Larvae of native and non-native crabs behave differently in response to chemical cues from potential fish predators and adult crabs

- Chemical cues altered behavior of crab megalopae swimming into flowing water.
- Cues from 3 fish species caused less swimming of native megalopae.
- Non-native megalopae responded less to mummichogs, which are absent in native range.

The behavior of planktonic larval invertebrates can play an important role in predator avoidance. Megalopae, the last larval stage in crab development, are strong swimmers yet are vulnerable to predation by fish. The behaviors of megalopae of the native mud crab Dyspanopeus sayi (Smith, 1869) and the Asian shore crab Hemigrapsus sanguineus (De Haan, 1835) in flowing seawater were analyzed in response to chemical cues from potential fish predators and adult crabs. Fish predators consisted of cunner (Tautogolabrus adspersus [Walbaum, 1792]), tautog (Tautoga onitis [Linnaeus, 1758]), and mummichog (Fundulus heteroclitus [Linnaeus, 1766]). Larvae were reared to early and late stages of megalopal development. Individual megalopae were dropped into a horizontal glass tube containing a stream of flowing artificial seawater (control) or seawater containing chemicals from fish or adult crabs. Behaviors of each megalopa were observed and categorized as swimming, walking, tumbling, or inactive. In control seawater, megalopae of both species swam upstream more than in seawater with fish or crab cues. Mud crab megalopae were inactive in the presence of chemicals from all fish species whereas Asian shore crab megalopae were inactive more in the presence of cues from tautog and cunner than from mummichog, a species unrelated to fish predators in its native range. Late-stage D. sayi megalopae walked on the bottom of the tube in response to confamilial adult cues more than H. sanguineus. Both species can therefore detect chemical cues in flowing water, even early in megalopal development, and H. sanguineus can distinguish cues from different potential fish predators.