A behavioral mechanism for dispersal in fiddler crab larvae (genus Uca) varies with adult habitat, not phylogeny

Paola C. López-Duarte, John H. Christy and Richard A. Tankersley

Limnol. Oceanogr., 56(5), 2011, 1879-1892 | DOI: 10.4319/lo.2011.56.5.1879

ABSTRACT: The genus Uca (fiddler crabs) includes both estuarine and coastal species, the zoeae of which must travel varying distances to reach the sea. Rapid seaward transport following hatching may be facilitated by tidal currents through an ebb-phased circatidal rhythm in vertical swimming by larvae. Assuming there is a cost to vertical swimming, we predicted that endogenous swimming rhythms would be stronger in zoeae of species that live far up estuaries (oligohaline and mesohaline areas) and weaker or absent in coastal species and that adult ecology, not phylogeny, would best explain interspecific variation in vertical swimming rhythms. Both predictions were confirmed. Ovigerous females of 15 Uca species in 4 subgenera were collected from 10 locations along the Atlantic and Gulf Coasts of the United States and the Pacific and Caribbean coasts of Panama. Swimming activity of zoeae-1 larvae was recorded under constant conditions for 72 h. Regardless of subgenus, estuarine species exhibited ebb-phased circatidal swimming rhythms that persisted for several cycles and matched the tidal patterns at the collection sites, while the swimming of coastal species was arrhythmic. Species-specific differences in the proximity of adult habitats and hatching areas to the sea accounted for differences in larval behavior that promotes seaward transport. Vertical swimming rhythms of larvae may have evolved together with other traits that permit estuarine living.

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