Why are Biologically Active Micro-Organisms Important to Isopods?
Terrestrial immigrants from the sea have to cope with two important nutritional problems: a shift in pathways of absorption, and the altered availability of nutrients. The more terrestrial species oflittoral crustaceans switch to food as the main source of water and salts. No serious difficulties accompany the oral uptake of mobile ions, but the vegetarians amongst the immigrants are required to assume a new attitude with regard to the assimilation or heavier metals. Some important heavy elements, notably Copper and Zinc, are soluble in sea water but form rather intractable organic complexes in plant tissues. Herbivorous isopods on land are incapable of extracting copper directly from their primary food sources. To compensate for this shortcoming, help is enlisted from microorganisms, which render the copper present in ill-digested fecal material available to the crustaceans. Moreover, in terrestrial and intertidal herbivorous crustaceans—as compared with their marine relatives—the storage capacity of the hepatopancreas, as well as the efficiency of Copper-assimilation, is augmented; compartmentalization of Copper-storage is more rigorously carried through; the movements of Copper and Zinc within the body are more strictly regulated, copper, for example, being exchanged between different compartments in the course of endogenously or exogenously induced phases of the animal's life cycle.
