The recently discovered family of “Yeti crabs” called Kiwaidae
Kiwa hirsuta is a unique crustacean species that was the first to be discovered in the family Kiwaidae, also known as “Yeti crabs”. This family of crustaceans was previously unknown to science and got its name from its white colour and the fact that it is covered with what looks like silky white hair. Until today, only 3 species of such crabs have been discovered, the first one was in 2005. These extremely weird looking creatures live around deep sea hydrothermal vents and cold seeps, meaning that they must be adapted to life in such extreme environments. The majority of the body of Kiwa hirsuta, including its cephalothorax, ventral surface and walking legs are covered with extremely dense setae. Hydrothermal vents and cold seeps are environments where there is a large variety of different complex symbiotic relationships between bacteria and the other species present. The reason for the high abundance of symbiosis near these deep vents is the extreme conditions there, such as high hydrostatic pressure, low levels of sunlight and high concentrations of toxic minerals constantly spewing out of the vents.
About 1500 kilometers south from Easter Island, off the coast of Chile, a French scientist called Michel Segonzac discovered the never before seen blind crab named Kiwa hirsuta. As mentioned before, a new family of crabs was created in order to classify the first yeti crab that was discovered in March 2005. This unique creature was found at a depth of over a mile and a half below surface, close to the local hydrothermal vents. The second species (Kiwa puravida) belonging to the Kiwaidae family was discovered in 2006, still at which point very little was known about them overall. The third species of Yeti crab was spotted during an expedition in 2010 on the hydrothermal vents on the East Scotia Ridge. Its scientific name is Kiwa tyleri, although it’s more commonly known as the “Hoff crab”. The name originates from the crabs “hairy” outlook resembling the bare chest of famous American actor David Hasselhoff. This species is extremely abundant around the base and the lower parts of the vent edifices in East Scotia Ridge, densities can reach up to 700 crabs per square meter.
It was recently discovered that in the hydrothermal vents around the East Scotia Ridge, Kiwa tyleri is the dominant macro-invertebrate. The first zoeal stage of the species is morphologically advanced and also provides evidence for its lecithotrophic larval feeding. Normally adult yeti crabs inhabit the warm areas around the “envelopes of vent chimneys”. In the case of ovigerous females possessing late embryos, they seem to be found away from the high temperatures around the vent sites. The implication that can be drawn from this is that embryogenesis in yeti crabs takes place away from the chemosynthetic environment that they normally inhabit in. The lecithotrophic larvae must then be released in the colder parts of the Southern Ocean. What allows the morphologically advanced larvae to survive in such areas is their lecithotrophy. Basically meaning that they can survive and grow through long developmental phases in an environment that is extremely seasonally varying in terms of its primary production. Yeti crab larvae are constantly exposed to the cold polar temperatures of the Southern Ocean due to the fact that they are demersal. They must therefore also possess extended larval developmental times. Yeti crabs are morphologically adapted and specialized for life in close proximity to hydrothermal vents in a polar environment setting. Most decapod crustaceans are unable to survive in low polar temperatures due to thermal issues. The scientific reason for this is that crabs and lobsters in general are unable to down regulate the concentration of magnesium in their blood to be below that of the surrounding sea water, which results in a condition similar to paralysis.
Endosymbiotic relationship with bacteria
The filamentous chemo-synthetic bacteria densely covers the setae of yeti crabs in clusters. Isotope and lipid analyses on Kiwa puravida provide evidence that the epibiotic bacteria is the crab’s main food source and is harvested by highly specialized setae hairs on its 3rd maxilliped. Yeti crabs have been found to swing their bacteria-covered chelipheds/walking legs rhythmically. Suspected reasoning for the swinging motion is thought to be to increase the flow of sulfide and methane towards the crab, which allows the symbiotic bacteria to feed. Scientists are fairly certain that differences in hydrothermal vent fluid composition directly affects both the diversity and composition of the epibiotic bacteria on the crab. The reason for the presence of bacteria can be logically explained as bacteria use a process similar to photosynthesis that’s called Chemosynthesis, in which they provide energy using the chemicals pumped out from the hydrothermal vents. Scientists have also discovered potential in the dense clusters of bacteria for both carbon fixation and “cycling of reduced and oxidized sulfur” because of the presence of the required enzymes. Basically, yeti crabs are dependent on chemo-synthetic bacteria for their primary production of energy, which is the reason that they inhabit areas so close to the highly chemical hydrothermal vents and cold seeps. This kind of endosymbiotic relationship can be called “farming”, in which both organisms mutually benefit by the “direct trophic transfer of energy from a symbiont to its host”.
Generally, the recent discovery of a new family of yeti crabs just goes to show how much of the oceans and its inhabitants we still do not know about. Especially in extreme marine environments where it is harder and more expensive to launch scientific expeditions to. I personally believe that we are still yet to find much more about the different species of yeti crabs in the future and that there’s still lots of new species out there to be discovered.