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Abstract HOW YOU GET TO THE SEA DEPENDS ON WHERE YOU START YOUR JOURNEY: VARIATION IN FIDDLER CRAB LARVAL DISPERSAL MECHANISMS John H. Christy The planktonic larvae of estuarine crabs often hatch at night on
large amplitude ebb tides and move rapidly to the coastal ocean
where they develop in relative safety from predation by inshore
planktivorous fish. Horizontal emigration of larvae from estuaries
to the sea is often facilitated by vertical migration of larvae:
up into the current on ebb tides and down to the bottom on flood
tides, resulting in a net ebb displacement. The larvae of the fiddler
crab Uca pugilator provide an excellent example
of such selective tidal stream transport (STST). In constant conditions
they show an ebb-phased circatidal rhythm in vertical migration;
in the field they can emigrate to the sea in one tidal cycle. The
genus Uca includes both estuarine and coastal species that
release larvae relatively far and near to the sea, respectively.
Thus, the adaptive value and strength of STST behaviors for emigration
likely varies among species in relation to where adults live on
the estuarine to coastal habitat gradient. We therefore examined,
under constant conditions for 72 h, the swimming activity of zoeae-I
larvae of fifteen Uca species from seven locations along
the Atlantic and Gulf coasts of the United States and the Pacific
and Caribbean coasts of Panama. Periodogram analysis of the time
series showed that estuarine species typically possessed ebb-phased
circatidal rhythms in swimming that matched the local tidal regime.
In contrast, lower estuarine and coastal species exhibited no clear
swimming rhythms. There was no evident phylogenetic signal in the
expression of these rhythms. Vertical swimming is likely to be energetically
costly. Whether and how species that do not vertically migrate spend
their cost savings on other life-history traits has yet to be explored.
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