noun
cyclops; water flea (Cyclops spp.)
Refers to tiny freshwater crustaceans of the genus Cyclops, often found in ponds and used as fish food. The name comes from the single eye-like spot.
池の水を顕微鏡で見ると、ケンミジンコがたくさん泳いでいた。
When I looked at the pond water under a microscope, there were many cyclops swimming around.
メダカの餌にケンミジンコを与えている。
I feed cyclops to the killifish.
ミジンコ (water flea) is a broader term for small freshwater cladocerans, while ケンミジンコ specifically refers to copepods of the genus Cyclops, which have a different body shape and a single eye-like spot.
The name combines 剣 (ken, 'sword') with 微塵子 (mijinko, 'water flea'), likely referring to the sword-like shape of the egg sacs or body. The exact derivation is uncertain.