Water exchange pertaining to host attachment sites and stream preference in crayfish-associated branchiobdellids, <Emphasis Type="Italic">Cambarincola fallax</Emphasis> and <Emphasis Type="Italic">Cambarincola ingens</Emphasis> (Annelida: Clitellata)

Water balance profiles were constructed for adults of two branchiobdellid species (Cambarincola fallax and Cambarincola ingens) to examine how they are differentially adapted for a freshwater habitat on different crayfish hosts. Both exhibited a high (75%) body water content and high net transpiration rate, reflecting that they are hyperosmotic to freshwater and display a consistent strategy that emphasizes water loss. In contrast to C. ingens, C. fallax was 6× smaller in body size, lost water 2× faster, and had heightened activation energy (E _a) for water permeability. Thus, C. fallax relies on high net transpiration rate with a focus on elimination, whereas the larger C. ingens are modified for water retention and rely on their low activation energy to suppress the amount of water that enters the body. Neither species showed evidence of a critical transition temperature (CTT), as indicated by an uninterrupted Boltzmann temperature function of passive water loss rates. No influence on favoring water balance was apparent in relation to the crayfish host with regard to preferred attachment sites (subrostral or branchial chamber) or preference for occupying different regions (pool vs. riffle) of the stream. Thus, water balance strategies between the two species are complementary, reflecting trade-offs that adjust for body size to promote survival in similar freshwater habitats. The ability of C. ingens and C. fallax to survive with a low water content, down to nearly 1/2 their body mass, however, enables them to cope with the excess water by permitting a high rate of water loss. Handling editor: K. Martens