Body transparent; rostrum sharply pointed in female, rounded in male. Carapace with a prominent spine (the so-called "Schalenstachel", shell spine) on posterior end (carapace spine in parthenogenetic female:P. avirostris parthenog. female ), with row of minute spines on both ventral and postero-dorsal edges, and much finer spinules between these spines. Postabdomen large, ending in a pair of long terminal claws with 2 spines (distal one much longer than proximal) in postero-basal portion. Dorso-posterior corner of carapace depressed, bearing a pair of long setae (so-called "Schwanzborsten", tail setae or caudal setae, see P. avirostris parthenog. female).
Head of female in ventral view: P. avirostris female head.
Antennules short with several distal aesthetascs (small sensory setae covered by delicate cuticle, often projecting from antennule, also known as olfactory hairs) in females (P. avirostris female antennule), but elongated and almost reaching posterior end of carapace in males. Antennae consisting of a large basipod, and 2-segmented exo- and endopods.
Exopods bearing 2 swimming setae on first segment and 6 on the second, endopods with 1 (on first segment) and 4 (on the second) swimming setae (setal formula: 2,6/1,4).
Six pairs of thoracic limbs (thoracic legs, or thoracopods), last one short and reduced; all limbs biramous, endopod indistinctly segmented with many minute plumose setae directing posteriorly, exopod consisting of a flat lamella with several thick plumose setae (P. avirostris thoracic limb). Endopod of male thoracic limb 1 with a sharp recurved claw on distal end (see terminal hook inP. avirostris male).
Differential sexual characters: in males a pair of copulatory appendages (penes) located posterior to thoracic limb 6.
Female: resting eggs usually produced by larger females in population; grayish, ovoid and somewhat flattened, may vary in size geographically, in the Inland Sea of Japan 0.21-0.29 mm (mean: 0.25 mm) long, 0.14-0.20 mm (mean 0.18 mm) wide, ca. 0.10 mm in depth (Onbé 1972).
Total length of Females: 0.4-1.2 mm.
Total length of males: 0.7-0.9 mm.