P3.113 Wednesday, Jan. 6 Orientation mechanisms of larval release migration by the sesarmid crab, Metasesarma aubryi LIN, C.-C.*; HUANG, H.-D.; LIU, H. C.; Dept. of Ecology, Providence Univ., Taiwan; National Museum of Natural Science, Taiwan; Dept. of Ecology, Providence Univ., Taiwan firstname.lastname@example.org
Metasesarma aubryi lives under the maritime forest and ovigerous females need migrate to surf for larval release. The breeding season of M. aubryi is variable, depending on the beginning and ending of the rainy season, between May and November. In the season, M. aubryi release larvae during the last quarter of the lunar cycle. In southern Taiwan, migration of ovigerous M. aubryi commences after midnight from the highland forest, and wait until the time before dawn for larval release. We hypothesize that M. aubryi use geomagnetic field, light, and chemical cues for navigation. In laboratory, crabs were tested in a circular arena for orientation mediated by geomagnetic and light cues; a four-chambered apparatus was employed for testing chemical cues from seawater, and congeners. In the dark, ovigerous crabs oriented seawards but randomly after larval release, which indicated geomagnetic fields as primary cues for seaward orientation. Spot lights from directions changed orientation of ovigenous crabs in the arena, which implied crabs may also use skylight in navigation. Scents of seawater and congeners showed weak influence in orientation of crabs. We conclude that, ovigerous crabs mainly use geomagnetic fields to navigate seawards; after larval release, they randomly move possibly for proximate shelters and may wait for slow resume of the landward orientation. Ovigerous crabs near the beach may use twilight at dawn as a signal for seaward orientation and larval release. Artificial illumination at the route may cause negative effect on migrating crabs.