Essay on Achieving Ionic Equilitbrium in Marine Fish

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Marine fish sustain an ionic equilibrium with seawater to keep their plasma around 350 mOsm/kg. The gills remove excess salts from the body (Evans et al., 2005), a process that indirectly causes water loss driven by the dehydrating effect of salinity. Therefore, water replacement by drinking becomes of absolute importance to sustain ion regulation (Fuentes and Eddy, 1997a). In addition to regulation by endocrine and environmental factors of the amount of water ingested (Fuentes et al., 1996; Fuentes and Eddy, 1997a; Fuentes and Eddy, 1997b; Guerreiro et al., 2004; Guerreiro et al., 2001), the processing of imbibed fluid has major impact in fish ion regulation. Ingested water is first processed in the esophagus where it undergoes selective…show more content…
In addition to apical mechanisms, the presence in the basolateral membrane of a Na+–HCO3- co-transporter (NBC) which belongs to the SLC4 gene family, leads to accumulation of cellular HCO3- to fuel apical secretion (Kurita et al., 2008; Taylor et al., 2010). Furthermore, a Bafilomycin-sensitive H+ pump participates in the regulation of apical HCO3– secretion in seawater adapted rainbow trout and Gulf toadfish (Grosell et al., 2009a; Guffey et al., 2011), that appears to be important for regional specialization of fluid processing in the sea bream intestine (Gregorio et al., 2013) .
Calcium as well as HCO3- immobilization in the luminal carbonate precipitates, results in reduced fluid osmolality and favors water absorption. Therefore, HCO3– secretion in marine fish intestine drives not only luminal precipitate formation, but also water absorption (Grosell, 2011). In keeping with this suggestion, a recent study in the Gulf toadfish demonstrates a role for the HCO3– –sensing soluble adenylyl cyclase (sAC) in intestinal water absorption (Tresguerres et al., 2010), a process also ocurring in the sea bream intestine (Carvalho et al., 2012).
The important contribution of fish intestinal carbonate precipitate production to the ocean carbon cycle has recently been highlighted (Wilson et al., 2009) and raises a new dimension to ecophysiological studies of the interaction between fish and their environment.

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