• Intestine convoluted
• Antennal setae formula 0-0-0-3 / 1-1-3
• Spine at superoposteal angle, juveniles have longer tailspines relative to adults
• Antennule long and slender, fringed with hairs below, lateral sense hair near base, olfactory setae unequal
• Postabdomen elongated
• Postabdominal claw with two basal spines

• Ophryoxus gracilis has a circumpolar distribution, with European populations appearing to be cold stenotherms while North American populations are able to tolerate a wider range of temperatures and are found at lower latitudes (1).

• Found in benthic and littoral habitats where it spends the majority of time resting on and moving slowly over the bottom material or plant surface (1).

• Feeds on bottom material and material that has settled on or is growing on aquatic vegetation. Majority of food is scraped from the substrate, though it is capable of filter feeding (1).

• Ophryoxus gracilis has the ability to swim weakly but more persistently than most other macrothricids. The first trunk limb is modified for gripping the substratum and rudimentary crawling is achieved through use of this limb assisted by the antennae, though persistant crawling has not been observed (1).

• Smirnov (1992) differentiates three subspecies based on the shape of their pre-labral outgrowth. Ophryoxus gracilis gracilis has a pointed pre-labral outgrowth while O. gracilis canadensis has a semi-globular pre-labral outgrowth. Ophryoxus gracilis kolymensis has a pre-labral outgrowth that is elongate with a blunt apex. The material he described for O. gracilis kolymensis was from Russia, and we have not observed this subspecies.

(1) FRYER, G. 1974. Evolution and adaptive radiation in the Macrothricidae (Crustacea: Cladocera): a study in comparative functional morphology and ecology. Philos. Trans. R. Soc. Lond., B, Biol. Sci. 269: 137-274.

(2) SMIRNOV, N.N. 1992. The Macrothricidae of the world. Guides to the identification of the microinvertebrates of the continental waters of the world. SPB Academic Publishing, The Hague: 143pp.