Life cycles of two limnetic cyclopoid copepods, Cyclops vicinus and Thermocyclops crassus, in two different habitats

Abstract

The life cycles of Cyclops vicinus and Thermocyclops crassus in two shallow eutrophic habitats, Junsainuma and Naganuma Ponds, Hokkaido, Japan, were investigated. Both ponds exhibited similar seasonal patterns of temperature, oxygen levels and pH during ice-free periods; however, oxygen levels were extremely lower under the ice in Naganuma Pond. Cyclops vicinus showed different life cycles in the two ponds; in Junsainuma Pond, it reproduced in winter and spring (January-May) and entered diapause during summer and autumn (June-October) as copepodite IV stage, while it reproduced in autumn (October-November) and spring (April-May), and entered diapause in summer (June-September) and winter (January and February) as copepodite V stage in Naganuma Pond. Thermocyclops crassus entered diapause during winter (December-April) as copepodite IV and V stages in both ponds, and egg-bearing females appeared only during the warm season, from early May to late October, when water temperatures were >10°C. Summer diapause in C.vicinus was suggested to be an adaptation against fish predation, whereas C.vicinus entered winter diapause in Naganuma Pond probably to avoid low oxygen levels. Thermocyclops crassus entered diapause in both ponds to avoid low water temperature. These results suggest that biotic and abiotic factors are important for leading to specific life cycles of cyclopoid copepods in small water bodies.