep water near Costa Rica. It also has hairy legs. In 2010, a third species of Kiwa was discovered near the coast of Antarctica around a hydrothermal vent. This species has hairs on its undersurface and has been named Kiwa tyleri, or the Hoff crab. The discovery of Kiwa araonae was reported in 2016, though the animal was first collected in 2013. This species lives by a hydrothermal vent on the Australian-Antarctic Ridge. In this article I describe the first three species mentioned above as representatives of their genus.
Another yeti crab that appears to deserve species status has been discovered by a hydrothermal vent in the Southeast Indian Ridge. This means that five species of the crab are known so far. It's interesting that an animal that wasn't known until 2005 appears to be fairly common.
Yeti crabs are sometimes known as yeti lobsters. They are neither true crabs nor true lobsters, however. They are actually squat łobsters and are classified as follows.
Squat lobsters are found in several families in the infraorder Anomura. A new family was created in this infraorder just for yeti crabs—the family Kiwaidae. True crabs are classified in the infraorder Brachyura while true lobsters are classified in the infraorder Astacidea.
General Features of Squat Lobsters
Squat lobsters are small to medium-sized animals with flattened bodies and a short abdomen that is tucked under their body. They have ten legs arranged in five pairs, although not all of the legs are visible. They also have a pair of long antennae on their head and a pair of compound eyes on stalks. The development of the eyes and the ability to see seem to be reduced in yeti crabs, however.
Squat lobsters have jointed legs, like all members of the phylum Arthropoda. The first pair of legs are enlarged and have a very noticeable claw at the end. The next three pairs of legs are smaller and have only a tiny claw at their tip. These legs are used for walking. The fifth pair of legs are very small and are usually folded under the body. They may be used for cleaning the gills, which are the animal's respiratory organs.
Plate Movement, Magma, and Hydrothermal Vents
Hydrothermal vents are found in deep water where plates in the Earth's crust are either moving away from each other or towards each other. In the first case, hot liquid rock called magma rises from deeper within the Earth at the boundary between separating plates. The magma eventually solidifies, filling in the gap between the plates and forming a ridge. In the second case, one of the colliding plates moves under the other one (subduction). The descending plate heats up as it moves downwards and eventually forms magma.
Hydrothermal vents form when sea water percolates downwards through cracks in the hot rock present in either of the above cases. The water is heated to a high temperature by magma as it moves downwards. The hot water then returns to the surface and gushes out of the sea floor into the colder ocean above it, forming a vent. The process is described in more detail below.
Formation and Features of Hydrothermal Vents
The main steps in the formation of a hydrothermal vent are as follows.
- Sea water enters the cracks and pores in a moving plate and is heated by magma.
- The sea water moves downwards due to gravity, becoming hotter and picking up dissolved minerals as it travels.
- The properties of the water change as it's heated to a high temperature and it becomes very buoyant. A knowledge of physics is needed to understand the change in properties.
- The hot water rushes to the surface and emerges from the sea floor in a gushing, mineral-rich geyser.
The temperature of the vent water may be as high as 400 degrees Celsius or 750 degrees Fahrenheit at the moment of release. The vent water doesn't boil, however, because of the pressure of the sea water above it.
The water released from a vent may form a "white smoker", which looks like a white cloud, or a "black smoker", which is black in colour. Black smokers are coloured by iron sulphide and are hotter than white smokers. White smokers contain barium, calcium, or silicon compounds.
Life Around Hydrothermal Vents
The hot, acidic water in a hydrothermal vent leaches minerals from rock, providing nutrients for the organisms living in the area. The minerals in the hot solution often precipitate as they contact the cold sea water, forming a chimney.
The vent water contains hydrogen sulphide. Bacteria produce food molecules from the energy stored in the chemical bonds inside the hydrogen sulphide molecules. This process is called chemosynthesis and forms the basis of the food chain in the area. Animals either eat the bacteria or obtain their food from the bacteria living in their tissues.
As scientists explore the areas around hydrothermal vents they are finding fantastic communities of animals that they haven't discovered anywhere else. The darkness of the ocean depths and the pressure created by the deep water haven't prevented a vibrant group of organisms from living around some vents. The organisms include squat łobsters, crabs, giant tube worms (shown in the video below), clams, mussels, barnacles, limpets, octopuses, and even fish. The vent species are different from the related species in shallower water, however.
Giant tube worms live around many hydrothermal vents. The worms live inside a mineral tube and feed on chemicals produced by bacteria in their body. They use their plumes to absorb nutrients for the bacteria. The plumes are red due to the presence of hemoglobin, the same pigment that colours our blood.
Photosynthesis and Chemosynthesis
The discovery that life can exist in deep, permanently dark water was an exciting one. It was once thought that life depended either directly or indirectly on the sun and photosynthesis. The discovery of chemosynthesis has changed this notion.
In photosynthesis, organisms use light energy to drive the reaction between carbon dioxide and water in order to make a sugar and oxygen. The sugar is a food molecule. Chemosynthesis is quite similar to photosynthesis, but in chemosynthesis organisms use the energy stored in a molecule such as hydrogen sulphide or methane to create food from simpler molecules.
The video below shows how chemosynthesis in hydothermal vents works.
Many people would probably consider Kiwa hirsuta to be the most attractive yeti crab. The crab is a pale creature just under 0.152 metres or six inches long. It has a few hairs on its undersurface, but most of its long, silky blond hairs are on its legs, especially its front claws. A "furry" crab is very strange site to see, since fur is associated with mammals, not crustaceans.
The role of the bacteria on the legs of Kiwa hirsuta isn't yet certain. The bacteria may be a source of food or they may remove poisonous minerals from the water around hydrothermal vents, enabling the crabs to live there. The crabs have been observed eating mussels and fighting over shrimp, so they may be carnivorous or omnivorous.
The crab was given the genus name "Kiwa" after the Polynesian goddess of crustaceans. "Hirsuta" is Latin for hairy. The crab is assumed to be blind, since it has membranes in place of eyes.
In 2010, an Oxford University team explored the sea floor in the Antarctic. The exploration was performed by a submersible robot vehicle named Isis. The vehicle visited and photographed a hydrothermal vent community at a depth of 2500 metres below the water's surface. Isis can travel to a depth of more than 6 kilometres.
The vehicle found a dense population of small, white yeti crabs on the ocean floor. The crabs were often arranged on top of each other in piles. In some areas the scientists counted 600 crabs in a square metre.
The Antarctic yeti crabs have long hairs on their underside. Filamentous bacteria are located on these hairs. The researchers are almost certain that the bacteria are used as food. The hairy "chest" of the crabs reminded the researchers of David Hasselhoff, a star in the old Baywatch TV series. They've nicknamed the creatures "Hoff crabs".
In June 2015, the Hoff crab was given the scientific name Kiwa tyleri. The species is named after Paul Tyler, a biologist at Southampton University in the UK. Tyler specializes in studying life in polar and deep sea environments.
Methane hydrates form in some cold seeps. These are potentially useful as fuel sources. In a methane hydrate, water molecules form a cage-like structure surrounding a methane molecule.
Types of Cold Seeps
Cold seeps are another feature found on the ocean floor. Unlike the situation in a hydrothermal vent, the fluid (liquid or gas) released from a cold seep has about the same temperature as the surrounding sea water and doesn't form a geyser.
There are believed to be two types of cold seeps—methane ones and brine ones. In methane seeps, methane and other hydrocarbons are produced in sediments below the ocean floor. These substances move upwards through fissures in rock and enter the ocean. The fluid in the seep often contains hydrogen sulphide as well as methane.
Brine seeps release a very salty and dense liquid. This dense water can collect in underwater depressions to form brine pools. The salt comes from within rock. The life forms mentioned below have been found around methane seeps, not brine ones.
Since hydrogen sulphide is often present in both hydrothermal vents and cold methane seeps, the same organisms may be found around each, including squat lobsters, giant tube worms, clams, and mussels. The creatures around the cold seep grow more slowly than those around the hydrothermal vent, however. Cold seeps also have some unique bacteria due to the presence of methane, which like hydrogen sulphide can be used as an energy source during chemosynthesis.
Unlike the two species of yeti crabs described above, Kiwa puravida is found around deep water cold seeps instead of hydrothermal vents. Its species name was derived from the phrase "pura vida", which literally means "pure life" and is popular in Costa Rica.
Kiwa puravida is a bacteria farmer. The bacteria on its hairs use the methane and possibly the hydrogen sulphide from a methane seep to produce food molecules. The crabs wave their claws rhythmically over a seep to create water currents and expose their bacteria to the nutrients in the fluid coming from the seep. They periodically run their claws through their mouths to feed on the bacteria. The mouth has comb-like structures that separate the bacteria from the hairs. Researchers have found that the crabs are almost entirely reliant on the bacteria for nourishment.
Learning More About Yeti Crabs
Yeti crabs and other creatures around hydrothermal vents and cold seeps are often difficult to study. Scientists need specialized equipment in order to explore the ocean floor in deep water. They are trying to learn more about the highly specialized organisms around vents and seeps, however.
It's important that the unusual vent and seep habitats are protected and that their unique and fascinating communities are allowed to thrive. This is necessary not only to maintain the amazing variety of life on Earth but also for other reasons. Understanding the biology and chemistry of vent and seep creatures may teach us about how life developed on Earth and may even lead to practical applications that benefit humans.
Discovery of Kiwa hirsuta from the Monterey Bay Aquarium Research Institute
Information about Kiwa tyleri from National Geographic
Discovery of Kiwa puravida from the Nature journal
Facts about hydrothermal vents from the NOAA (National Atmospheric and Oceanic Administration) website
Information about cold seeps and the organisms that live around them from NOAA
A new species of yeti crab discovered around the Australian-Antarctic Ridge from the Journal of Crustacean Biology, Oxford University Press
© 2012 Linda Crampton