Floating and algal rafting as potential dispersal mechanisms in brooding invertebrates

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Abstract
Many benthic marine invertebrates brood their young to a crawl-away stage and thus lack a swimming dispersal phase on their life histories. Results of flotation experiments on brooding infaunal tanaids, amphipods and bivalves from intertidal habitats show that many of these animals, especially small individuals, may remain suspended on the water surface from a few hours to a few days, indicating that floating is a potential dispersal mechanism for these species. Because animals suspended on the water surface are likely to contact algae or logs to which they can attach, floating also increases the probability of dispersal by rafting. The presence of eel grass reaching the surface initially accelerated the rate at which organisms returned to the bottom, but did not alter the final results. The non-wetting characteristic of the crustacean species studied decreased with time, so that most of the animals were eventually able to resubmerge. In addition, data on abundance of brooding organisms in algal mats on a mudflat and in drift algae suggest export of algae and associated fauna, including several brooding species, from the mudflat occurs during spring tides. With respect to dispersal by floating or rafting, small adult size probably contributes to the evolutionary success of shallow-water brooders. The increasing representation of brooding species in benthic communities at higher latitudes is considered with respect to Thorson''s (1950) hypothesis that low temperatures (slow development direct rate) and short periods of algal productivity at high latitudes limit planktotrophy and favor non-pelagic direct development. Two alternative hypotheses are suggested: (1) the geographic distribution of brooding species is correlated with rafting opportunities; (2) the abundance of potential pelagic predators on planktotrophic larvae at high latitudes selects for brooding or lecithotrophy in less fecund species and, conversely, intense benthic predation pressure at low latitudes restricts brooding and favors planktonic larval forms.