noun
three-spined stickleback
A small freshwater fish, Gasterosteus aculeatus, known for the male's nest-building behavior.
春になると、イトヨのオスは巣を作り始める。
In spring, male three-spined sticklebacks start building nests.
The name likely derives from 糸 (ito, 'thread') and 魚 (yo, an old reading for 'fish'), referring to the fish's slender shape. The exact origin is uncertain.