Kiwa (crustacean)

Kiwa is a genus of marine decapods living at deep-sea hydrothermal vents and cold seeps. The animals are commonly referred to as "yeti lobsters" or "yeti crabs”, after the legendary yeti, because of their "hairy" or bristly appearance. The genus is placed in its own family, Kiwaidae, in the superfamily Chirostyloidea. The genus Kiwa is named after the god of shellfish in Polynesian mythology. Based on the presence of sulphur-oxidising bacteria on the setae of both K. hirsuta and the new South West Indian Ridge species, they may both feed on bacteria in addition to scavenging. For K. puravida, the bacteria have been identified and the feeding behaviour observed, as well as a cyclical rhythmic motion of the crab documented that is suspected to increase the flow of methane and hydrogen sulfide, the bacterial food, towards the bacteria. The two sexes of the new South West Indian Ridge species prefer different temperatures, with males seeming to prefer warmer water and egg-carrying females and juveniles preferring the coldest. Chemosynthetic bacteria is growing which is affecting the habitat for Yeti Crabs

Because yeti crabs live in extreme environments like cold seeps and hydrothermal vents, the species has adapted certain behaviors and qualities to aid their survival. These adaptations include the crab's growth of their setae, propodus and claws. The internal anatomy, diet, behaviors and adaptations of the Yeti Crab are outlined below.

Range and Habitat
Various species of yeti crabs have been found at varying locations. Four species have been described: Kiwa hirsuta discovered in 2005 on the Pacific-Antarctic Ridge, Kiwa puravida discovered in 2006 at cold seeps in the East Pacific (all other species are from hydrothermal vents), Kiwa tyleri, known colloquially as the "Hoff crab", from the East Scotia Ridge, and Kiwa araonae from the Australian-Antarctic Ridge. Two similar but undescribed species are known from vents on the South West Indian Ridge and at the Galápagos respectively. Analysis of DNA has confirmed the distinction of the species, them having diverged from each other millions of years ago. The third undescribed species of Kiwa was discovered in 2010 in the Atlantic sector of the Southern Ocean at vents on the East Scotia Ridge. Compared with the first two species, it has proportionally much shorter chelae, with the majority of the bacteria-growing setae concentrated on the ventral carapace.

Kiwa araonae's discovery added about 6,500 kilometers to the Kiwaidae's known range on mid-ocean ridges. From morphological observations and molecular phylogenetic investigations, K. araonae and K. puravida are closely related. Although K. araonae and K. puravida are separated by around 12,000 kilometers, they are similar. Research pertaining to fossils around the Eocene-Oligocene boundary in the eastern Pacific suggests a decrease in deep temperatures and increased ventilation in each habitat may have influenced species divergence. Likewise fossils found in the mid-Miocene (located in the tropical East Pacific), indicate that the evolution of one species of yeti crab, K. puravida, may coincide with lower deep sea temperatures.

Internal anatomy
Internal anatomy has been studied in Kiwa puravida.

Digestive system: The stomach (gastric mill + pylorus) is situated in the upper anterior portion of the cephalothorax. The gut extends as a tube to the telson. A pair of short anterior ceca emanates from the transition between pylorus and gut, appearing as short tubes. The hepatopancreas fills most of the cephalothorax and parts of the pleon and is formed by two bunches of tubular diverticles which are each connected to the pylorus via a main duct. The stomach is embedded ventrally by the hepatopancreas whose anterior diverticles reach into the rostrum.

Antennal glands: The antennal glands are flatenned and located anterolaterally in the cephalothorax, near the second antennae. the lobes of the antennal bladder are situated anteriorly and laterally to the stomach and cover a large portion of the hepatopancreas.

Reproductive system: The testes are paired and restricted to the cephalothorax. They are connected to the vasa deferentia on each side which run to the gonopores on the coxae of the eighth thoracic segment.

Nervous system: The brain is the anteriormost part of the central nervous system. It is relatively small compared to the rest of the body. Emanating from the brain several nerves run to the sensory organs (eyes, antennulae, antennae). A pair of circumesophageal ("surrounding the esophagus") connectives connect the brain with the cephalothoracic ganglion. The latter is a compaction of several neuromeres in the lower part of the anterior cephalothorax. These neuromeres correspond morphologically with the body segments of the mandibles and the 1st and 2nd maxillae, the thoracic segments I-VIII and the first pleonal segment. The 2nd to 6th pleonal neuromeres form separate ganglia. They are arranged in an irregular segmental pattern with the sixth pleonal segment reaching into the sixth pleonal segment.

Circulatory system: The heart is located in the upper portion of the cephalothorax below the carapace and above the anterior portion of the gut. The heart is suspended by several ligaments within the pericardial sinus which is bordered by the pericardial septum. Three pairs of openings (ostia) connect sinus and interior heart space. The interior heart is crossed by asymmetrical arranged muscular bundles which are part of the heart muscle (myocard). Seven arteries which can be categorized into five artery systems (two paired, three unpaired) emanate from the heart and run to the respective organs and body regions. After having left the arteries and having washed around the tissues the "blood" (hemolymph) is channelled to the gill (branchial) sinus via crevices (lacunae) and channels (sinus). After its oxygenation the hemolymph is returning to the pericardial sinus via the branchio-pericardial sinus.

Muscle systems: The general structure and organization of the various muscle systems in Kiwa (puravida) is congruent with that of other decapods.

Diet
Kiwa crustaceans play a crucial role in the deep-sea ecosystem, serving as primary consumers and contributing to the biodiversity within hydrothermal vent communities. The absence of sunlight at hydrothermal vents poses a challenge to how vent species obtain energy required to sustain life. Since sunlight cannot provide the necessary fatty acids and carbon isotopes necessary for their survival, Yeti crabs rely solely on symbiotic bacteria as their main food source. Yeti crabs feed on chemosynthetic bacteria that reside on hair-like setae on the outer appendages of the crustacean. Other organisms, such as Shinkaia crosnieri and Rimicaris exoculata exhibit similar dietary patterns as the Yeti crab.

Feeding behavior
Scientists have observed that K. puravida have a unique approach when it comes to farming food. This method has made the species become known as the "dancing crab". Yeti crabs have been observed waving their claws around near cold seeps, which makes them appear as if they are dancing. This dance is actually a form of farming for the species. By waving their claws around, the crab stirs up water around the symbiotic bacteria living in the setae on their claws. This ensures the bacteria are receiving enough chemicals that will provide energy to grow. By growing their own food, the yeti crab has a readily available resource of food in the harsh environment of the deep sea.

Reproductive behavior
Little is known about the mating behavior between yeti crabs. Much of what we know so far has been studied in Ki puravida. Scientists have found that male crabs exhibit larger claws than females, suggesting that antagonistic interactions may occur between males competing for a female mate. This resembles a precopulatory guarding type mating system, which describes a set of mating behaviors found in decapods which consists of long interactions between a set of mates and higher levels of aggression in males. In another species, K. tyleri, males were observed to be found in groups closest to hydrothermal vents, while females were seen further away from these dense areas. This may be because egg-bearing females need to escape the high levels of heat near vents and move to a colder area in order to protect the development of their eggs.

Adaptations
One adaptation of the yeti crab species is the growth of its setae to farm bacteria in the species K. puravida. There is some variation regarding the different types and various states of setae. There are two different types of setae that are organized adjacent to one another. Rows of narrow and flexible setae enclose the rows of more restricted, thicker setae. Because females slightly deviate from the vent environment during egg fertilization and larval development, brooding females contain more deteriorated, brown setae, as a result of their reduced carapace health away from the vent environment.

An additional adaptation found in the species Kiwa (tyleri) is the formation of a spine on the propodus (the end of the crab leg). This is beneficial for yeti crabs because they help the crustacean to cling to steep chimneys of hydrothermal vents. The stout, compact build of the crab also helps it traverse hydrothermal vent environments.

Scientists have found that male yeti crabs have larger claws than females, which may indicate that bigger claws are sexually selected for reproductive advantages. Previous research has suggested the claws are a mechanism of protection as scientists observed yeti crabs fighting with their claws or waving their claws to fend off predators. Bigger claws may also provide a better fitness advantage because they provide more surface area for symbiotic bacterial farming.