Habitat
preference
In
resolving population structure of marine fishes, most attention has focused on
the dispersive larval stage. However the movements and feeding activities of
adults play a role in shaping population structure, especially for fishes in
the pelagic zone. For example, population structure in wide-ranging
tunas, billfishes, and pelagic sharks is usually measured on the scale of ocean
basins: East versus West Atlantic in the Bluefin Tuna Thunnus
thynnus (Carlsson et al. 2007), North versusSouth Atlantic in the White Marlin Tetrapturus
albidus (Graves & McDowell 2006), Indian versus Pacific in the
Swordfish Xiphias gladius (Lu et al. 2006), and Atlantic versus Indian-Pacific
in the Whale Shark Rhincodon typus (Castro et al. 2007).
A few demersal
(bottom-dwelling) fishes conduct reproductive or seasonal migrations, but
most are sedentary, and for this reason the corresponding habitat preferences
are seldom considered in predicting population structure. However, habitat
preference can have a strong influence on the distribution of genetic diversity
in fishes. Usually ecosystem specialists (those with very specific feeding
or habitat requirements) have more population structure than generalists, as
demonstrated by genetic comparisons of reef .fishes across the Amazon
barrier. This turbid plume of fresh water was long regarded as a barrier
that divided the West Atlantic reef fauna into northern (Caribbean) and
southern (Brazilian) provinces. However, fresh water is less dense than salt
water, and may form a surface layer with a saltwater “wedge” below. Trawl surveys
conducted under the Amazon plume demonstrated the presence of many marine
fishes that are usually associated with coral reefs (Collette & Rützler
1977). An mtDNA survey of West Atlantic wrasses (genus Halichoeres)
across the Amazon barrier demonstrates a strong connection between habitat use
and genetic structure. Halichoeres maculipinna, a reef species with
specialized diet and feeding morphology, has an ancient evolutionary separation
between Brazil and the Caribbean (sequence divergence d =0.065 in
cytochrome b). In contrast, H. bivittatus is found in a variety
of habitats in addition to coral reefs and shows no strong genetic separation
across the Amazon barrier (Rocha et al. 2005a). Notably, H. bivittatus was
collected in the trawl surveys under the Amazon plume, whereas H.
maculipinna was not. Combined, these genetic and field studies indicate
that habitat preference and speciesecology can be as important as geography and
larval dispersal in defining the distribution of genetic diversity in fishes
(Choat 2006).
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