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G.O. Sars, 1870

Description
Generic features. Statocyst usually present. Pleopods of female reduced, rudimentary; of male variable. Endopod (and exopod) of uropod without spines. Outer margin of antennal scale without setae; ending in terminal tooth. Eyeplate quadrangular with median cleft and with margins serrated in all species of the North Sea. Eyes rudimentary in the form of flat plates without definite stalks. Eyes fused to form a median plate. Telson rounded. Lateral margins of telson and apex armed with a graduated series of spines and a pair of median setae. Marsupium consists, as in the Genus Amblyops, of two well-developed incubatory brood lamellae.
— Species. Body compact, sublinear in shape. Carapace only slightly wider than the abdomen, anterior margin evenly rounded; posterior margin emarginate leaving the last part of the seventh thoracic somite exposed in dorsal view.
Eyes consisting of two subrectangular fused plates, barely extending to the distal end of the first segment of the antennular peduncle. Eyeplate with serrations through the whole antero-lateral and lateral margins.
Antennal scale with outer margin entire, terminating in a strong spine; apex obliquely truncate, extending considerably beyond the terminal spine of the outer margin to a length varying from one-fourth to one-third of the total length of scale; strong spine on the outer distal corner of the sympod.
Antennular peduncle with the first and third segments subequal in length in the female, second segment very short; in the male third segment longer than first.
Antennal peduncle about equal in length to the antennular peduncle; third segment the longest.
Uropod with endopod shorter and more slender than the exopod; no spines on the ventral inner surface of endopod.
Telson very slightly shorter than the sixth abdominal somite, apex entire, broadly rounded, armed with a pair of median plumose setae and with 4, occasionally 5 or very rarely 6, pairs of spines of which the median is the longest and measures about one-fourth of the length of the telson; second pair of spines shorter than the median and the third pair slightly shorter still. Lateral margin on distal half armed with 3-7 spines which are usually not symmetrically arranged, and increase in length towards the apex.

Colour
Eyes with pigment diffused as in Pseudomma roseum without definite branching chromatophores (visual elements). Body and appendages are reddish pink, shading into purple. The antennal scale as well as the uropods, are edged with quite a broad band of purple and the hind margin of the first, third and fourth abdominal somites respectively is marked by a band of intense rose colour. The posterior margins of the other abdominal somites are not so strongly marked. On the carapace there is a pair of twisted, irregular, ribbon-like markings of purple whilst the lateral posterior region is of a deep rose pink.

Size
Length of adult females up to 12 mm, males smaller at about 10 mm.

Depth range
Hyperbenthic; 200-1400 m.

Remarks
Pseudomma affine can be distinguished from P. roseum by the following:

- eyeplates: in P. affine the denticulations cover the whole lateral margins.
- apex of antennular scale is never greater than 1/3 of the total length of the scale in P. affine.
- in P. affine there are at least 4 pairs of apical spines on the telson. Apex of the telson is more broadly rounded and the lateral margins are somewhat concave.
- in European waters the body length of P. affine is half as long as P. roseum.

Pseudomma affine is a bottom-living species and is more abundant at greater depths.

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

World distribution
North Atlantic 40? - 69°N; ?Mediterranean; shelf, slope. Form extreme north of Norway (W. Finnmark) to the Bay of Biscay, Iceland and Greenland and the East coast of North America.

[After Tattersall and Tattersall, 1951]

Pseudomma affine