''Polyphemus'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

water flea The Diplostraca or Cladocera, commonly known as water fleas, are a superorder of small crustaceans that feed on microscopic chunks of organic matter (excluding some predatory forms). Over 1000 species have been recognised so far, with many more ...

s, and the only genus in the family Polyphemidae. There are two species, ''P. exiguus'' and ''P. pediculus'', although

allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...

has resulted in a number of

cryptic species In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...

of ''P. pediculus''. ''Polyphemus exiguus'' inhabits open zones in the

Caspian Sea The Caspian Sea is the world's largest inland body of water, often described as the List of lakes by area, world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad s ...

, while ''Polyphemus pediculus'' exists throughout the

Holarctic The Holarctic realm is a biogeographic realm that comprises the majority of habitats found throughout the continents in the Northern Hemisphere. It corresponds to the floristic Boreal Kingdom. It includes both the Nearctic zoogeographical regi ...

. It lives in diverse conditions, from small ponds to lakes and

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environme ...

such as the

Gulf of Saint Lawrence , image = Baie de la Tour.jpg , alt = , caption = Gulf of St. Lawrence from Anticosti National Park, Quebec , image_bathymetry = Golfe Saint-Laurent Depths fr.svg , alt_bathymetry = Bathymetry ...

and the

Gulf of Finland The Gulf of Finland ( fi, Suomenlahti; et, Soome laht; rus, Фи́нский зали́в, r=Finskiy zaliv, p=ˈfʲinskʲɪj zɐˈlʲif; sv, Finska viken) is the easternmost arm of the Baltic Sea. It extends between Finland to the north and E ...

. It can be found quite far offshore. ''Polyphemus'' are a predatory genus of water flea. The two species have four pairs of legs with exopodites, or outer branches. The legs are adapted for catching mobile prey, generally smaller species of water flea such as young ''

Daphnia ''Daphnia'' is a genus of small planktonic crustaceans, in length. ''Daphnia'' are members of the order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory swimming style resemb ...

'' and ''

Bosmina ''Bosmina'' is a genus in the order Cladocera, the water fleas. Its members can be distinguished from those of '' Bosminopsis'' (the only other genus in the family Bosminidae) by the separation of the antennae; in ''Bosminopsis'', the antennae a ...

''. ''Polyphemus'' has two compound eyes that are fused to form a single unit, with a zoned set of receptors. This zoned structure is paired with an eye-control system that allows the ''Polyphemus'' to visually distinguish target size to avoid predators and track prey. ''P. pediculus'' is approximately in length. ''P. exiguus'', while similar in morphology, is smaller.

References

{{Taxonbar, from1=Q7226744, from2=Q11846999 Cladocera Branchiopoda genera