''Polybius henslowii'' is a species of

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

, the only species in the genus ''Polybius''. It is a capable swimmer and feeds in open water in the north-east

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

and western

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...

Description

The

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

of ''P. henslowii'' is almost circular, wide and long. The first pair of

pereiopod The decapod (crustaceans such as a crab, lobster, shrimp or prawn) is made up of 20 body segments grouped into two main body parts: the cephalothorax and the pleon (abdomen). Each segment may possess one pair of appendages, although in various g ...

s (walking legs) carry

claws A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or tarsus ...

, and the remaining four pairs are flattened with fringed edges. This contrasts with other members of the family

Portunidae Portunidae is a family of crabs which contains the swimming crabs. Description Portunid crabs are characterised by the flattening of the fifth pair of legs into broad paddles, which are used for swimming. This ability, together with their strong ...

, which have only the last pair of legs adapted for swimming. ''P. henslowii'' is red-brown on the upper surface, and paler beneath.

Distribution

''Polybius'' is found in the north-east

from the

British Isles The British Isles are a group of islands in the North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner and Outer Hebrides, the Northern Isles, ...

Morocco Morocco (),, ) officially the Kingdom of Morocco, is the westernmost country in the Maghreb region of North Africa. It overlooks the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to ...

, and in the western

. Very abundant swarms have been found along the coasts off

Galicia (Spain) Galicia (; gl, Galicia or ; es, Galicia}; pt, Galiza) is an autonomous community of Spain and historic nationality under Spanish law. Located in the northwest Iberian Peninsula, it includes the provinces of A Coruña, Lugo, Ourense, and ...

and off

Portugal Portugal, officially the Portuguese Republic ( pt, República Portuguesa, links=yes ), is a country whose mainland is located on the Iberian Peninsula of Southwestern Europe, and whose territory also includes the Atlantic archipelagos of ...

. Rare specimens have been caught in the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...

east of

Shetland Shetland, also called the Shetland Islands and formerly Zetland, is a subarctic archipelago in Scotland lying between Orkney, the Faroe Islands and Norway. It is the northernmost region of the United Kingdom. The islands lie about to the no ...

, and in the

Skagerrak The Skagerrak (, , ) is a strait running between the Jutland peninsula of Denmark, the southeast coast of Norway and the west coast of Sweden, connecting the North Sea and the Kattegat sea area through the Danish Straits to the Baltic Sea. The ...

. Its range has expanded into the southern North Sea, possibly as a result of

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

. It can be found on

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...

y or

gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...

ly sediments at depths of up to .

Ecology and behaviour

''Polybius henslowii'' is a capable swimmer, and can be found swimming near the ocean surface, where it feeds on a mixture of

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...

, fish, and other crustaceans, including specimens of the same species. Indeed, ''P. henslowii'' has been observed to form

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...

swarms. These swarms are mostly composed of females, excluding that aggregations are for mating, while they might be related to feeding. When they swim inshore, along the coastal upwelling areas of Galicia (NW Spain), they rely more on benthic prey, such as

polychaetes Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are mad ...

and detritus and are influenced by terrestrial organic matter coming from the rivers. In Galicia (NW Spain), ''P. henslowii'' is an important food source for the yellow-legged gull, ''

Larus michahellis The yellow-legged gull (''Larus michahellis'') is a large gull found in Europe, the Middle East and North Africa, which has only recently achieved wide recognition as a distinct species. It was formerly treated as a subspecies of either the Casp ...

''. In summer,

loggerhead turtle The loggerhead sea turtle (''Caretta caretta'') is a species of oceanic turtle distributed throughout the world. It is a marine reptile, belonging to the family Cheloniidae. The average loggerhead measures around in carapace length when full ...

s (''Caretta caretta'') off

North Africa North Africa, or Northern Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, and it is sometimes defined as stretching from the Atlantic shores of Mauritania in ...

feed almost exclusively on ''P. henslowii''.

Taxonomy

''Polybius henslowii'' was first described by

William Elford Leach William Elford Leach Royal Society, FRS (2 February 1791 – 25 August 1836) was an English zoologist and marine biologist. Life and work Elford Leach was born at Hoe Gate, Plymouth, the son of an attorney. At the age of twelve he began a me ...

in 1820, as the only species in his new genus ''Polybius''. Although a number of other species were added to the genus over time, they have all since been split off into new genera such as '' Liocarcinus'', ''

Necora The velvet crab, or alternately ''velvet swimming crab'', ''devil crab'', “fighter crab”, or ''lady crab,'' ''Necora puber'', is a species of crab from the North-East Atlantic and the Mediterranean. It is the largest of the swimming crab fam ...

'' and '' Macropipus''. The genus name "''Polybius''" may be in reference to the Greek historian

Polybius Polybius (; grc-gre, Πολύβιος, ; ) was a Greek historian of the Hellenistic period. He is noted for his work , which covered the period of 264–146 BC and the Punic Wars in detail. Polybius is important for his analysis of the mixed ...

; the

specific epithet In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

' honours

John Stevens Henslow John Stevens Henslow (6 February 1796 – 16 May 1861) was a British priest, botanist and geologist. He is best remembered as friend and mentor to his pupil Charles Darwin. Early life Henslow was born at Rochester, Kent, the son of a solici ...

, Professor of Botany at Cambridge University, who had collected the specimens used by Leach from a herring fisherman in

North Devon North Devon is a local government district in Devon, England. North Devon Council is based in Barnstaple. Other towns and villages in the North Devon District include Braunton, Fremington, Ilfracombe, Instow, South Molton, Lynton and Lynmouth ...

in 1817. This was the first of several species that were named in Henslow's honour.

Common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contrast ...

s for the species include "sardine swimming crab" and "Henslow's swimming crab".

References

{{Taxonbar, from=Q3934386 Portunoidea Crustaceans of the Atlantic Ocean Crustaceans described in 1820