Giant isopod
Encyclopedia
A giant isopod is any of approximately nine species
of large isopods (crustacean
s related to the shrimp and crabs) in the genus
Bathynomus. They are thought to be abundant in cold, deep waters of the Atlantic
. Bathynomus giganteus, the species upon which the generitype
is based, is the largest known isopod and is the one most often referred to by the common name
"giant isopod".
French
zoologist
Alphonse Milne-Edwards
was the first to describe the genus in 1879 after fishing 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. Females were not recovered until 1891.
Giant isopods are of little interest to most commercial fisheries
owing to the typical scarcity of catches and because ensnared isopods are usually scavenged beyond marketability before they are recovered. The species are noted for resemblance to the common woodlouse
or pill bug
, to which they are related. The few specimens caught in the Americas
with baited traps are sometimes seen in public aquaria
.
(cf. giant squid
); most other isopods range in size from 1 to 5 cm (0.393700787401575 to 2 ). Their morphology resembles that of their terrestrial cousin, the woodlouse
: their bodies are dorso-ventrally compressed, protected by a rigid, calcareous exoskeleton
composed of overlapping segments. Like the woodlouse
, they also possess the ability to curl up into a "ball", where only the tough shell is exposed. This provides protection. 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). The large eyes are compound with nearly 4,000 facets
, sessile and spaced far apart on the head. There are two pairs of antennae
.
The uniramous thoracic legs
or pereiopods are arranged in seven pairs, the first of which are 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 natatory legs and rami, flat respiratory structures acting as gill
s. The isopods are a pale lilac in colour.
Giant isopods are important scavenger
s in the deep-sea benthic
environment; they are found from the gloomy sublittoral zone at a depth of 170 metres (557.7 ft) to the pitch darkness of the bathypelagic
zone at 2140 metres (7,021 ft), where pressures are high and temperatures are very low – down to about 4 °C (39.2 °F). Over 80 percent are found at a depth between 365 and 730 m (1,197.5 and 2,395 ). 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 be active predators of slow-moving prey such as sea cucumbers, sponges, radiolaria
ns, nematode
s, and other zoobenthos, and perhaps even live fish. They are known to attack trawl catches. As food is scarce in the deep ocean biome
, giant isopods must make do with what fortune brings; they are adapted to long periods of famine and have been known to survive over eight weeks without food in captivity. When a significant source of food is encountered, giant isopods gorge themselves to the point of compromising their locomotive ability. One study examining the contents of 1651 giganteus intestines found that fish were most common there, followed by cephalopod
s and decapods
, particularly carideans and galatheids
.
In 1990, the Scavengers of East Australian Seas expedition (SEAS) started to document the scavenging crustaceans along the east coast of Australia by setting traps. The deeper the water, the fewer number of species they 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 it is thought that Bathynomus existed more than 160 million years ago, before the break-up of the supercontinent Pangaea
, so it did not evolve independently in all three locations, but since then it would be expected that Bathynomus would show divergent evolution in the various locations. However, the SEAS study found that the giant isopods in all three locations were almost identical. Andrew Parker in his book In the Blink of an Eye (from where this description of the SEAS expedition is taken) links this lack of evolution to the extremely low light levels of their habitat.
when sexually active, the pouch being formed by overlapping oostegites or brood plates grown from the medial border of the pereopods. The fertilized eggs — thought to be the largest of all marine invertebrate
s — are retained safely within the marsupium for an unknown period. A brooding female is at risk of losing its eggs if it overindulges in food to the point of bloating.
The young isopods emerge from the marsupium as miniatures of the adults, known as mancae
. This is not a larva
l stage: the mancae are fully developed, lacking only the last pair of pereopods.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of large isopods (crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s related to the shrimp and crabs) in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Bathynomus. They are thought to be abundant in cold, deep waters of the Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
. Bathynomus giganteus, the species upon which the generitype
Biological type
In biology, a type is one particular specimen of an organism to which the scientific name of that organism is formally attached...
is based, is the largest known isopod and is the one most often referred to by the common name
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...
"giant isopod".
French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
zoologist
Zoology
Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...
Alphonse Milne-Edwards
Alphonse Milne-Edwards
Alphonse Milne-Edwards was a French mammalologist, 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 .Milne-Edwards obtained a medical degree in 1859 and became assistant to his father...
was the first to describe the genus in 1879 after fishing a juvenile male B. giganteus from the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
; 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
Charles Wyville Thomson
Sir Charles Wyville Thomson was a Scottish zoologist and chief scientist on the Challenger expedition.-Career:...
and others. Females were not recovered until 1891.
Giant isopods are of little interest to most commercial fisheries
Fishery
Generally, a fishery is an entity engaged in raising or harvesting fish which is determined by some authority to be a fishery. According to the FAO, a fishery is typically defined in terms of the "people involved, species or type of fish, area of water or seabed, method of fishing, class of boats,...
owing to the typical scarcity of catches and because ensnared isopods are usually scavenged beyond marketability before they are recovered. The species are noted for resemblance to the common woodlouse
Woodlouse
A woodlouse is a crustacean with a rigid, segmented, long exoskeleton and fourteen jointed limbs...
or pill bug
Armadillidium
Armadillidium is a genus of the small terrestrial crustacean known as the woodlouse. Armadillidium are also commonly known as pill woodlice, pill bugs or roly-poly, and are often confused with pill millipedes such as Glomeris marginata. They are characterised by their ability to roll into a ball...
, to which they are related. The few specimens caught in the Americas
Americas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
with baited traps are sometimes seen in public aquaria
Aquarium
An aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants...
.
Physical description
Maturing to an average length between 19 and 36 cm (7.5 and 14.2 ), with a maximum weight and height of approximately 1.7 kilograms (3.7 lb) and 76 centimetres (29.9 in) respectively in B. giganteus, giant isopods are a good example of deep-sea gigantismDeep-sea gigantism
In zoology, deep-sea gigantism, also known as abyssal gigantism, is the tendency for species of crustaceans, invertebrates and other deep-sea-dwelling animals to display a larger size than their shallow-water counterparts...
(cf. giant squid
Giant squid
The giant squid is a deep-ocean dwelling squid in the family Architeuthidae, represented by as many as eight species...
); most other isopods range in size from 1 to 5 cm (0.393700787401575 to 2 ). Their morphology resembles that of their terrestrial cousin, the woodlouse
Woodlouse
A woodlouse is a crustacean with a rigid, segmented, long exoskeleton and fourteen jointed limbs...
: their bodies are dorso-ventrally compressed, protected by a rigid, calcareous exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
composed of overlapping segments. Like the woodlouse
Woodlouse
A woodlouse is a crustacean with a rigid, segmented, long exoskeleton and fourteen jointed limbs...
, they also possess the ability to curl up into a "ball", where only the tough shell is exposed. This provides protection. 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). The large eyes are compound with nearly 4,000 facets
Ommatidium
The compound eyes of insects, mantis shrimp and millipedes are composed of units called ommatidia . An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlaid with a transparent cornea...
, sessile and spaced far apart on the head. There are two pairs of antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
.
The uniramous thoracic legs
Arthropod leg
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...
or pereiopods are arranged in seven pairs, the first of which are 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 natatory legs and rami, flat respiratory structures acting as gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s. The isopods are a pale lilac in colour.
Ecology
Scavenger
Scavenging is both a carnivorous and herbivorous feeding behavior in which individual scavengers search out dead animal and dead plant biomass on which to feed. The eating of carrion from the same species is referred to as cannibalism. Scavengers play an important role in the ecosystem by...
s in the deep-sea benthic
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
environment; they are found from the gloomy sublittoral zone at a depth of 170 metres (557.7 ft) to the pitch darkness of the bathypelagic
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...
zone at 2140 metres (7,021 ft), where pressures are high and temperatures are very low – down to about 4 °C (39.2 °F). Over 80 percent are found at a depth between 365 and 730 m (1,197.5 and 2,395 ). They are thought to prefer a muddy or clay substrate and lead solitary lives.
Although generalist scavengers, these isopods are mostly carnivorous
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
and feed on dead whales, fish, and squid; they may also be active predators of slow-moving prey such as sea cucumbers, sponges, radiolaria
Radiolarian
Radiolarians are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as zooplankton throughout the ocean, and their skeletal remains cover large portions of the...
ns, nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s, and other zoobenthos, and perhaps even live fish. They are known to attack trawl catches. As food is scarce in the deep ocean biome
Biome
Biomes are climatically and geographically defined as similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, and are often referred to as ecosystems. Some parts of the earth have more or less the same kind of abiotic and biotic factors spread over a...
, giant isopods must make do with what fortune brings; they are adapted to long periods of famine and have been known to survive over eight weeks without food in captivity. When a significant source of food is encountered, giant isopods gorge themselves to the point of compromising their locomotive ability. One study examining the contents of 1651 giganteus intestines found that fish were most common there, followed by cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s and decapods
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...
, particularly carideans and galatheids
Squat lobster
Squat lobsters are decapod crustaceans of the families Galatheidae, Chirostylidae and Kiwaidae, including the common genera Galathea and Munida. They are not lobsters at all, but are more closely related to porcelain crabs, hermit crabs and then, more distantly, true crabs...
.
In 1990, the Scavengers of East Australian Seas expedition (SEAS) started to document the scavenging crustaceans along the east coast of Australia by setting traps. The deeper the water, the fewer number of species they 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 it is thought that Bathynomus existed more than 160 million years ago, before the break-up of the supercontinent Pangaea
Pangaea
Pangaea, Pangæa, or Pangea is hypothesized as a supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration....
, so it did not evolve independently in all three locations, but since then it would be expected that Bathynomus would show divergent evolution in the various locations. However, the SEAS study found that the giant isopods in all three locations were almost identical. Andrew Parker in his book In the Blink of an Eye (from where this description of the SEAS expedition is taken) links this lack of evolution to the extremely low light levels of their habitat.
Reproduction
Study of the seasonal abundance of B. giganteus juveniles and adults suggests a peak in reproductive capacity in the spring and winter months. This is apparently due to a shortage of food during the summer. Mature females develop a brood pouch or marsupiumBrood pouch (Peracarida)
The marsupium or brood pouch, is a characteristic feature of Peracarida, including the orders Amphipoda, Isopoda and Cumacea. It is an egg chamber formed by oostegites, which are appendices which are attached to the coxae of the first pereiopods...
when sexually active, the pouch being formed by overlapping oostegites or brood plates grown from the medial border of the pereopods. The fertilized eggs — thought to be the largest of all marine invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s — are retained safely within the marsupium for an unknown period. A brooding female is at risk of losing its eggs if it overindulges in food to the point of bloating.
The young isopods emerge from the marsupium as miniatures of the adults, known as mancae
Mancae
The manca are post-larval juveniles in some crustaceans. The manca stage is the defining characteristic of a clade called Mancoida which comprises all the member of the Peracarida except the Amphipoda. Mancae closely resemble the adult form, but for the absence of the last pair of pereiopods...
. This is not a larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l stage: the mancae are fully developed, lacking only the last pair of pereopods.