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.[1] Giant isopods are noted for their resemblance to the much smaller terrestrial woodlice, 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.
Morphology
Size of Bathynomus giganteus compared to a human handTop, bottom and frontal view of Bathynomus vaderi
Giant isopods are an example of deep-sea gigantism (cf.giant squid), as they are far larger than other isopod groups. 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][6] 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).[6]
Their morphology resembles that of their terrestrial relatives the woodlice. 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, exposing only the tough dorsal shell. This protects them 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.[7] 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. Giant isopods are a pale lilac or pinkish in coloration.[1]
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][8]
Giant isopods like Bathynomus giganteus store substantial organic reserves in their midgut gland and fat body, with lipids forming a significant component, particularly in the fat body where triacylglycerols make up 88% of total lipids.[9]
Range and habitat
Bathynomus giganteus
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 known species occurs 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]
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.
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[18] (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.[18]
Giant isopods are important scavengers in the deep-sea benthic environment. They 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.[19]
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.[20][21] When a significant source of food is encountered, giant isopods gorge themselves to the point that they have difficulty moving. 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]
Reproduction and life cycle
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 mancas. This is not a larval stage; the mancas are fully developed, lacking only the last pair of pereiopods.[citation needed] The relative seasonal abundance of B. giganteus mancas and juveniles suggests a peak in reproductive capacity in the autumn and winter.[22]
As of 2021 there were only two documented instances of ovigerous (egg-bearing) giant isopod females being captured. This suggests that ovigerous individuals may seclude themselves in pits dug in the ocean floor substrate and cease foraging during the incubation of the eggs.[22]
A study of B. giganteus and B. miyarei found that larger, sexually mature individuals tended to be found in shallower regions.[22]
The approximate lifespan of B. giganteus has been estimated to be 7.7 years for females and 6 years for males, and that of B. miyarei has been estimated at 6 years for females and 9 years for males.[22]
Relationship to humans
Giant isopods are occasionally eaten as novelty food, such as in East Asian cuisine like ramen;[23] in Vietnam, however, they are actively sought as a delicacy.[24] Relative to total size, there is not very much "meat" to be harvested.[citation needed]
Giant isopods are killed in large numbers as bycatch in the commercial fishing of other species such as the monkfish Lophius gastrophysus. During 14 industrial fishing cruises in 2001, 55,475 giant isopods were caught as bycatch.[22] If thrown back into the sea following capture as bycatch, the post-release mortality of giant isopods is estimated to lie between 50-100%, due to the stress of capture and spending time on board.[25] Commercial fisheries typically do not monitor the bycatch of giant isopods, making it difficult to ascertain the scale of bycatch killings.[22]
Giant isopods are known to attack and eat fish caught in trawl nets.[26]
Large quantities of plastic were found during analysis of the stomach contents of three giant isopods in the Gulf of Mexico.[4]
Fossil 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.[29]
↑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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