Whilst searching for something completely different (a phenomenon that has occurred sufficiently often recently for me to consider naming it "the Monty Python teleology slip", but I digress), I found a nice BBC article on research aimed at finding more clues to the tetrapod ancestry problem.

This piece of research started out when a different project, studying the behaviour of a fish species called the Mangrove Rivulus (a modern, living fish which leads an amphibious lifestyle, and hence is thought to provide clues about the vertebrate colonisation of the land in the late Devonian), led to one of those unexpected offshoots, courtesy of a lab accident. As Dr Alice Gibb, the co-author of the resulting paper (patience, everyone!) said, "When we tried to move these fish in the lab [using nets], they would jump out of a net and back into the tank". Consequently, it occurred to her to ask if this jumping ability, from land back into water, was restricted to those fishes that had developed some capacity for mobility on land, or whether this was a more universal feature. So, time to run some experiments.

The interesting result was, that this capacity, to jump back into the water successfully, appears to be a trait shared by a wide range of fish taxa, and that it involved the same mechanics across taxa, including taxa whose most recent common ancestor existed at least 150 million years ago.

Now, after keeping you all in suspense, it's time for the paper! Namely:

Fish Out Of Water: Terrestrial Jumping By Fully Aquatic Fishes by Alice C. Gibb, Miriam A. Ashley-Ross, Cinnamon M. Pace and John H. Long Jr, Journal of Experimental Zoology Part A, 313A (early online publication 4th October 2011) [Full paper downloadable from here]

Gibb, 2011 wrote:Many teleosts that live at the water’s edge will voluntarily strand themselves to evade predators or escape poor conditions—this behavior has been repeatedly observed in the field for killifishes (Cyprinodontiformes). Although most killifishes are considered fully aquatic and possess no obvious morphological specializations to facilitate terrestrial locomotion, individuals from several different species have been observed moving across land via a ‘‘tail flip’’ behavior that generates a terrestrial jump. Like aquatic fast starts, terrestrial jumps are produced by high-curvature lateral flexion of the body (stage one), followed by contralateral flexion of the posterior body (stage two). Here, terrestrial jumps and aquatic fast starts are quantified for two littoral teleosts: Gambusia affinis (a killifish, Cyprinodontiformes) and Danio rerio (a small carp, Cypriniformes) to determine if the tail flip is produced by other (non-killifish) teleosts and to test the null hypothesis that the tail flip is a fast start behavior, performed on land. Both Danio and Gambusia produce tail flip-driven terrestrial jumps, which are kinematically distinct from aquatic escapes and characterized by (1) a prolonged stage one, during which the fish bends, lifting and rolling the center of mass over the caudal peduncle, and (2) a relatively brief stage two, wherein the caudal peduncle pushes against the substrate to launch the fish into the aerial phase. The ability of these fully aquatic fishes to employ the same structure to produce distinct kinematic patterns in disparate environments suggests that a new behavior has evolved to facilitate movement on land and that anatomical novelty is not a prerequisite for effective terrestrial locomotion.

Right, what does this mean? Basically, the research article aims to provide evidence that the mobility of fish on land could have arisen before any anatomical features facilitating this, and that the behaviour in question is of ancient origin. Well, I know of several fishes that are mobile on land without having any specialised limb modifications (Clarias catfishes being a notorious example to anyone living in Florida), and so it doesn't really surprise me to learn, that fish species habitually gathering close to bodies of land have developed some means of getting back in the water, if they end up on land as a result of some previous excursion (e.g., an escape from a predator). Interestingly, Rivulus species, of which the amphibious Rivulus marmoratus is one, are all well known amongst aquarists for being miniature fish versions of Polaris missiles in the aquarium, with a tendency to launch themselves out of the aquarium at a moment's notice if the aquarium is left uncovered, and indeed, Rivulus cylindraceus, the Cuban Rivulus, was documented climbing out of water onto floating aquatic vegetation for periods of up to several minutes, or sticking itself velcro fashion to the aquarium sides, as long ago as 1936 (see William T. Innes' Exotic Aquarium Fishes, where this is documented).

All of the group of Cyprinodonts known colloquially in the aquarium world as "Killifishes" (a contraction from the Dutch for "creek fish"), members of five or so Cyprinodont Families, are frequently described in the aquarium literature as being rocket-propelled jumpers, with species such as the Blue Gularis, Fundulopanchax sjoestedti, being notorious for their propensity for aerial excursions, despite possessing none of the adaptations associated with various freshwater flying fishes (e.g., the Hatchet Fishes of the Family Gasteropelecidae) or the various Walking Fishes (Clarias catfishes, Anabas testudinens or the Mudskippers). Of course, any prolonged spell spent on land would also require the acquisition of a means of breathing atmospheric air (Clarias and Mudskippers are not the only fishes possessing this - species as diverse as Corydoras catfishes and the various Lungfishes, such as Protopterus annectens and the various Lepidosiren species, also possess the means to utilise atmospheric air), but given that modern Actinopterygians exhibit what is, in effect, an ancient behaviour, it should come as no surprise (together with all the other evidence) that at some point in the past, some of the extant fishes of the relevant era gave rise to ever better land-mobile descendants. What makes this research important, is that as the authors state, anatomical novelties were not needed for the earliest stages of land mobility (a fact illustrated by the modern Clarias catfishes, and the European Eel), and relevant means of establishing this are now published in the peer reviewed literature. I'm surprised no one asked the question before, to be honest, but it's nice to see someone finally doing so!

Along those lines, we have the Spangled Perch.

It has admirable dispersal abilities which allows it to rapidly colonise habitats not readily accessible to other fish species. During heavy rain it has been observed swimming across flooded paddocks and along wheel ruts on tracks.

It's pretty easy to see this gives the fish an advantage. The better adapted it gets to moving on land, the further it can breed.

http://www2.mdbc.gov.au/subs/fish-info/ ... Perch.html

Ah, it's a relative of the Targetfish, Terapon jarbua.

The Targetfish is a species that finds its way into the aquarium, though usually, it's only dedicated keepers that have one, because of the large size it reaches (36 cm), the somewhat boisterous habits it exhibits, and the fact that it prefers to live in shoals, requiring the prospective keeper to furnish a very large aquarium for six or more of these fishes if they are to be properly housed. Hardiness wise, Targetfishes are bomb proof - they're fully euryhaline, will live in anything ranging from full fresh water to full sea water and any brackish mixture in between, will stand levels of pollutants in a marine aquarium that will kill off most other fishes in short order, and whilst persuading some marine fishes to feed is a problem for marine aquarists, the only problem you have with Targetfishes is rationing their food, because they'll devour just about any animal based food at a rate of knots. If you're looking for a tough, survive-anything fish for a marine aquarium, Terapon jarbua fits the bill nicely, provided you can live with its large adult size, or aggression toward smaller and weaker fishes (fishes small enough to fit in its mouth will be eaten).

I'll check up on Leiopotherapon unicolor, which, from the image, appears to be anything but "uni-coloured" ... thus far, it appears to be a fish capable of withstanding a wide salinity range, but which prefers fresh water to brackish if given a choice. It also appears to be another one of those bullet-proof fishes hardiness wise.

Actually, Clownfishes can be pretty aggressive when they want to be. This is because they share the same reproductive habits as that other well-known Family of strongly territorial fishes, the Cichlidae (indeed, Cichlids and Pomacentrids share a common ancestor, and are anatomically distinguished only by subtle articulations of the skull bones in the region of the eye sockets). Clownfishes are monogamous pair-forming fishes, that defend a territory where the eggs are to be laid and the young fishes cared for, and when the Clownfishes are in parental mode, they are utterly fearless. They'll go for your fingers if you wiggle them too close to the egg mass. Plus, your shrimps had better be decent sized ones, such as Boxing Shrimp, or else the Clownfishes might decide to turn them into a snack.

Meanwhile, back on track, one of the mudskippers was the subject of a recent scientific paper, in which the authors established that the species in question has a very interesting way of solving a problem arising during reproduction. Fish eggs usually obtain oxygen from the surrounding waters, but the mudskipper in question lives in waters with a low oxygen tension, and consequently, the eggs might not receive enough oxygen from the water. The fish's solution? It constructs a U shaped burrow, open to the water at one end, closed at the other. Then, it gulps air from the surface, and delivers the air bubbles into the closed section of the burrow. It then lays its eggs into that chamber, out of the water. The eggs are then kept oxygenated by regular trips to the surface on the part of the parent fish, bringing new air to replace the old, and are kept moist by appropriate splashing motions. Once the eggs are ready to hatch, the parent fish removes the air from the chamber, flooding the section containing the eggs, whereupon they hatch, and the fry can make their way out of the burrow, and take their first gulps of fresh air at the surface.

The paper detailing this behaviour is:

Mudskippers Brood Their Eggs In Air But Submerge Them For Hatching by Atsushi Ishimatsu, Yu Yoshida, Naoko Itoki, Tatsusuke Tekeda, Heather J. Lee and Jeffrey B. Graham, The Journal of Experimental Biology, 210: 3946-3954 (15th November 2007) [Full paper downloadable from here]

Ishimatsu et al, 2007 wrote:Summary

Intertidal mudflats are highly productive ecosystems that impose severe environmental challenges on their occupants due to tidal oscillations and extreme shifts in habitat conditions. Reproduction on mudflats requires protection of developing eggs from thermal and salinity extremes, O2 shortage, dislodgement by currents, siltation and predation. Mudskippers are air-breathing, amphibious fishes, and one of few vertebrates that reside on mudflats. They lay their eggs in mud burrows containing extremely hypoxic water, raising the question of how the eggs survive. We found that the Japanese mudskipper Periophthalmus modestus deposits its eggs on the walls of an air-filled chamber within its burrow. To ensure adequate O2 for egg development, the burrow-guarding male mudskipper deposits mouthfuls of fresh air into the egg chamber during each low tide, a behaviour that can be upregulated by eggchamber hypoxia. When egg development is complete the male, on a nocturnal rising tide, removes the egg-chamber air and releases it outside the burrow. This floods the egg chamber and induces egg hatching. Thus, P. modestus has developed a reproductive strategy that allows it to nurture eggs in this severe habitat rather than migrating away from the mudflat. This requires that mudskipper eggs be specialized to develop in air and that the air-breathing capacity of the egg-guarding male be integrated in a complex behavioural repertoire that includes egg guarding, ferrying air to and from the egg chamber, and sensing O2 levels therein, all in concert with the tidal cycle.

I'll let you examine the rest of the paper in detail, as it's yet another scintillating example of the sort of biological phenomenon that only makes sense, as Dobzhansky said, in the light of evolution.

Two other papres relevant to this are:

Roles Of Environmental Cues For Embryonic Incubation And Hatching In Mudskippers by Atsushi Ishimatsu and Jeffrey B. Graham, Integrative & Comparative Biology, 51(1): 38-48 (30th April 2011) [Abstract available here, full paper requires subscription or institutional access]

Ishimatsu & Graham, 2011 wrote:Abstract

Reproduction on mudflats requires that eggs are protected from different environmental challenges during development and hatch when environmental conditions are favorable for survival of juveniles. Mudskippers are air-breathing, amphibious gobies of the subfamily Oxudercinae, and one of a few vertebrates that reside on mudflats. They excavate burrows in mudflats and deposit eggs in them. However, these burrows are filled with extremely hypoxic water, in which eggs could not survive. To secure embryonic development within their burrows, the burrow-guarding parental fish (a male or mating pair) store fresh air in an egg chamber, located near the bottom or at mid-depth in a burrow, by transporting mouthfuls of air during each low tide. The Japanese mudskipper, Periophthalmus modestus, is the best-studied species regarding reproductive strategies. The air-supplying behavior appears to be predominantly governed by the oxygen levels within egg chambers, but also by some other factor that is possibly related to the tidal cycle. When embryonic development is complete, the burrow-guarding male P. modestus removes the air from the egg chamber and releases the air outside the burrow on a nocturnal rising tide. Consequently, the tide floods the egg chamber and induces hatching. Because P. modestus eggs only have a 5–6 day window for hatching competence, the male's initial selection of the position for the burrow in the intertidal zone and the timing of spawning relative to the tidal cycle are both important factors in hatching success. This is particularly crucial for those burrows in higher intertidal zones, which may be reached only by spring high tides. Not much is known for other mudskippers, but it is likely that they also employ similar reproductive strategies. The objective of this review is to summarize available information on reproductive strategies of mudskippers, and to discuss future directions to better elucidate mechanisms and adaptive significance for the reproduction of mudskippers. Further comparative studies with both mudskippers and other oxudercine gobies dwelling mudflats could shed new light on how vertebrates solved problems of reproduction when they expanded habitats to environments in an air-water interface.

Direct Evidence For Aerial Egg Deposition In The Burrows Of The Malaysian Mudskipper, Periophthalmodon schlosseri by Atsushi Ishimatsu, Tatsusuke Takeda, Yko Tsuhako, Tomas T. Gonzales and Khay Huat Khoo, Ichthyological Research, 56(4): 417-420 (13th June 2009) [Full paper downloadable from here]

Ishimatsu et al, 2009 wrote:The presence of mudskipper eggs in an air-filled chamber was confirmed by direct endoscopic observation of intact burrows of Periophthalmodon schlosseri in a mudflat in Penang, Malaysia. For all five burrows from which video images of egg chambers were successfully obtained, the presence of air was unequivocally demonstrated by the existence of an air-water interface inside the chambers. Of these burrows, eggs were found in two, but not in the others. Eggs were laid uniformly in a monolayer on the inner top surface of the chamber. The much brighter color of the surface mud of the egg chambers than the surrounding mud, irrespective of the presence or absence of the eggs, suggested that the surface mud had been oxidized by deposited air.

A supplementary article can be read on this subject here (downloadable PDF).

Of course, we have one other known instance of a fish laying eggs out of water, namely Copella arnoldi, a South American Characin that leaps out of water to lay its eggs on the underside of overhanging plant leaves, keeping them moist by periodically splashing them. Indeed, this behaviour has been observed many times in the aquarium, as the fish has been bred in captivity since the late 1930s, and many hobby aquarists have enjoyed the pleasure of seeing these acrobatic fishes mating in their home aquaria. Indeed, it's not that difficult to persuade the fish to breed in captivity, and Tropical Fish Hobbyist magazine published a photographic account of this fish's breeding back in 1977 - I have the issue in question.