A giant isopod is any of the almost 20 species of large isopods in the genusBathynomus. They are abundant in the cold, deep waters of the Atlantic, Pacific, and Indian Oceans.[1][2]Bathynomus giganteus, the species upon which the generitype is based, is often considered the largest isopod in the world, though other comparably poorly known species of Bathynomus may reach a similar size (e.g., B. Kensleyi).[1] The giant isopods are noted for their resemblance to the much smaller common woodlouse (pill bug), to which they are related.[3]
French zoologist Alphonse Milne-Edwards was the first[4] to describe the genus in 1879[5] after his colleague Alexander Agassiz collected a juvenile male B. giganteus from the Gulf of Mexico. This was an exciting discovery for both scientists and the public, as at the time the idea of a lifeless or "azoic" deep ocean had only recently been refuted by the work of Sir Charles Wyville Thomson and others.[citation needed] No females were recovered until 1891.
Giant isopods are of little interest to most commercial fisheries, but are infamous for attacking and destroying fish caught in trawls.[6] Specimens caught in the Americas and Japan are sometimes seen in public aquariums.[3]
Description
A frontal view of Bathynomus giganteus, showing its large, highly reflective compound eyesThe underside of Bathynomus giganteus
Giant isopods are a good example of deep-sea gigantism (cf.giant squid), as they are far larger than the "typical" isopods that are up to 5cm (2.0in). Bathynomus can be divided into "giant" species where the adults generally are between 8 and 15cm (3.1 and 5.9in) long and "supergiant" species where the adults are typically between 17 and 50cm (6.7 and 19.7in).[1][7] One of the "supergiants", B. giganteus, reaches a typical length between 19 and 36cm (7.5 and 14.2in);[4] an individual claimed to be 76cm (30in) long has been reported by the popular press, but the largest confirmed was c.50cm (20in).[7]
Their morphology resembles that of their terrestrial relative, the woodlouse. Their bodies are dorsoventrally compressed, protected by a rigid, calcareous exoskeleton composed of overlapping segments. Like some woodlice, they can curl up into a "ball", where only the tough shell is exposed. This protects from predators trying to strike at the more vulnerable underside. The first shell segment is fused to the head; the most posterior segments are often fused, as well, forming a "caudal shield" over the shortened abdomen (pleon).[4]
The large eyes are compound with nearly 4,000 facets, sessile, and spaced far apart on the head.[8] They have two pairs of antennae. The uniramousthoracic legs or pereiopods are arranged in seven pairs, the first of which is modified into maxillipeds to manipulate and bring food to the four sets of jaws. The abdomen has five segments called pleonites, each with a pair of biramous pleopods. These are modified into swimming legs and rami, flat respiratory structures acting as gills. The isopods are a pale lilac or pinkish.[1][2]
The individual species generally resemble each other but can be separated by various morphological features, notably the number (7–13) and shape (straight or upturned) of the spines on the pleotelson ("tail"), shape (simple or bifid) of the central spine on the pleotelson, and the shape and structure of the uropods and pereopods.[1][9]
Range
Giant isopods have been recorded in the West Atlantic from the US state of Georgia to Brazil, including the Gulf of Mexico and the Caribbean.[1] The four known Atlantic species are B. obtusus, B. miyarei, B. maxeyorum, and B. giganteus, and the last of these is the only species recorded off the United States.[1][10] The remaining Bathynomus species are all restricted to the Indo-Pacific.[1]
No species occur in both the Atlantic and Indo-Pacific. Previous records of B. giganteus from the Indo-Pacific are now considered misidentifications of other species.[1] Giant isopods are unknown from the East Atlantic or East Pacific.[1] The greatest species richness (five species) is found off eastern Australia, but it is possible other regions that are not as well-sampled match this figure.[1] In general, the distributions of giant isopods are imperfectly known, and undescribed species may exist.[1]
Ecology
A giant isopod and shrimp
Giant isopods are important scavengers in the deep-sea benthic environment. They are mainly found from the gloomy sublittoral zone at a depth of 170m (560ft) to the pitch darkness of the bathyal zone at 2,140m (7,020ft), where pressures are high and temperatures are very low.[11] A few species from this genus have been reported from shallower depths, notably B. miyarei between 22 and 280m (72 and 919ft),[12] the poorly known B. decemspinosus between 70 and 80m (230 and 260ft), and B. doederleini as shallow as 100m (330ft).[1]
The depth record for any giant isopod is 2,500m (8,200ft) for B. kensleyi, but this species also occurs as shallow as 300m (980ft).[1] Over 80% of B. giganteus are found at a depth between 365 and 730m (1,198 and 2,395ft).[13] In regions with both "giant" and "supergiant" species, the former mainly live on the continental slope, while the latter mainly live on the bathyal plain.[1] Although Bathynomus have been recorded in water as warm as 20°C (68°F), they are primarily found in much colder places.[14] For example, during a survey of the deep-sea fauna of Exuma Sound in the Bahamas, B. giganteus was found to be common in water between 3.25 and 13°C (37.8 and 55.4°F), but more abundant towards the lower temperature.[15]
In contrast, preliminary studies indicate that B. doederleinii stops feeding when the temperature falls below 3°C (37°F).[16][17] This lower temperature limit may explain their absence from temperate and frigid regions where seas at the depth preferred by Bathynomus often are colder.[16][17] They are thought to prefer a muddy or clay substrate and lead solitary lives.
Although generalist scavengers, these isopods are mostly carnivorous and feed on dead whales, fish, and squid. They may also prey on slow-moving animals such as sea cucumbers, sponges, radiolarians, nematodes, and other zoobenthos, and perhaps even live fish. They are known to attack trawl catches. One giant isopod was filmed attacking a larger dogfish shark in a deepwater trap by latching onto and eating its face.[18]
As food is scarce in the deep-ocean biome, giant isopods must take advantage of whatever food they have available. They are adapted to long periods of famine and have been known to survive over 5 years without food in captivity.[19][20] When a significant source of food is encountered, giant isopods gorge themselves to the point that they could barely move. A study examining the digestive system contents of 1651 specimens of B. giganteus found that fish were most commonly eaten, followed by cephalopods and decapods, particularly carideans and galatheids.[4]
Giant isopods collected along the east coast of Australia by setting traps exhibit a variation in diversity with water depth. The deeper the water, the fewer number of species found, and the larger the species tended to be. The giant isopods found in very deep waters off Australia were compared to those found off Mexico and India. From the fossil record, Bathynomus is thought to have existed more than 160 million years ago, so it did not evolve independently in all three locations, but since then Bathynomus likely would show divergent evolution in the various locations. However, the giant isopods in all three locations are almost identical in appearance[21] (although some differences are seen, and they are separate species).[1] This reduced phenotypic divergence is linked to the extremely low light levels of their habitat.[21]
Reproduction
A study of the seasonal abundance of B. giganteus juveniles and adults suggests a peak in reproductive capacity in the spring and winter. This is observed to be due to a shortage of food during the summer. Mature females develop a brood pouch or marsupium when sexually active, the pouch being formed by overlapping oostegites or brood plates grown from the medial border of the pereiopods. The young isopods emerge from the marsupium as miniatures of the adults, known as mancae. This is not a larval stage; the mancae are fully developed, lacking only the last pair of pereiopods.
Human consumption
There have been occasional attempts at utilizing giant isopods as novelty food, such as prepared in East Asian cuisine like ramen. Relative to total size, there is not very much "meat" to be harvested. The meat is sometimes described as resembling lobster and/or crab in taste, with a somewhat firmer, chewier texture.[22]
Classification
The genus currently contains the following species:[23]
Fossilized specimens of Bathynomus are known extending back to at least the Early Oligocene (Rupelian) of Italy, with other fossils being known from Japan and Spain.[25]
Related Research Articles
Alphonse Milne-Edwards was a French mammalogist, ornithologist, and carcinologist. He was English in origin, the son of Henri Milne-Edwards and grandson of Bryan Edwards, a Jamaican planter who settled at Bruges.
Isopoda is an order of crustaceans. Members of this group are called Isopods and include both terrestrial and aquatic species such as woodlice. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen that are used in respiration. Females brood their young in a pouch under their thorax.
Bathynomus giganteus is a species of aquatic crustacean, of the order Isopoda. It is a member of the giant isopods (Bathynomus), and as such it is related—albeit distantly—to shrimps and crabs. It was the first Bathynomus species ever documented and was described in 1879 by French zoologist Alphonse Milne Edwards after the isopod was found in fishermen's nets off the coast of the Dry Tortugas in the Gulf of Mexico.
Reef lobsters, Enoplometopus, are a genus of small lobsters that live on reefs in the Indo-Pacific, Caribbean and warmer parts of the Atlantic Ocean.
The Gnathiidae are a family of isopod crustaceans. They occur in a wide range of depths, from the littoral zone to the deep sea. The adults are associated with sponges and may not feed. The juvenile form is known as a 'praniza', and it is a temporary parasite of marine fish. These forms are not larvae; Gnathiidae instead become parasitic during the manca stage. Mancae of the Gnathiidae closely resemble the adult form, however they lack the final pair of pereiopods.
Sphaeromatidae is a family of isopods, often encountered on rocky shores and in shelf waters in temperate zones. The family includes almost 100 genera and 619 known marine species. Within these genera, there are groups that share distinctive morphologies; further research may reclassify these genus-groups as separate families.
The Cymothoidae are a family of isopods in the suborder Cymothoida found in both marine and freshwater environments. Cymoithoids are ectoparasites, usually of fish, and they include the bizarre "tongue-biter", which attaches to a fish's tongue, causing it to atrophy, and replaces the tongue with its own body. Ceratothoa oestroides is one of the most devastating ectoparasites in Mediterranean aquaculture. Around 40 genera and more than 380 species of cymothoid are recognised. Species of the Cymothoidae are generally found in warmer waters and rarely in the cool and cold climates.
Ceratoserolis is a genus of isopods in the family Serolidae from the Southern Ocean around Antarctica and some Sub-Antarctic Islands. They prefer to live on soft bottoms and range of least between 24 and 950 m (80–3,120 ft) in depth. They are superficially similar to the unrelated, extinct trilobites and reach up to about 8 cm (3.1 in) in length. They were once considered to be part of the genus Serolis and for a long time only Ceratoserolis trilobitoides was recognized. The validity of the other species has been disputed, but there are some morphological and genetic differences between them and C. trilobitoides, and there are indications that additional, currently unrecognized species of Ceratoserolis exist.
In zoology, deep-sea gigantism or abyssal gigantism is the tendency for species of deep-sea dwelling animals to be larger than their shallower-water relatives across a large taxonomic range. Proposed explanations for this type of gigantism include necessary adaptation to colder temperature, food scarcity, reduced predation pressure and increased dissolved oxygen concentrations in the deep sea. The harsh conditions and inhospitality of the underwater environment in general, as well as the inaccessibility of the abyssal zone for most human-made underwater vehicles, have hindered the study of this topic.
Archaeoniscus is a genus of prehistoric isopods that first appeared during the Bajocian stage of the Middle Jurassic. It is a widespread genus with a paleogeographic distribution encompassing the continental margin environments of the central Atlantic Ocean and the western Tethys Ocean. Fossils of Archaeoniscus suggest that this genus lived in diverse aquatic habitats, including the marine, paralic, and freshwater environments. While earlier descriptions suggested that it may have had an ectoparasitic association with fishes, some researchers argue that at least two species, A. aranguthyorum and A. coreaensis, lived a benthic free-living lifestyle based on morphological characteristics that are either unsuitable for or unrelated to parasitic behavior.
Cirolanidae is a family of aquatic isopods.
Cymothoida is the name of a suborder of isopod crustaceans with a mostly carnivorous or parasitic lifestyle. It contains more than 2,700 described species in four superfamilies. Members of the suborder are characterised by their specialised mouthparts which include a mandible with a tooth-like process which is adapted for cutting or slicing.
Aega psora is a species of isopod crustacean that parasitises a number of fish species in the North Atlantic. It is a serious ectoparasite of larger species of fish, particularly when they are injured.
The Aegidae are a family of isopod crustaceans. The adults are temporary parasites of fish, feeding on their hosts' blood before dropping off to digest the meal. They differ from members of the family Cirolanidae in having only three pairs of hook-like pereiopods, whereas in Cirolanidae all seven pairs of pereiopods are hooked. The family was first described by Adam White in 1850.
Glyptonotus antarcticus is a benthic marine isopod crustacean in the suborder Valvifera. This relatively large isopod is found in the Southern Ocean around Antarctica. It was first described by James Eights in 1852 and the type locality is the South Shetland Islands.
Ceratothoais a genus of isopod ectoparasites of teleost fish, first described by James Dwight Dana in 1852. Infection by Ceratothoa can cause anaemia, lesions, growth retardation, emaciation, and mortality in their fish hosts.
Anuropus is a genus of isopods in the suborder Cymothoida. As of 2021, it is the only genus in the family Anuropidae.
Bathynomus yucatanensis is a species of marine crustacean that was discovered in the Gulf of Mexico. It is a member of the order of Isopoda, similar to Bathynomus giganteus.
Natatolana neglecta is a species of crustacean in the family Cirolanidae, and was first described by Hans Jacob Hansen in 1890 as Cirolana neglecta, It was redescribed as Natatolana neglecta by Stephen Keable and Niel L. Bruce in 1997.
Bathynomus brucei is a species of giant isopod first described in 2006. It was first located off of the coast of Australia, east of Flynn reef, though the range extends from waters of south-east of Asia to waters around the north of Australia.
References
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Lowry, J. K. and Dempsey, K. (2006). The giant deep-sea scavenger genus Bathynomus (Crustacea, Isopoda, Cirolanidae) in the Indo-West Pacific. In: Richer de Forges, B. and Justone, J.-L. (eds.), Résultats des Compagnes Musortom, vol. 24. Mémoires du Muséum National d’Histoire Naturalle, Tome 193: 163–192.
↑ Chamberlain, Steven C.; Meyer-Rochow, V. Benno; Dossert, William P. (August 1986). "Morphology of the compound eye of the giant deep-sea isopod Bathynomus giganteus". Journal of Morphology. 189 (2): 145–156. doi:10.1002/jmor.1051890205. PMID3746916. S2CID37070286.
↑ Kou, Qi; Chen, Jun; Li, Xinzheng; He, Lisheng; Wang, Yong (July 2017). "New species of the giant deep-sea isopod genus Bathynomus (Crustacea, Isopoda, Cirolanidae) from Hainan Island, South China Sea". Integrative Zoology. 12 (4): 283–291. doi:10.1111/1749-4877.12256. hdl:173321/20801. PMID28054459.
↑ Shipley, Oliver N.; Bruce, Niel L.; Violich, Mackellar; Baco, Amy; Morgan, Nicole; Rawlins, Scott; Brooks, Edward J. (2 August 2016). "A new species of Bathynomus Milne Edwards, 1879 (Isopoda: Cirolanidae) from The Bahamas, Western Atlantic". Zootaxa. 4147 (1): 82–88. doi:10.11646/zootaxa.4147.1.6. PMID27515606.
↑ Cocke, B. T. (1987). Morphological variation in the giant isopod Bathynomus giganteus (suborder Flabellifera: family Cirolanidae) with notes on the genus (Master's thesis). Texas A&M University. p.129 pp.
↑ Helgering, L.; Niele, H.; Mulders, R.; Smid, P. (10 February 2014). "Bathynomus giganteus' gigantisme". University of Amsterdam. Retrieved 14 January 2016.
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