The vast majority of the excreted ammonia originates from the catabolism of proteins and amino acids. There is little evidence to support nitrogenous excretion as amino-nitrogen because consumable amino acids released by crustaceans are more likely a result of passive lost via amino-acid-permeable structures (Dagg, 1976
) and via feces (Claybrook, 1983). Some additional ammonia is produced in reactions involving purine and pyrimidine bases (Claybrook, 1983) (Fig. 1). However, ammonia derived via this uricolytic pathway is considered to contribute only a small portion of the total ammonia excreted compared with the predominant production from amino acids (Hartenstein, 1970; Schoffeniels and Gilles, 1970). Ammonia derives, for the most part, from deamination of glutamine, glutamate, serine and asparagine by the specific enzymes glutaminase, glutamate dehydrogenase, serine dehydrogenase and asparaginase, respectively (King et al., 1985; Krishnamoorthy and Srihari, 1973; Greenaway, 1991, 1999).
Organs of ammonia excretion in aquatic crabs
In aquatic crabs, primary urine is formed via ultrafiltration in the antennal gland, which is thought to play the key role in regulation of body water and divalent cations (e.g. Mg2+, Ca2+; Mantel and Farmer, 1983), but not to contribute significantly to the excretion of nitrogenous waste products (Regnault, 1987). For instance, in the blue crab Callinectes sapidus, total ammonia is excreted in the urine via the antennal gland system (Cameron and Batterton, 1978).
The main site for ammonia excretion by aquatic crabs is the phyllobranchiate gill (Claybrook, 1983; Kormanik and Cameron, 1981; Regnault, 1987), featuring a single-cell-layered epithelium covered by an ion-selective cuticle (Avenet and Lignon, 1985; Lignon, 1987; Onken and Riestenpatt, 2002, Weihrauch et al., 2002). The gills of aquatic crabs are multifunctional organs. In addition to their function in excretion of nitrogenous waste products, they are also responsible for respiratory gas exchange (Burnett and McMahon, 1985), regulation of acid–base balance (Henry and Wheatly, 1992) and osmoregulatory ion transport (Towle, 1981; Lucu, 1990, Riestenpatt et al., 1996, Towle and Weihrauch, 2001). Several transporters and enzymes putatively linked and involved in ammonia transport have been shown to be present in the branchial epithelium of crabs, as summarized in Fig. 2 and Table 2.
Ammonia excretion in aquatic crabs
In solution, both forms of ammonia, non-ionic ammonia (NH3) and the ammonium ions (NH4+) exist in a pH-dependent equilibrium. As a weak base (pK 
9.48 at 20°C and NaCl=250 mmol l–1; Cameron and Heisler, 1983) and at a physiological pH of pH 7.8, 98% of total ammonia exists in the ionic form NH4+, whereas only 2% is present as non-ionic NH3. However, the higher lipid solubility of NH3 makes it more diffusible through phospholipid bilayers. Kormanik and Cameron (1981) reported that ammonia excretion of seawater adapted blue crabs Callinectes sapidus occurred mainly by diffusion of non-ionic NH3. An excretion mechanism based predominately on NH3 diffusion is not likely, however, because membrane permeability of NH3 is much lower than that of CO2 (Knepper et al., 1989). Indeed, some plasma membranes of animal epithelia are relatively impermeable to NH3 as shown for frog oocytes (Burckhardt and Frömter, 1992), the renal proximal straight tubules (Garvin et al., 1987) and colonic crypt cells (Singh et al., 1995). Accordingly, other authors have obtained experimental evidence for at least partial excretion of ammonia in its ionic form (NH4+) in Callinectes sapidus (Pressley et al., 1981) and Carcinus maenas (Lucu, 1989; Siebers et al., 1995).
Studies on isolated perfused gills of several aquatic crabs showed that ammonia can be excreted actively against a 4–8-fold inwardly directed ammonia gradient across both the anterior and the posterior gills to a similar degree despite their different morphological and physiological characteristics (Copeland and Fitzjarrell, 1968; Goodmann and Cavey, 1990; Weihrauch et al., 1998, 1999; Towle and Weihrauch, 2001) (Fig. 3). Under physiologically relevant conditions, the potential for active branchial ammonia excretion is significantly greater in the marine Cancer pagurus than in freshwater-acclimated Chinese mitten crabs Eriocheir sinensis, despite the much larger ionic conductance of Cancer pagurus gills (
250–280 mS cm–2) compared with that of Eriocheir sinensis gills (4 mS cm–2) (Fig. 4). It is noteworthy that the posterior gills of Carcinus maenas (thought to play the dominant role in osmoregulatory NaCl uptake) and also the anterior gills (thought to be primarily responsible for gas exchange) are equally capable of active ammonia excretion (Weihrauch et al., 1999).
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