noun
beach flea; sand flea; sand hopper
Refers to amphipods of the family Talitridae, commonly found on sandy beaches. The term is used in biological and casual contexts.
夏の砂浜にはハマトビムシがたくさんいる。
In summer, there are many sand hoppers on the sandy beach.
ハマトビムシは夜になると活発に動き回る。
Sand fleas become active and move around at night.
Compound of 浜 (hama, 'beach') + 跳虫 (tobimushi, 'jumping insect'), referring to its habitat and leaping behavior. The exact derivation is straightforward.