Body smooth. Length of largest male 14 mm, largest female 13 mm, smallest ovigerous female 3.7 mm.
Antenna 1 variable in length, from one-half to the length of the body. Antenna 2 approximately equal in length to the peduncle of antenna 1, flagellum of 3-5 articles.
Mandibular palp three-segmented, setal formula for terminal article varies from 1-1-1 to 1-6-1. Left mandible with 5-toothed incisor, 5-toothed lacinia mobilis, 2 additional plates, setal row of 7 or 8 short blunt setae and 3 longer apically serrate setae. Right mandible with 5-toothed incisor, lacinia mobilis smooth, 2 additional plates, setal row of 6 or 7 short setae and 2 longer apically serrate setae. Outer lobe of labium wing like, both lobes quite setose. Palp of maxilla 1 with 3 or 4 apical spines of which 2 sometimes serrate and with 3 or 4 proximal setae, outer lobe with 6 apical spines which become bifid or serrate in larger individuals. Outer lobe of maxilliped with 1 apical seta, 1 short apical spine, and 2-5 proximal setae along medial margin; inner lobe as long as outer lobe and nearly fused along medial margins, mediodistal corner with 1 very large serrate spine, 1 small spine, and 1 slightly lateral marginally serrate spine, lateral margin of lobe sometimes toothed; article 2 of palp robust.
Propodus of gnathopod 1 triangular with 4 or 5 grasping spines, grasping margin without serrations but with numerous short spines; dactylus not serrate. Propodus of gnathopod 2 with 1 large and 1 small proximal grasping spine, palm with numerous short spines and few setae. Carpus of gnathopod 2 shorter than the merus, propodus with the greatest width proximal and with proximal grasping spines.
Gills elliptical, gill on pereionite 2 the smallest.
Pereiopods 3 and 4, six-segmented, palm of propodus with 3 short spines. Pereiopod 5, five-segmented, palm of propodus without spines, dactylus approximately one-half of the propodus length, basis approximately twice the propodus length. Pereiopods 6 and 7, six-segmented with 2 proximal grasping spines, 1 medial spine, and 1 distal spine.
Abdomen of male with 2 pairs of biarticulate appendages and a pair of pyriform appendages, each pyriform appendage terminating in a seta; female with 2 pairs of biarticulate appendages, a pair of lobes and an anterior raised projection.
P. marina is widely distributed in the warmer parts of the Atlantic. It has been collected in plankton tows at the surface but also from 660 metres. The habitat does not seem specific because it has been reported on green and brown algae, sea grass, sponges, hydroids and bryozoans. P. marina is predatory on Caprella acanthifera, grasping the prey with gnathopod 2 and the prey seems almost incapable of the violent movement. It seems, therefore, that the poison tooth may introduce some venom.
Northern Norway to the Mediterranean Sea; British Islands; Azores; Canary Islands; Mediterranean and Black Seas; tropical West Africa; South Africa; Brazil.