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(G.O. Sars, 1964)

Description
Generic features. General form somewhat robust with the cephalothorax broadly oblong in dorsal view, breadth more than half the length and considerably wider than the abdomen. The abdomen is flexed sharply upward at its junction with the cephalothorax. This is particularly marked in ovigerous females giving the animals an obese appearance in lateral view which, no doubt, suggested the specific name. Abdomen cylindrical and not tapering posteriorly; last somite about one and a half times as long as the fifth. Antennular peduncle, first segment short with distal outer margin straight and not drawn out into a projection as in the Genus Erythrops; second segment short with distal margin very oblique so that the outer margin is much shorter than the inner. Antennal scale short with small suture. Eyes nearly globular, not compressed dorso-laterally. First thoracic limbs with very large hatchet-shaped epipod. Uropods with exopod considerably longer than endopod. Inner margin of endopod of uropod with spines. Telson narrowly triangle; lateral margin naked. Apex of telson armed with a pair of plumose setae. Marsupium consisting of three pairs of brood lamaellae.
— Species. Apex of telson armed with a pair of plumose setae. Eyes large, without ocular papilla. Carapace short and broad, lateral margin deeply concave leaving the bases of the limbs completely exposed; anterior margin produced into a rightangled point between the bases of the eyestalks; antero-lateral angles rounded; cervical portion inflated; posterior margin concave leaving the last thoracic somite exposed in dorsal view.
Antennal scale short, extending only slightly beyond the peduncle of the antennule; outer margin naked, terminating in a strong tooth; apex considerably produced as a narrow parabola which is nearly one-third of the length of the whole scale, small distal suture present. Apex and inner margin setose all round. The outer distal angle of the sympod comes to a sharp point but is not produced into a spine.
Eyes large, set widely apart, globular and not flattened dorso-ventrally; stalks small and narrow proximally but widening rapidly cornea large occupying two-thirds of the whole organ; pigment a deep tawny yellow which is insoluble in spirit and remains unchanged for years in preserved specimens.
First thoracic limb robust and well developed with a particularly large hatchet-shaped epipod which is much larger than in any of the species of Erythrops. Second thoracic limb with the merus very slightly arcuate and longer than the carpus. Third to the eighth thoracic limbs of the same form as in Erythrops but somewhat more robust and shorter. Pleopods of the female reduced to simple unjointed plates which become progressively longer on the posterior segments; in the male the first pair is rudimentary as in the female and the remaining pairs are well developed and biramous.
Telson narrowly triangular, longer than the last abdominal somite and considerably longer than its breadth at the base; lateral margins unarmed, straight or very slightly concave; the narrow truncate apex armed with two pairs of slender spines, of which the inner pair is more than twice as long as the outer, and a pair of median, diverging, delicate, plumose setae.
Uropods with exopod linear with broadly rounded apex, nearly half as long again as the tapering endopod; inner margin of endopod armed with a regular row of even spines (there is some discussion about the number of these spines: 16 or more, possibly up to 25). Statocyst large.

Colour
Milky white ornamented with beautiful red pigment particularly on the ventral surface where it occurs in the form of irregular spots and shading. The limbs and the jaws are a bright pink. The eye pigment is yellowish brown and is very stable, remaining unchanged in spirit specimens.

Size
Both sexes up to 12-13 mm long.

Depth range
Hyperbenthic to bathypelagic; 95-1200 m. Bottom-living species often found on ooze and mud.

Remarks
In spite of the well-developed exopods of the thoracic limbs, the movements of the animals are not quick. When they are kept in captivity, they either remain quiescent at the bottom or move slowly along the floor of the vessel with the abdomen held high up, forming a distinct angle with the plane of the cephalothorax, whilst the exopods perform rapid whirling movements.

Distribution in the North Sea
Northern North Sea, off W Norway, Skagerrak.

World distribution
E North Atlantic: 28-79°N, shelf to abyssal. Probably also present in the Mediterranean, Indian Ocean.

[After Tattersall and Tattersall, 1951]

Parerythrops obesa