Despite the general preference of aquatic
invertebrates for detritus, some herbivory
does occur, particularly when alternative food sources are scarce. Crayfish are
omnivorous, adults eating mostly vegetation and detritus; they prefer fully
submerged species to emergent forms and can affect macrophyte numbers both
through direct consumption and by cutting plant stems near the base.
True grazers on macrophytes are found
among mammals and birds. Barasingha
(Cervus duvaucelii), the endangered swamp deer of southeast Asia, relies heavily
upon submerged macrophytes as a source of sodium and will actively seek
otherwise nutritionally poor species from ponds and ditches; moose or elk (Alces
alces), which occurs widely in North America and northern Eurasia, is another deer
species which habitually feeds upon aquatic plants, apparently choosing sodium-
rich species.
Perhaps the most effective mammalian grazers,
however, are the sirenians, mainly
marine but including the American manatee (Trichechus manatus) which moves
freely between marine, estuarine and freshwater habitats, and the only exclusively
freshwater species, the Amazonian manatee (Trichechus inunguis). This latter
species, which weighs up to 450 kg, feeds upon aquatic and semi-aquatic macro-
phytes, consuming about 8% of its body weight per day. The movements of this
species are determined by the hydrological cycle of the Amazon. As the rivers rise,
between December and June, manatees spread into the vdrzea and igapo
(flooded forest) areas, to feed on the lush new growth of macrophytes; when river
levels drop, manatees migrate into perennial lakes where, if macrophytes are in
short supply, they will eat detritus off the lake bed, or simply not feed at all.
