Ophryoxus gracilis differs from other cladocerans by its habitus, including the slender first antennae attached to rostrum and the long and slender postabdomen, which tapers towards the end. It is associated with electrolyte poor water and is rare outside Scandinavia.
Seen from the side, the body form of Ophryoxus gracilis differs from other cladocerans, and we can easily identify this species by habitual criteria. Its body is rather elongate, widest approximately at the middle, tapering to the posterior end. Its head is comparatively small with a short rostrum to which the extraordinarily long and slender first antennae is attached. The postabdomen is long and slender and tapers towards the end. The postabdominal claw has two strong spines close to the base, the outermost one is longer than the innermost one. The colour of O. gracilis varies from bright olivaceous-green to pale yellow-brown.
Female: Length 1.5–2.0 mm
Male: Length 1.0–1.2 mm
Ecology and distribution
O. gracilis is a common littoral species found in 20 % of the localities. It has a northern distribution and is scarcely found outside Scandinavia. It occurs with almost the same frequency in all lakes below 1000 m a.s.l. Above this latitude it is rare. Though it is found in small ponds, it is most common in the littoral of lakes. It is looked upon as an indicator species of relatively electrolyte poor water, and is also weakly acid sensitive.
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