noun
Marphysa sanguinea; rock worm; Korean rockworm
A species of large marine polychaete worm, often used as bait in fishing. The Japanese name literally means 'rock beach worm'.
See also: 岩虫
釣り餌としてイワイソメをよく使う。
I often use rock worms as fishing bait.
イワイソメは岩場の砂泥中に生息する。
Rock worms live in sandy mud among rocky shores.
岩虫 (いわむし) is a broader term for various rock-dwelling marine worms, while イワイソメ specifically refers to Marphysa sanguinea.
Compound of 岩 (iwa, 'rock') + 磯 (iso, 'beach') + 目 (me, 'order') or 蚯蚓 (mimizu, 'earthworm'). The exact derivation is uncertain; the name likely describes its habitat and worm-like appearance.