The 5-7mm long carapace is the colour and shape of a coffee bean, slight sexual dimorphism (M. castanea Habitus 2, M. castanea Habitus 4). Near the midpoint of the carapace is a transparent window in the pigmentation of the shell through which can be seen the facets of the compound eyes (M. castanea 1, M. castanea 2, M. castanea 3). The rostrum is quite small with a row of strong spines on its inner surface; near the ventral point is a swelling carrying about 15-20 spines. Posteriorly the carapace is drawn out to form a blunt point, and in ventral view the shell is spindle-shaped. The first antennae are 7-jointed, and the terminal segments carry long setae with sensory filaments along their length. In males, one of the shorter seta has suction cups. These setae are held splayed in front of the animal as it swims.
The second antenna has a reduced endopodite and there is little sexual dimorphism. The protopodite is a third the total length of the carapace. The caudal furca has nine (occasionally eight) pairs of claws. Females brood the young and carry upto 60 developing embryos.
Very characteristic species, unlikely to be confused with any other. Poulsen, 1962 gave a full description of var. rotunda , which is an Indo-Pacific form of this species lacking the posterior process of the carapace; this form may eventually be found in the Atlantic.
Occurs in all oceans, but appears not to extend polewards of the polar fronts. Juveniles are shallow mesopelagic animals, sometimes being taken as shallow as 100m, whereas adults are deep mesopelagic and are most abundant at depths of 700-1000m. 1, 2, 3 (R.R.S. Discovery Map).
None designated; status of original material uncertain.
Original specimens were caught at 32°N36°W.