Gigantocypris | |
---|---|
Preserved specimens at Oxford University Museum | |
Scientific classification | |
Kingdom: | |
Phylum: | |
Subphylum: | |
Class: | |
Order: | |
Family: | |
Genus: | Gigantocypris Skogsberg, 1920 [1]
|
Species | |
See text |
Gigantocypris, sometimes known as giant ostracod[2] or giant seed shrimp,[3] is a genus of ostracod crustaceans in family Cypridinidae,[1] and among the most well-known members of the class Ostracoda (together with Vargula hilgendorfii).[4] Its members are extremely large for ostracods, measuring up to 3.2 cm (1.3 in) across,[5] have a globular shape, are typically semi-transparent orange or reddish, and have a large pair of mirror-like eyes that are used to locate their small animal prey.[6][7] They are found worldwide in dark, deep and cold oceans.[8]
Gigantocypris are ubiquitous in open oceans around the world, ranging from tropical to polar regions.[6][9][10] Although locally abundant,[6] the distributions of the individual species are often not well known.[8] Gigantocypris dracontovalis is found worldwide, mostly deeper than the other species.[6][11] Gigantocypris agassizii is essentially a widespread Pacific species, and Gigantocypris muelleri a widespread Atlantic and Southern Ocean species, but there are also a few possible records of the former in the Atlantic, and a few records of the latter in the Pacific and Indian oceans.[8][10] Gigantocypris australis and Gigantocypris danae are only known from the Southwestern Pacific and Western Indian Ocean, respectively.[8]
Gigantocypris have been reported at depths between 150 and 3,500 m (490–11,480 ft).[7][8] They are typically found from 600 to 2,300 m (2,000–7,500 ft).[8][10] There are indications that young tend to occur shallower than adults.[10] They live in water that is dark (below the sunlight zone) and cold, less than 15 °C (59 °F),[7] with most records between about 2 and 5 °C (36–41 °F).[8] In water that is 15 °C (59 °F) or warmer, their swimming becomes weak and erratic.[7]
Gigantocypris include the largest ostracods, at up to 3.2 cm (1.3 in) across.[5] The largest are G. agassizii and G. australis.[10][12] Another relatively large species is G. muelleri, which reaches up to 2 cm (0.8 in),[13] but typically is about 1.0–1.8 cm (0.4–0.7 in), with Southern Ocean individuals averaging largest.[10] The smallest species, such as G. dracontovalis, typically reach 0.8–1.2 cm (0.3–0.5 in).[6][11][14] Females grow larger than males.[7][14]
Their body is suspended within a semi-translucent, globular carapace.[8] Depending on the exact species, living individuals typically are orange, orange-red or violet-red,[6][15] but they can also be colourless.[10] Specimens preserved in alcohol become whitish.[4] Calcium carbonate is absent in their shell, and their body is fragile with a watery body that often is damaged when collected for scientific studies.[6][10][14][16] They have a water content of about 95%, far above that reported for other crustaceans and more similar to jellyfish.[17]
Despite living in the darkness below the sunlight zone, they are equipped with a pair of large eyes which, rather than using lenses to focus light onto a retina, use parabolic mirrors.[18] The parabolic mirror eyes typically have a diameter of about 3 mm (0.12 in), look out through transparent sections of the carapace,[19] and appear silvery or golden in colour.[10][14] Their eyes are the most elaborate known from ostracods,[20] and are better at gathering light than the eyes of any other animal (although the resolution of the image produced by the eyes is likely poor).[19][21] It is thought that Gigantocypris use them to find bioluminescent prey animals.[4] They are known to feed on other ostracods, copepods, arrow worms and small fish (primarily fish larvae).[7][15][22] Exactly how they catch their prey is unclear, but studies show that the outer part of their mandibles can be extended out through the slit (opening) of their globular carapace.[7] Gigantocypris swim by "rowing" with two featherlike antennae, each with nine long setae.[7][15] Another pair of long antennae, believed to be used for sensing, extend out in front of the animal when swimming.[7] Both their swimming and sensing antennae can be retracted into the globular carapace through its slit.[7] They have a near-neutral buoyancy (marginally negative, sinking) and are able to swim smoothly (not in jerks) at a relatively high speed, indicating that they are active predators.[7] It is speculated that their relatively large heart—the largest among ostracods in both total and relative size—supports their active behavior, as well as their large eyes.[7] When brought to the ocean surface, they have a slightly positive (floating) buoyancy, and their swimming is highly unstable and tumbling, but they are able to re-adjust to a near-neutral buoyancy and normal swimming pattern in less than a day.[7] They change their buoyancy by adjusting the sulphate content of their haemolymph.[10] They sometimes fall prey to other animals such as squid,[23] fish like grenadiers and chub mackerels,[24][25] and prions.[26]
The female Gigantocypris has a brood pouch, located inside the carapace, in which the eggs and embryos develop. When "born", the young resemble miniature adults.[8] Adult males are uncommon compared to adult females.[8]
The World Register of Marine Species recognize six valid species in the genus Gigantocypris.[1][27] One of these, G. pellucida (described simultaneously with G. agassizii, both based on East Pacific specimens[28]), is often not considered valid.[8] In contrast, possibly undescribed species are known, and Atlantic and Southern Ocean G. muelleri may represent separate species.[10]