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(Montagu, 1808)

Coxal plate 2 short and rounded. Epimeral plate 3 posterodistal angle with minute tooth.
Head, lateral lobes small, prominent, truncated; eyes small and rounded. Antenna 1 peduncle article 3 almost as long as article 2, peduncle articles with numerous long setae on posterior margin, flagellum also setose, flagellar articles distinct; accessory flagellum small. Antenna 2 longer and more robust than 1, especially in male, peduncle article 4 little shorter than 5, in male peduncle articles 3-4 sparsely setose, in female these articles with many long setae, flagellar articles distinct, peduncle article 5 and flagellum often with dense tufts of short setae. Gnathopod I propodus oval, narrowing distally, palm strongly oblique, straight or weakly concave delimited by 2-3 small spines, dactylus with inner margin serrate. Female gnathopod 2 much larger than 1, basis robust, anterior margin devoid of setae, propodus large with anterior margin strongly convex, palm delimited by 2-3 small spines, palmar margin concave medially with rounded proximal lobe and angular distal process. Male gnathopod 2 extremely large and robust, development variable, basis elongate with anterior margin devoid of setae, propodus rather oval in small specimens with palm delimited by short process, in large males propodus very long and slender with enormous proximal process. Pereopods 3-4 only moderately setose, basis with only few small setules on anterior margin, merus broad with anterodistal angle overlapping small carpus. Pereopod 5 basis expanded, merus broad; pereopod 6-7 robust basis moderately expanded, posterodistal angle produced and narrowly rounded. Uropods l-2 outer ramus slightly shorter than inner; uropod 3 inner ramus straight, outer ramus with hooked apical spine and 2-3 reverted denticles.

Up to 12 mm.

Greyish with distinct reddish brown markings.

Depth range from to metres. Reported from most coastal regions of the North East Atlantic, and often locally abundant. This species is an important fouling organism; the animals construct tubes amongst algae or hydroid growths, or on solid surfaces in areas of strong water currents such as pilings, buoys, rafts or the hulls of ships. The colonies or 'nests' may become very large and can cause congestion of water pipes and ducts which run into the sea from Power Stations or similar installations.

North Sea. Cosmopolitan in temperate and warm-temperate waters; widespread and frequently recorded in Atlantic, Pacific and Indian Oceans, both north and south of the equator.

Jassa falcata