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(Milne-Edwards, 1837)

Description
— Generic features. Statocyst usually present. Carpo-propodus divided into 6-8 subsegments. Pleopods of male rudimentary; fourth with a very long exopod armed with "pincer" setae. Apex of antennal scale rounded. No sternal processes. Antennal scale with rounded apex. Endopod (and exopod) of uropod without spines. Outer margin of antennal scale with setae; terminal tooth absent. Telson linguiform; lateral margins of telson and apex armed with a graduated series of spines and a pair of median setae.
— Species. General appearance is long and slender with the abdomen nearly straight.
Antennule with very long peduncle, second segment small, distal segment swollen and almost equal in length to the first.
Antennal scale extending only slightly beyond the antennular peduncle, narrowly lanceolate with rounded apex, about eight times as long as broad, with an oblique suture cutting off the terminal portion.
Eyes large and pyriform, somewhat widely separated and projecting considerably beyond the lateral margins of the carapace, eyestalk stout and hispid.
Thoracic limbs with propodite usually three-segmented, the proximal one the longest, merus subequal in length to the carpus, nail distinct, long and very slender. Pleopods similar to those in the genus Neomysis. Fourth pleopod of the male reaching to the middle of the last abdominal somite, exopod two-segmented, with the first segment about seven times as long as the second, which terminates in two long, unequal, spinous setae of which the longer is twice as long as the segment
Telson elongate, entire, widely dilated at the base and then suddenly constricted with the distal portion linguiform, apex bluntly rounded, proximal margin armed in the region of the constriction with two spines on each side, posterior two-thirds of margin and apex densely armed with numerous, unequal spines arranged in series, with larger spines separated by series of three, four or five smaller ones gradually increasing in size distally. Apex with two small central spines clanked by two larger ones. These spines arming the telson are somewhat peculiar. The external sheath of the spine is suddenly thickened towards the distal end, reducing the lumen in such a way as to present the appearance of an axial line. This has led some observers to think that the tip of the spine was trigonal.
Uropods with endopod nearly equal in length to the telson, swollen at the base and narrowing sharply distally, inner margin armed in the region of the statocyst with a row of very small spines, of which the distal two or three are considerably larger than the others; exopod long and narrow, one and a half times as long as the endopod. Pigment blackish brown, sparse.

Colour
Blackish brown, sparse.

Size
Adult up to 9 mm long.

Depth range
Hyperbenthic; from 8 to 150 m.

Acanthomysis longicornis is a filter-feeder, living on very minute organisms and on detritus.

Distribution in the North Sea
Southwestern North Sea.

World distribution
Atlantic < 26-55°N, Mediterranean, Indo-W Pacific; coastal to shelf. Widely distributed in the waters of the Mediterranean and off the shores of west Europe as far as north as the British Isles.

[After Tattersall and Tattersall, 1951]

Acanthomysis longicornis