The acid-water fish of the wallum

In the acid, tea-dark water of the wallum — its swamps, its slow blackwater streams, the tannin-stained hollows between the sand ridges — live two fish you could cup in your hands, and that live almost nowhere else on Earth. One is the honey blue-eye (Pseudomugil mellis), a small, glinting, amber-and-gold fish of the coastal sand country; the other is the Oxleyan pygmy perch (Nannoperca oxleyana), a stubby little perch of the same sour waters. Both are tiny, both are restricted to the wallum blackwater, and both are now nationally threatened. They are the animal counterpart to the wallum's carnivorous plants and its acid frogs: specialists that have turned a hostile chemistry into a home.

The water they live in is genuinely punishing. Leached from the peaty, acidic sand, wallum blackwater runs sour — often well down toward the acidity of vinegar — and for most freshwater animals that is simply lethal, stripping the salts out of their bodies and burning delicate tissue (see blackwater-acidity). So why would a fish evolve to live there? For the same reason the acid frogs do, and it is the deep logic of the whole sand country applied to animals (see species-dependence): the sour water is hostile to almost everything, including the things that would otherwise eat these fish or crowd them out. Many of the larger predators cannot follow them in. And, most importantly, the water is largely hostile to the mosquitofish — Gambusia holbrooki, the pugnacious little import that has invaded so much of Australia's fresh water and that nips fins and eats the eggs and young of native fish and frogs wherever it goes. In the most acidic wallum waters, the mosquitofish struggles. So the blackwater that looks like a death trap is, for these two natives, closer to a fortress: by evolving to endure the acid, they have bought themselves a refuge their worst enemies mostly cannot enter.

Now the calibration, because it is the whole honest heart of the story and the easy version gets it wrong. The acid is a partial refuge, not a sealed wall. Low pH stresses and limits the mosquitofish; it does not absolutely exclude it. Gambusia can and does get into wallum waters, especially wherever the acidity is diluted or the chemistry softened, and recovery planning for these species still calls for active mosquitofish control — which it would not, if the acid did the job on its own. So read the fortress as a moat that is deep but fordable: it keeps most of the enemy out most of the time, and that margin is enough for the fish to persist, but it is a margin, not a guarantee.

Which is exactly why these fish are so fragile, and why you protect them by protecting the water rather than the fish. A fortress works only while its walls stand, and the walls here are made of chemistry. Anything that sweetens or dilutes the acid blackwater — run-off from a new estate, fertiliser, a drain that lets ordinary water in — lowers the drawbridge and lets the mosquitofish and the competitors march straight through the gap the acid was holding shut. Anything that drops the water table can dry the breeding sites out altogether. So to keep the honey blue-eye and the Oxleyan pygmy perch, you do not manage the fish at all; you protect the strange, sour, acid water and the whole hungry wallum system that brews it — the same conclusion the acid frogs force, reached from the water this time instead of the land.