Ammonia excretion in the marine polychaete Eurythoe complanata (Annelida)

Ammonia is a toxic waste product from protein metabolism and needs to be either converted into less toxic molecules or, in the case of fish and aquatic invertebrates, excreted directly as is. In contrast to fish, very little is known regarding the ammonia excretion mechanism and the participating excretory organs in marine invertebrates. In the current study ammonia excretion in the marine burrowing polychaete Eurythoe complanata was investigated. As a potential site for excretion the 100-200 micrometer long, 30-50 micrometer wide and up to 25 micrometer thick dentrically branched, well ventilated and vascularized branchiae (gills) were identified. In comparison to the main body, the branchiae showed considerably higher mRNA expression levels of Na⁺/K⁺-ATPase, V-type H⁺-ATPase, cytoplasmatic carbonic anhydrase (CA-2), a Rhesus-like protein, and three different AMTs. Experiments on the intact organism revealed that ammonia excretion did not occur via apical ammonia trapping, but was regulated by a basolateral localized V-type H⁺-ATPase, carbonic anhydrase and intracellular cAMP levels. Interestingly, the V-type H⁺-ATPase seems to play a role in ammonia retention. A one week exposure to 1 mmol l⁻¹ NH₄Cl (HEA) did not cause a change in ammonia excretion rates, while, the 3 branchial expressed AMTs were in tendency down-regulated. This indicates a shift of function in the branchial ammonia excretion processes under these conditions.