In mudskippers the excretion, regardless of the degree of terrestriality (e.g. in Periophthalmus, Periophthalmodon and Boleophthalmus spp.) and like in the majority of fish species, is essentially realised through the gills, in the form of toxic ammonia and, to a less extent, urea.
As a matter of 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).

Mudskippers adopt different metabolic strategies to defend against ammonia toxicity upon emergence.
Both Periophthalmodon schlosseri and Boleophthalmus boddarti tolerate high environmental ammonia concentrations (Peng et al., 1998). Moreover, in these species a decrease in the rate of proteolysis and of amino acid catabolism slows down the accumulation of ammonia (Lim et al., 2001).

Pn. schlosseri produces energy through partial amino acid catabolism (i.e. amination and transamination pathways: alanine synthesis) when out of water. This process increases its efficiency with the duration of emergence (Ip et al., 2001, 2004). This enables this species to be very active on land, whilst not consuming its glycogen reserves, the main energy source while in 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, with a consequential decrease of the energy charge. This species is therefore unable to engage in intense and extended exercise while out of water (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 inside burrows.
In particular, ammonia excretion is inhibited by alkaline conditions.
Recently it has been demonstrated in Pn. schlosseri its unique 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 further decreases at high ammonia environmental concentration.

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 to re-establish a favorable chemical gradient and prevent further NH4+ intake.

Pn. schlosseri not only actively excretes NH4+ in ammonia loading conditions, but is even able to excrete protons (H+) and lower environmental pH, thus keeping a low NH3 concentration around its body and preventing a back flux of NH3 through the gills (Ip et al., 2004).

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

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