Mesochra rapiens is mainly a brackish water species, found in oligohaline and tidal freshwaters estuaries. It is a short and robust animal with a rather large rostrum.
Mesochra rapiens has a body which is short and robust. Both exopod and endopod in first leg are 3-segmented. It has an especially large eye. The operculum is smooth while caudal ramus is as long as wide, some narrowed at the distal end. The antennule consists of seven segments. In the male the endopod in leg 5 is characterized by having two rather strong and thick, smooth spines and one short slender spine. According to the literature, the male operculum has 5–6 spines.
Female: Length 0.50–0.63 mm
Male: Length 0.38–0.47 mm
Ecology and distribution
Female individuals of M. rapiens were found by Sars (1911) in a brackish water tarn (Southern Norway). Lately M. rapiens has been found in four additional localities: lakes, pond and rock pool in Eastern and Northern Norway situated from 4 to 38 m a.s.l. All habitats apart from Lake Isesjøen in Eastern Norway situated about 5.5 km away from the shore, are closely located to marine water. The species has Palearctic and Nearctic distribution.