Delicate Blue-eye (Pseudomugil tenellus)

The Pseudomugil Blue-eyes are an interesting group of fishes, and one whose relationship to other families might not be readily apparent at first glance. These small, slender-bodied creatures could easily be mistaken for being a type of live-bearer, or maybe a rasbora, or perhaps even a ricefish. But the similarities to these are only superficial and don’t reflect any particularly close kinship.

The shape of the dorsal fins on these little fishes is helpful in this regard. The separate spiny dorsal and soft dorsal fins are a trait we see among the rainbowfishes (Melanotaeniidae), which are a fairly close relation here. But even closer is the small family Telmatherinidae, whose most familiar member is the Celebes Rainbowfish (Marosatherina ladigesi). And if we step back a little further in this group’s evolutionary history, we’ll run into the silversides (Atherinidae), which aquarists are most likely to recognize as being the silvery feeder fishes that are sold frozen.

The interesting thing about this diverse group, the Atheriniformes, is that there have been multiple transitions from an ancestrally marine habitat into freshwaters. And, for this reason, we often encounter an unusual amount of salt-tolerance, even in species which normally spend little of their time near saltwater. For instance, within the 16 species of Pseudomugil, there are some (e.g. P. signifier, P. cyanodorsalis) which can thrive in full marine conditions, whereas others (e.g. P. gertrudae) are often encountered in highly acidic blackwater swamps.

But many species fall somewhere in the middle, with a broad tolerance for water conditions. The Delicate Blue-eye is one of these, being known from a number of acidic freshwater swamps and slow-moving streams near the coastline of Northern Australia and neighboring portions of New Guinea. During the last ice age, some 10,000 years ago, these regions were all connected into a single landmass due to the drastically lowered sea levels of the time.

The genus as a whole is entirely limited to these two countries, and the same largely applies to the other rainbowfish families previously mentioned. Australia, of course, doesn’t have much in the way of native freshwater fishes outside these groups, so they have succeeded in filling an otherwise untapped ecological niche. These are, in a sense, the barbs and tetras and livebearers of this continent.

As for aquarium care, each Pseudomugil has its own ideal water parameters. For the Delicate Blue-eye, just about anything will do (though it’s unclear if full marine conditions are tolerated). These are shoaling fishes, so keeping a moderately sized group is highly, highly recommended. Having more females than males is wise, though aquarists naturally gravitate towards the more colorful males, whose fins are larger and richly pigmented in shades of amber, with contrasting margins of black and white. The natural diet of these fishes in mostly comprised of small crustaceans, like water fleas (cladocerans) and midge larvae (chironomids), but they are quite unfussy in captivity and will take to nearly any food offered.