''Alicella gigantea'' is an

amphipod Amphipoda () is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphipod species cur ...

inhabiting the

deep sea The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combination of low tempe ...

, the largest species ever observed, with some individuals reaching up to long; this species is an example of abyssal gigantism. The

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

''Alicella'' lives only at great depths; the first specimens were collected at the end of the 19th century from the

Madeira Abyssal Plain Madeira Abyssal Plain, also called Madeira Plain, is an abyssal plain situated at the center and deepest part of the Canary Basin. It is a north-northeast to south-southeast elongated basin that almost parallels the Mid-Atlantic Ridge. Its western b ...

, and subsequent specimens have been found on other

abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. They ...

s of both the

Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...

and

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

s, as well within

ocean trench Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic t ...

es and

fracture zone A fracture zone is a linear feature on the ocean floor—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on eit ...

Taxonomy

The genus name ''Alicella'' refers to the ship '' Princess Alice'', which collected the first specimens in 1897. The ship itself is named after

Alice Heine Marie Alice Heine (10 February 1857 – 22 December 1925) was an American-born Princess consort of Monaco, Princess of Monaco, by marriage to Prince Albert I, Prince of Monaco, Albert I of Monaco. Marcel Proust used her as a model for the Princes ...

, who was the wife of

Albert I, Prince of Monaco Albert I (Albert Honoré Charles Grimaldi; 13 November 1848 – 26 June 1922) was Prince of Monaco from 10 September 1889 until his death in 1922. He devoted much of his life to oceanography, exploration and science. Alongside his expeditions, ...

This species was first described in 1899 by

Édouard Chevreux Édouard Chevreux (10 November 1846, in Paris – 10 January 1931, in Bône) was a French carcinologist. Chevreux specialised in the study of Amphipoda, an order of malacostracan crustaceans. With Louis Fage (1883–1964), he was co-author of th ...

from two specimens collected by the ''Princess Alice'' while on an expedition at the

off the

Canary Islands The Canary Islands (; ) or Canaries are an archipelago in the Atlantic Ocean and the southernmost Autonomous communities of Spain, Autonomous Community of Spain. They are located in the northwest of Africa, with the closest point to the cont ...

in 1897. These specimens were collected using triangular traps that were set at in depth.Chevreux, E. (1899)
Sur deux espèces géantes d'amphipodes provenant des campagnes du yacht Princesse Alice
''Bulletin de la Société Zoologique de France''. 24, 152-158; figs. 1-6. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

and

paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype (biology), isotype ...

specimens are a juvenile of indeterminate sex and a juvenile male respectively; they are currently deposited within the

Oceanographic Museum of Monaco The Oceanographic Museum (), is a museum of marine sciences in Monaco City, Monaco. This building is part of the Institut océanographique, which is committed to sharing its knowledge of the oceans. History The Oceanographic Museum was i ...

. Chevreux placed this species in the family

Lysianassidae Lysianassidae is a family (biology), family of marine Amphipoda, amphipods, containing the following genera: *''Acontiostoma'' Stebbing, 1888 *''Alibrotus'' Milne-Edwards, 1840 *''Allogaussia'' Schellenberg, 1926 *''Ambasia'' Boeck, 1871 *''Amba ...

. In 1906, the species description was revised by

Thomas Roscoe Rede Stebbing The Reverend Thomas Roscoe Rede Stebbing (6 February 1835, London – 8 July 1926, Royal Tunbridge Wells) was a British zoology, zoologist, who described himself as "a serf to natural history, principally employed about crustacean, Crustacea". ...

, but little new information was provided. A second species was described as ''Alicella scotiae'' in 1912 by

Charles Chilton Charles Frederick William Chilton MBE (15 June 1917 – 2 January 2013) was a British presenter, writer and producer who worked on BBC Radio. He created the 1950s radio serials ''Riders of the Range'' and ''Journey into Space'', and also ...

from specimens collected during the

Scottish National Antarctic Expedition The Scottish National Antarctic Expedition (SNAE), 1902–1904, was organised and led by William Speirs Bruce, a natural scientist and former medical student from the University of Edinburgh. Although overshadowed in terms of prestige by Robe ...

. However, this species was later recognized to actually be '' Eurythenes obesus'', and therefore was synonymized with that species. It underwent a

taxonomic revision In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon), and these groups are given ...

in 1987, where the authors redescribed both original type specimens and described new specimens collected during the SEABED 2 and DEMERABY abyssal campaigns. In 2008, this genus was moved from the

to a new family, the

Alicellidae Alicellidae is a family (biology), family of Amphipoda, amphipod crustaceans, which live as scavengers in the deep sea (at depths of ), often in association with hydrothermal vents. The family includes the following genera: *''Alicella'' Chevre ...

and was selected as the

type genus In biological taxonomy, the type genus (''genus typica'') is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearin ...

. This family contains six other genera, all of which are deep sea

scavenger Scavengers are animals that consume Corpse decomposition, dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a he ...

Description

Many other amphipods possess either red to orange coloration, which is thought to help avoid predators (as the red

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

of light is quickly absorbed by water, and never reaches the

). ''Alicella gigantea'' however are uniformly white, which may reflect their lack of predators. Although there is very little

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

, there are very small differences in the shape and size of antenna segments between males and females. ''A. gigantea'' can be best distinguished from other Alicellidae by the combination of having the first

gnathopod An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

(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. 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 the labrum and epistome (plate like structures) being inconspicuous and blunt. The incisor edge (cutting edge of the mandible) is straight with some inner corner teeth, one middle tooth and the rakers (blade like structures) absent. The

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

palp Pedipalps (commonly shortened to palps or palpi) are the secondary pair of forward appendages among chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicera ...

is attached distally to the molar, while the molar (grinding section of the mandible) itself is rather large, simple and covered in small hairs. The first

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

(mouthpart

appendages An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

) have an inner plate with many

setae In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae ...

(hair like structures) occupying the

medial Medial may refer to: Mathematics * Medial magma, a mathematical identity in algebra Geometry * Medial axis, in geometry the set of all points having more than one closest point on an object's boundary * Medial graph, another graph that repr ...

edge (the surface pointed towards the middle of the body) 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 the

maxilliped An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

(appendages modified for feeding) are strongly developed. The second maxilla palp is longer than the outer plate. The first coxa (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 the

dactylus The dactylus is the tip region of the tentacular club of cephalopods and of the leg of some crustaceans (see arthropod leg). In cephalopods, the dactylus is narrow and often characterized by the asymmetrical placement of suckers (i.e., the vent ...

(claw) is large. For the second gnathopod, article six is slightly shorter than article (both of which are elongated and linear) and the seventh article is an overlapping obsolescent palm. The dactylus of the third to seventh pereopods (leg like structures) are quite short. The inner ramus (branches at the end of an appendage) of the second

uropod Uropods are posterior appendages found on a wide variety of crustaceans. They typically have functions in locomotion. Definition Uropods are often defined as the appendages of the last body segment of a crustacean. An alternative definition sugge ...

(appendages on the last segment of the body) is unnotched. The third uropod has a regular peduncle (segment at beginning of appendage) and the outer ramus is articulated. The

telson The telson () is the hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment (biology), segment of the arthropod body, or an additional division that is not a true segm ...

(rearmost segment on the body) is elongated and deeply cleft.

Gigantism

''A. gigantea'' are the largest known amphipods in the world and can reach between in length. There have been several suggested reasons and mechanisms for this gigantism. Deep sea habitats have reduced temperatures and very high levels of hydrostatic pressure. To counteract these pressures, ''A. gigantea'' (and other deep sea amphipods) are hypothesized to have increased cell size and life spans, which in turn leads to abnormally large body growth. One study found that genes related to “growth regulation” were overrepresented in ''A. 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. Another potential reason for the large size of ''A. gigantea'' is that it could have undergone a whole genome duplication, which could potentially increase the size of the species (and explain their large genome size). One study reported that ''A. gigantea'' had a high level of selenium in its leg muscles. This trace element is linked with growth and metabolic activity so might partly explain why this species grows so large.

Genetics

There have been few studies into the genetics of ''Alicella gigantea''. In 2020, a study examined the relationships of deep sea amphipod species;

sequences In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is call ...

of 16S, COI,

Histone 3 Histone H3 is one of the five main histones involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal end, N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'b ...

, and 28S found that ''A. gigantea'' formed a clade with '' Tectovalopsis'' and '' Diatectonia''. However, in contrast to this, a 2015 study found that ''Alicella'' formed a clade with just '' Cyclocaris'' and ''Tectovalopsis'' (but ''Diatectonia'' sequences were not used in this study), which conflicts with the 2020 study. Most recently, a 2025 study examined the population genetics of ''A. gigantea'' over its entire known range. One of the key genetics findings of this study was that most populations shared a single haplotype for each gene, which indicates that this species distribution is a widespread and connected one.

Genome

In 2013, the size of the whole genome has been estimated to be about 34.79

gigabase A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...

pairs in length, which is considerably larger than genome size estimates for other species of deep amphipods in the same study. Because of the large size of the genome when compared to other deep sea amphipods, it has been shown that ''A. gigantea'' has a notably faster genome size diversification rate. It was hypothesized that this could be due to a whole genome duplication, but the authors cautioned this would require further study to prove. In 2019, the complete

mitochondrial genome Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the DNA contained in ...

of ''A. gigantea'' was sequenced with a total length of 16,851 base pairs. The study found that the genome had 13 protein-coding genes, 2 ribosomal genes, 22

transfer RNA Transfer ribonucleic acid (tRNA), formerly referred to as soluble ribonucleic acid (sRNA), is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes). In a cell, it provides the physical link between the gene ...

genes and 2 noncoding gene regions.

Distribution and habitat

''Alicella gigantea'' is a marine species that has primarily been recorded in the lower abyssal and hadal depths between in depth, which would restrict them to

oceanic trench Oceanic trenches are prominent, long, narrow topography, topographic depression (geology), depressions of the seabed, ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers ...

es and

s, such as the

Kermadec Trench The Kermadec Trench is a linear ocean trench in the south Pacific Ocean. It stretches about from the Louisville Seamount Chain in the north (26°S) to the Hikurangi Plateau in the south (37°S), north-east of New Zealand's North Island. Togethe ...

in the southwest Pacific. However, there is a single record of a juvenile specimen at in depth that was collected with a fish trap. Similar to this, there is a record of a specimen being recovered from the stomach of a

black-footed albatross The black-footed albatross (''Phoebastria nigripes'') is a large seabird of the albatross family Diomedeidae from the North Pacific. All but 2.5% of the population is found among the Northwestern Hawaiian Islands. It is one of three species of al ...

in the

Hawaiian Islands The Hawaiian Islands () are an archipelago of eight major volcanic islands, several atolls, and numerous smaller islets in the Pacific Ocean, North Pacific Ocean, extending some from the Hawaii (island), island of Hawaii in the south to nort ...

. It has been suggested that this record may be due to the high

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

content of this species causing it to float upwards in the water column. A 2025 study found ''A. giganteas depth range to be , which suggests that 59% of the world's oceans (and all six major ocean bodies) is suitable habitat for this amphipod. It is a

cosmopolitan Cosmopolitan may refer to: Internationalism * World citizen, one who eschews traditional geopolitical divisions derived from national citizenship * Cosmopolitanism, the idea that all of humanity belongs to a single moral community * Cosmopolitan ...

species and has been recorded in North

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

, North

, and the South West Pacific Ocean, though there are numerous gaps in their distribution that are likely due to the scarcity of this species. A 2025 study sampling ''Alicella'' specimens worldwide confirmed low

genetic divergence Genetic divergence is the process in which two or more populations of an ancestral species accumulate independent genetic changes ( mutations) through time, often leading to reproductive isolation and continued mutation even after the populations h ...

; all specimens sampled are to be considered the same species, though most collection sites for this study were in Pacific ocean trenches, which may skew the results.

Biology

Like many other amphipods, adult ''Alicella gigantea'' are primarily

s and feed on

carrion Carrion (), also known as a carcass, is the decaying flesh of dead animals. Overview Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters (or scavengers) include crows, vultures ...

. Because of this behaviour, they are most frequently caught using baited traps. Due to their dependence on carrion as a food source, the species may be susceptible to changes occurring at the ocean's surface. The

gut microbiome Gut microbiota, gut microbiome, or gut flora are the microorganisms, including bacteria, archaea, fungi, and viruses, that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the g ...

of ''A. gigantea'' is dominated by ''

Candidatus In prokaryote nomenclature, ''Candidatus'' (abbreviated ''Ca.''; Latin for "candidate of Roman office") is used to name prokaryotic taxa that are well characterized but yet- uncultured. Contemporary sequencing approaches, such as 16S ribosomal R ...

Hepatoplasma''. One study compared the gut microbiome of ''A. gigantea'' with that of two other hadal amphipods and found that the particular gut assemblage was unique to each species. It has also been discovered that hadal amphipods such as ''A. gigantea'' have large amounts of probiotic gut microbiota. It was suggested that this could support this species ability to survive in the deep sea. Presently, it is unknown if these amphipods gut microbes are inherited from their parents or picked up from the surrounding environment. Although its size is thought to give it protection from most deep sea predators, ''A. gigantea'' are known to be preyed upon by the rough abyssal grenadier ('' Coryphaenoides yaquinae'').

Life cycle

Like all amphipods, female ''A. gigantea'' brood their eggs in a pouch. It has been suggested that females probably have several broods over their lifetimes. The eggs are oval in shape and are in length. The shell of the eggs is composed of two

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It is also present around the embryo of other animals, like insects and molluscs. Structure In humans and other therian mammals, the cho ...

layers: the exochorion which is fibrillar in structure, and the endochorion layer which has a porous structure where the pores are on average less than 10

μm The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System ...

. As juveniles, their diet has been inferred to be mostly

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and zooplankton debris and then transitions into carrion and algae as they mature. ''Alicella gigantea'' grows at a much faster rate compared to other amphipods. The size of ''A. gigantea'' also allows them to avoid being preyed on by predators such as ''

Notoliparis kermadecensis ''Notoliparis kermadecensis'' (from Greek: ''noton'', back, and ''liparos'', fat) is a species of snailfish (Liparidae) that lives in the deep sea. Endemic to the Kermadec Trench in the Southwest Pacific, it is hadobenthic with a depth range be ...

'', a liparid snailfish that preys on smaller amphipods. Analysis of ¹⁴C signatures indicates that hadal amphipods such as ''Alicella gigantea'' have an unusually long life span of over 10 years.

Human impact

Despite their apparently isolation from the surface, chemical contamination caused by humans has been detected in ''Alicella gigantea''. In one study, pesticides such as

DDT Dichlorodiphenyltrichloroethane, commonly known as DDT, is a colorless, tasteless, and almost odorless crystalline chemical compound, an organochloride. Originally developed as an insecticide, it became infamous for its environmental impacts. ...

and

chlordane Chlordane, or chlordan, is an organochlorine compound that was used as a pesticide. It is a white solid. In the United States, chlordane was used for termite-treatment of approximately 30 million homes until it was banned in 1988. Chlordane was b ...

were detected in ''A. gigantea'' specimens, whilst in another study trace elements such as cadmium and chromium were detected in high concentrations which were suggested to be linked to human activity.

Gallery

File:7000m Super-G in hand.jpg, Specimen File:Alicella gigantea specimen.png, '' ditto'' File:Alicella gigantea from 6200m.jpg, ''ditto'', from depth File:Alicella gigantea feeding.jpg, Feeding at a bait station File:Live Alicella gigantea feeding.jpg, ''ditto''

References

{{Taxonbar, from=Q18091951, from2=Q2836751 Amphilochidea Monotypic amphipod genera Amphipods of the Pacific Ocean Amphipods of the Atlantic Ocean Crustaceans described in 1899