| Alicella gigantea | |
|---|---|
 | |
| Specimen collected from theJapan Trench in 2022 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Malacostraca |
| Order: | Amphipoda |
| Parvorder: | Lysianassidira |
| Superfamily: | Alicelloidea |
| Family: | Alicellidae |
| Genus: | Alicella Chevreux, 1899 |
| Species: | A. gigantea |
| Binomial name | |
| Alicella gigantea | |
Alicella gigantea is a giant, deep sea livingspecies ofamphipod. Sometimes referred to as the "supergiant amphipod", the largest of thesecrustaceans reach up to 34 cm (13 in) in length. The only species within the genusAlicella, it lives only at 4,850–7,000 m (15,910–22,970 ft) in depth. The species is white in colour and is distinguished from other deep-sea amphipods by minute anatomical differences, apart from size in the larger specimens. The large size of the species is often presented as an example ofabyssal gigantism, though the specifics of this trait remain under investigation. Genetic studies into the species have found that it has an exceptionally largegenome, which may be linked to its large body size.
The species was first collected and described in the 1890s from theMadeira Abyssal Plain off theCanary Islands. Although it is rarely detected,A. gigantea is acosmopolitan species and can potentially occur in 59% of the world'soceans. Their diet varies with age, but they are primarilyscavengers ofcarrion. Like other amphipods, femaleA. giganteabrood their eggs in pouches. Individuals of this species are believed to have an unusually long lifespan, reaching over 10 years in age. Despite their relative isolation from the surface, human pollutants such asDDT andchlordane have been detected in specimens.
The first two specimens ofAlicella gigantea were collected by thePrincess Alice, a ship named afterAlice Heine, wife ofPrince Albert I of Monaco.[2] These specimens were collected while on an expedition at theMadeira Abyssal Plain off theCanary Islands in 1897 using triangular traps that were set at 5,285 metres (17,339 ft) in depth.[3][2] The species was then subsequently described byÉdouard Chevreux, who named the genus after the ship from which they were collected. Chevreux placed this species in the familyLysianassidae. Theholotype (a single specimen upon which aspecies description is made) andparatype (additional specimens on which a species description is made) specimens are a juvenile of indeterminate sex and a juvenile male respectively; they are currently deposited within theOceanographic Museum of Monaco.[4][5] In 1906, the species description was revised byThomas Roscoe Rede Stebbing.[6]
A second species was described asAlicella scotiae in 1912 byCharles Chilton from a specimen 20 mm (0.79 in) long collected during theScottish National Antarctic Expedition. It was described as a new species because of subtle differences in the morphology of the mouthparts.[7] However, this species was later recognised to actually beEurythenes obesus, and therefore wassynonymized with that species, makingA. gigantea the only species in its genus.[8]
| Phylogeny[9] |
A. gigantea underwent ataxonomic revision in 1987, when the authors redescribed both original type specimens and described new specimens collected during the SEABED 2 and DEMERABY abyssal campaigns.[3] In 2008, this genus was moved from the Lysianassidae to a new family, theAlicellidae. It was selected as itstype genus. This family contains six other genera, all of which are deep-sea scavengers.[10]
There have been few studies into the genetics ofAlicella gigantea. In 2020, a study examined the relationships of deep sea amphipod species;sequences of16S,COI,Histone 3, and28S found thatA. gigantea formed aclade (group of organisms that include all descendants of a common ancestor) withTectovalopsis andDiatectonia.[9] In contrast to this, a 2015 study found thatAlicella formed a clade withCyclocaris andTectovalopsis althoughDiatectonia sequences were not used in this study.[11]
Since redwavelengths of light are quickly absorbed by water and thus do not reach thedeep sea, mostamphipods have red to orangecoloration, which helps them avoidpredators.Alicella gigantea, however, are mostly white, which may reflect their lack of predators.[12] Although there is minimalsexual dimorphism, there are very small differences in the shape and size ofantenna segments between males and females.[3]
A. gigantea can be best distinguished from other Alicellidae by the combination of having the firstgnathopod (leg-like appendage modified for feeding), simple in structure, and the first urosomite (segment that makes up part of the abdomen) with a rounded hump.[9]
The peduncle (the first three segments of the antennae) of the second antennae is short, and its first article (segment) is strongly swollen. The mouthparts form a squarish bundle with thelabrum and epistome (plate-like structures) being inconspicuous and blunt. The incisor edge (cutting edge of the mandible) is straight with some inner corner teeth. There is one middle tooth, and the rakers (blade-like structures) are absent. Themandiblepalp is attached to the front end of the molar, while the molar (grinding section of the mandible) itself is rather large, simple and covered in small hairs. The firstmaxilla (mouthpartappendages) have an inner plate with manysetae (hair-like structures) occupying the inner edge, while the first maxilla palp (appendage sprouting off the maxilla) is two-jointed and large. The second maxilla has a medial facial row of setae, and the inner and outer plates of themaxilliped (appendages modified for feeding) are strongly developed. The second maxilla palp is longer than the outer plate.[13]
The firstcoxa (first segment of the gnathopod) is expanded at the anterior end and is visible. The first gnathopod is small and simple. The third article of this gnathopod is elongated, article five is longer than the sixth, and thedactylus (claw) is large. In the second gnathopod, article six is slightly shorter than article five (both of which are elongated and linear), and the seventh article is an overlapping obsolescent palm. The dactyli of the third to seventh pereopods (leg-like structures) are quite short. The innerramus (branches at the end of an appendage) of the seconduropod (appendages on the last segment of the body) is unnotched. The third uropod has a regular peduncle (segment at the beginning of the appendage) and the outer ramus is articulated. Thetelson (rearmost segment on the body) is elongated and deeply cleft.[13]
A. gigantea is the largest known amphipod and can reach between 240–340 mm (9.4–13.4 in) in length.[13][14] As such, it is sometimes referred to as the "supergiant amphipod".[12][15] For comparison, other deep sea amphipods such asEurythenes gryllus andTectovalopsis wegeneri have been recorded at 126 mm (5.0 in) and 33.9 mm (1.33 in) in length respectively.[16][17] There have been several suggested reasons and mechanisms for thisabyssal gigantism.[14][18]
Deep-sea habitats have reduced temperatures and very high levels ofhydrostatic pressure. To counteract these pressures,A. gigantea are hypothesised to have increased cell sizes and lifespans, which in turn lead to abnormally large body growth. One study in 2021 found that genes related to "growth regulation" were over-represented inA. gigantea when compared to smaller amphipods. This indicates that size control or growth regulation mechanisms may be responsible for the large size of the species.[14]
One study reported thatA. gigantea had a high level ofselenium in its leg muscles. Thistrace element is linked with growth and metabolic activity and therefore might partly explain why this species grows so large.[19] Another potential reason for the large size ofA. gigantea was suggested by a 2013 study: It could have undergone a wholegenome duplication, which could potentially increase the size of the species and explain its large genome size.[20]
In 2017, the size of the whole genome was estimated to be about 34.79 gigabase pairs in length, which is considerably larger than genome size estimates for other species of deep amphipods in the same study. Due to the large size of its genome compared to other deep-sea amphipods, it has been shown thatA. gigantea exhibits a faster rate of genome size change. It was hypothesised that this could be due to a whole-genome duplication.[20]
In 2019, the completemitochondrial genome ofA. gigantea was sequenced with a total length of 16,851 base pairs.[21][22] The study found that the genome had 13protein-coding genes, 2ribosomal genes, 22transfer RNA genes and 2noncoding gene regions.[22]
Alicella gigantea is a rarely encounteredmarine species that has primarily been recorded in thelower abyssal andhadal depths between 4,850–7,000 metres (16,000–23,000 ft) in depth,[18] which would restrict them tooceanic trenches andfracture zones, such as theKermadec Trench in the southwest Pacific.[15] However, there is a single record of a juvenile specimen at 1,720 metres (5,640 ft) in depth that was collected with a fish trap.[13] There also is a record of a specimen from the stomach of ablack-footed albatross in theHawaiian Islands.[23] It has been suggested that this record may be due to the highlipid content of this species causing it to float upwards in the water column.[12] A 2025 study of 195 collections found thatA. gigantea occur in depths between 3,890–8,931 m (12,760–29,300 ft), which suggests that 59% of the world's oceans (and all sixmajor ocean bodies) are suitable habitat for this amphipod.[15]
It is acosmopolitan species. It has been recorded in the North Atlantic, North Pacific, and South West Pacific oceans. However, numerous gaps in their distribution are likely due to incomplete sampling of this species.[18] A 2025 study examined the genetics ofA. gigantea populations over its entire known range, showing that while populations had several haplotypes, most shared a single commonhaplotype (group of genes that are inherited from a parent) for each gene. This indicates that there is significant gene flow between populations.[15]
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To enable movement, marine amphipods swim using theirpleopods, which are pulsated more quickly to increase their speed. They are also capable of walking on solid surfaces using their pereopods, but this method is not as fast.[24]
Like many other amphipods, adultAlicella gigantea are primarilyscavengers and feed oncarrion.[3][22] Because of this behaviour, they are most frequently caught usingbaited traps.[18]
Thegut microbiome ofA. gigantea is dominated byCandidatusHepatoplasma.[25] One study in 2022 compared the gut microbiome ofA. gigantea with that of two other hadal amphipods and found that the particular gut assemblage was unique to each species.[25] It has also been discovered that hadal amphipods such asA. gigantea have large amounts of "probiotic" gut microbiota (microorganisms that are beneficial to the host).[26] Presently, it is unknown if these amphipods' gut microbes are inherited from their parents or picked up from the environment.[12]
The size ofA. gigantea also allows them to avoid being preyed on by predators such asNotoliparis kermadecensis, aliparid snailfish that preys on smaller amphipods.[18] However,A. gigantea ranging from 40 to 100 mm (1.6 to 3.9 in) in length have been recorded from stomach contents of theCoryphaenoides yaquinae, the rough abyssal grenadier.[13][12]
Like all amphipods, femaleA. giganteabrood their eggs in a pouch. It has been suggested that females probably have several broods over their lifetimes.[13] The eggs are oval in shape and are 6.95 to 14.88 mm (0.274 to 0.586 in) in length. The shell of the eggs is composed of twochorion layers: the exochorion, which isfibrillar in structure, and the endochorion, which has a porous structure with pores averaging more than 10μm in diameter.[27]
As juveniles, their diet consists mostly of bacteria andzooplankton debris, transitioning into carrion and algae as they mature.[28] Analysis of14C signatures indicates that deep sea amphipods such asAlicella gigantea have an unusually long lifespan of over 10 years.[29]
Despite their apparent isolation from theocean's surface,manmade pollutants have been detected inAlicella gigantea.[12] In one study in 2020,pesticides such asDDT andchlordane were detected inA. gigantea specimens, whilst in a 2022 study trace elements such ascadmium andchromium were detected in high concentrations which were suggested to be linked to human activity.[30][19] Due to their dependence on carrion as a food source, the species may be susceptible to changes occurring at the surface such as chemical pollution andoverfishing.[28]
