"One foot in the sea, and one on shore,
To one thing constant never".

(Shakespeare W. - Much Ado About Nothing)

The vernacular name of 'mudskippers' (or 'mudhoppers') is used for several related species of fishes (Teleostei, Gobiidae, Oxudercinae: see also Systematics & Biogeography), characterised by peculiar amphibious habits.

Semi-terrestrial activities include foraging, courtship, homing, tidal movements and territorial behaviour (Brillet, 1969a; 1969b; 1970; 1975; 1976; 1980a; 1980b; 1981; 1983; 1984a; 1984b; Berti et al., 1994; Clayton & Vaughan, 1982; 1986; 1988; Clayton, 1987; Clayton & Wright, 1989; Clayton & Snowden, 2000; Ikebe & Oishi, 1996; Kobayashi et al., 1971; Nursall, 1981; Sponder & Lauder, 1981).

These species are currently included in the gobiid subfamily Oxudercinae (10 genera, 40 species; Murdy, 1989; Jafaar & Larson, 2008) and are distributed on tropical and subtropical intertidal soft-bottomed shores of Western Africa and the Indo-Pacific region. The highest species richness and number of endemisms are found in the Indo-Malayan region, which probably is the centre of origin of this group (Murdy, 1989).

The degree of adaptation to terrestrial environment and amphibious life of some of these species, unrivalled among all aquatic chordates, is approximated only by some members of the blennid family (Teleostei: Perciformes: Blennioidei: Blennidae), also called 'rockskippers', that live on rocky intertidal shores (Horn et al., 1999).

Boleophthalmus pectinirostris (Ariake Sound, Japan, 2003). Photo: © BlueNativeFactory (2006), with permission

In particular, periophthalms (genus Periophthalmus) and boleophthalms (genus Boleophthalmus) include species that inspired popular and scientific imagination since the dawn of the Natural Sciences.

Two of the eight species of gobies that C. Linnaeus originally described in his Systema Naturae in 1766 are now included into this group (more than 2,000 nominal gobiid species, comprised in more than 200 genera, have been described up to date: Nelson, 1994).

These fishes are excellent examples of how vertebrates could have undertaken and continue to undertake evolutionary paths towards semi-terrestriality within transitional aquatic ecosystems. The air-water interface has always been one of the most definite and efficient and absolute ecological barriers for vertebrates to overcome.

Within the subfamily Oxudercinae there exists a gradual continuum of specific complexes of adaptations to the amphibious lifestyle.

The wide set of adaptations spans from the most rudimentary ones of those species that always live in demersal or semi-demersal conditions (e.g. Parapocryptes sp.), to extreme ones, that allow certain species to live in the high supratidal zone, where the substrate is submerged only once or twice a month (e.g. Periophthalmus novemradiatus: pers. obs.).

Apocryptes bato.
Photo: Frank Schäfer, © Der Aquaristik-Laden (2002)

left: dorsal and lateral view of the skull of a typical mudskipper (Periophthalmus sp.); right: dorsal and lateral view of the skull of an extinct panderichthid tetrapod of the Middle Devonian;
note the peculiar and apparently convergent position of the nasal openings (no) and the presence of orbital ridges (ri): the eyes are located dorsally while the nostrils are ventrally oriented. This condition is probably adaptive in very shallow waters, as those found in intertidal habitats in proximity of the water edge and at low tide.
Modified from Schultze (1999), with permission from Elsevier.

A recent scientific debate, which involves eco-physiologists and palaeoecologists (Graham & Lee, 2004; Schultze, 1999), is centred on the possibilities of evaluating this group as a model for the evolutionary sea-land transition of Devonian protoamphibians, ancestors of all present tetrapods (= terrestrial vertebrates, including man: see also The Devonian Opportunity).

Mudskippers are no more related to tetrapods than a perch or a snapper (see also Fish Coming Ashore). Nonetheless, the study of their evolutionary history and adaptations to amphibious life could help shed light on the eco-evolutionary mechanisms at the base of the first steps of the tetrapod transition from sea (or other aquatic environment) to land.

Strongly convergent adaptations are the output of intense selective pressures acting independently on species that do not share recent common ancestors (see also Fish Coming Ashore). In particular, this is the case of originally aquatic organisms which adapted to intertidal ecosystems; many resident intertidal species present typical convergent adaptations, such as the existence of early dispersalist life stages, of air breathing organs, of climbing and burrowing behaviours, or of amphibious behaviours.

This precious natural legacy is currently threatened by human demographic expansion, exacerbated by the recent trends of the global economy.

Countries as Niger, Iran, India, Malaysia, Indonesia, Philippines, and China, to name just a few, are systematically destroying the natural habitats of these species: coastal tidal mudflats and mangrove forests.

Mudskippers are remarkable and often considerably abundant components of these transitional ecosystems, and are susceptible to the impact of human action both in the sea and, even more drastically, on land (Polgar, 2008).

However, the domino effects of this progressive destruction are acting not only at an ecological and cultural level.
The economically and socially damaging consequences of this policy, both in the short and long term, are becoming ever more apparent.

Tsunami disaster after destruction of the coastal protective belts of mangrove forests. Banda Aceh, Indonesia, 2005
© IABC, (10/2005), with permission