As in the majority of fish species, and regardless of the degree of terrestriality (e.g. in Periophthalmus, Periophthalmodon and Boleophthalmus spp.) excretion in mudskippers is essentially realised through the gills, in the form of ammonia and, to a less extent, urea.
In fact, a tendency towards ureotely (a relatively expensive metabolic process, widespread among living sarcopterygians - see also Fish Coming Ashore), has never been demonstrated in any oxudercine gobies (Gregory, 1977; Evans et al., 1999).

In general, mudskippers are highly resistant to ammonia loadings.
Peng et al. (1998) measured a tolerance of ammonia concentrations of about 450 μM NH3 for at least 7 days in Periophthalmodon schlosseri. Such values are comparable only to Oreochromis alcalicus grahami (= Alcalotilapia grahami, a cichlid which lives in African alkaline lakes, in waters with pH>9.5), but at much lower pH values.
The same authors measured that Boleophthalmus boddaerti (= B. boddarti) tolerated about 40 μM NH3; concentrations comparable to those tolerated by Periophthalmus cantonensis (= P. modestus: Iwata, 1988), and to concentrations that were lethal for a sample of other freshwater, estuarine and tide pool gobioid fishes (about 45 μM NH3 for 24 hr: Iwata, 1988).

Both Pn. schlosseri and B. boddarti can decrease the rate of proteolysis and of amino acid catabolism, thus slowing down the accumulation of ammonia (Lim et al., 2001).

Emergence and exposure of the gills to air drastically limits excretory functions, and mudskippers adopt different metabolic strategies to defend against ammonia toxicity upon emergence.

When out of water, Pn. schlosseri detoxifies ammonia by accumulating alanine and producing energy through partial amino acid catabolism (amination and transamination pathways). The efficiency of this process increases with the duration of emergence (Ip et al., 2001, 2004a). This physiological adaptation makes this species capable of intense and prolonged terrestrial activity, whilst not consuming its glycogen reserves, the main energy source under water.

This strategy is probably adopted also by Periophthalmus modestus (Iwata et al., 1981; Iwata, 1988).

Agonistic encounter of two males of Boleophthalmus boddarti.
Photo: Akinori Kamiya "Yamaneko", Can Gio, Viet Nam, 2004; © umisuzume 2006, with permission

Boleophthalmus boddarti, a more aquatic species, consumes instead glycogen reserves while out of water, with a consequential decrease of the energy charge. This species is therefore unable to engage in intense and extended terrestrial exercise (Ip et al., 2001).

Specific mechanisms are also necessary in the highly variable pH water conditions of estuarine brackish habitats where usually mudskippers live, and in the highly polluted water volumes inside burrows.
In particular, ammonia excretion is inhibited by alkaline conditions.
It has been demonstrated in Pn. schlosseri the capacity of excreting NH4+ through the gills against high concentration gradients, even at pH 9.0 (Chew et al., 2003; Randall et al., 2004). The cellular membranes of its skin also have a low permeability to ammonia, that decreases at higher ammonia environmental concentrations.

B. boddarti instead, is unable to actively eliminate NH4+ ions, but its high tolerance to ammonia enables it to accumulate them in its muscles, liver and plasma, therefore re-establishing a favorable chemical gradient and preventing further NH4+ intake.

Pn. schlosseri not only actively excretes NH4+ in ammonia loading conditions, but is also able to excrete protons (H+) and lower environmental pH in small volumes of water (e.g. inside burrows), thus reducing the NH3 concentration around its body and preventing a back flux of NH3 through the gills (Ip et al., 2004b).

These adaptations are probably crucial for fishes that take refuge and lay eggs in small volumes of polluted and alkaline water (Randall et al., 1999; Randall & Tsui, 2002; Chew et al., 2003; Ip et al., 2004a).

Boleophthalmus boddarti "bathing" after the long period of submersion during high tide (pers. obs.); a tactic to get rid of the accumulated catabolites?
Photo: G. Polgar, Kukup, Malaysia, 2006.