Thermocyclops crassus is a small copepod having a powerful appearance. It may have been overlooked because it co-occur with Mesocyclops leuckarti and T. oithonoides which are much more common. All records are from lakes southeast of Oslo.
The body of Thermocyclops crassus is rather short and stout and its furca is about twice as long as wide. With respect to these criteria, it is possible to distinguish this species from Mesocyclops leuckarti and T. oithonoides which it may co-occur with. To separate T. crassus from T. dybowskii, the ratio between the outer and the inner setae attached to the furca can be used. The ratios are respectively 2:3 in T. dybowskii compared with 1:3 in T. oithonoides. Its colour is very pellucid with a faint yellowish tinge.
Female: Length 0.9–1.1 mm
Male: Length 0.7 mm
Ecology and distribution
G.O. Sars found this species twice, southeast of Oslo. Since then there have been only a few records of T. crassus, all within the catchment of Lake Vansjø where Sars found the species too. T. crassus is quite common in lakes upstream of Lake Vansjø (25–129 m a.s.l.) and there is one record from the vegetated shore of the main river in this catchment. pH in these sites is close to neutral (6.3–7.1) and conductivity varies between 5 and 22 mS/m.
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