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Leydig, 1860

Description
Female carapace relatively narrow in lateral view, dorsal part never strongly humped; largest dorso-ventral width at, or above middle of the body, dorso- frontal curve of carapace convex, never concave. Carapace always with spine (mucro), that is sometimes reduced to only a backwards protruding corner of the carapace [B.longispina f. maritima-f]. Rostrum usually short and blunt, however, sometimes rather long. Frontal part of the head before the eye sometimes strongly humped; eyes usually large. Antennulae short to moderately long, last segment with 5-22 incisures; antennulae mostly weakly curved, sometimes almost projecting downwards, rarely weakly S-curved. [Antenna not depicted].
Male carapace with low, narrowing dorsal side; short, rarely long mucro (carapace spine) [B.longispina f. maritima-m]. Frontal part of the head humped; antennulae
weakly curved. Base of furcal claw [fc] armed with usually eight, rarely six equally sized teeth (characteristic for males of the forma maritima). [Antenna not depicted].
Colourless and completely transparent in clear waters, otherwise yellowish.

Ecology
Optimal salinity range 2.5-5.3 ä, maximum salinity 8-10ä. Occurs in the plankton from May till November.

Size
Female 0.4-1.2 mm; male 0.4-0.66 mm.

Depth range
Surface to 5 m.

Distribution in the North Sea
B. longispina forma maritima is endemic in the Baltic Sea, but occurs also in the Kattegat and rarely in the southern Skagerrak.

World distribution
B. longispina is distributed in the northern and middle holarctic, and inhabits oligotrophic and mesotrophic freshwater lakes etc., but also occurs in the marine littoral. The species contains various formae of which f. maritima is especially flourishing in the oligohaline parts of the Baltic.

[After Flößner, 1972]

Remark
B. longispina is a very variable species, with numerous local races that have no systematic importance. However, the various forms are dived into two major groups: an arctic-subarctic group and an alpine group; the present f. maritima belongs to the first group (Flößner, 1972).
According to Costello and Emblow, 2000, the present f. maritima P.E. Müller, 1867 is recognised as a subspecies of Bosmina coregoni Baird, 1857.
The apparent very similar species B. longispina and B. coregoni differ in the length of the antennula: almost half as long as the body in longispina and more than half as long as the body in coregioni. In maritima a mucro is always present, though sometimes reduced to a protruding corner of the carapace. In coregoni the mucro is more variable, either a long spine, or a pointed corner of the carapace (Flößner, 1972).

Bosmina longispina