Pseudomugil signifer is a small, colourful, endemic native fish species inhabiting freshwater streams and estuaries in coastal drainages along much of the east coast of Australia and offshore islands. They are the most common and abundant freshwater/estuarine species along the east coast of Australia. They were first discovered near Sydney, New South Wales in the 1860’s and were the first blue-eye species to be scientifically described. A few years’ later specimens were collected from northern Queensland. Various name changes followed, and in the late nineteenth and early twentieth centuries, they were separated into two species, the northern Pseudomugil signatus and the southern Pseudomugil signifer. Also, it was suggested that a population resident on the offshore Low Isles and some northern mainland populations were distinctive and were afforded subspecific status as Pseudomugil signatus affinis.
The various populations exhibit remarkable morphological variation throughout their range, which is evident from their confused taxonomic history. In 1979, researchers using electrophoretic analyses and a large sample of specimens taken from 14 localities along the east coast determined that all populations were scientifically indistinguishable from one another. Although they did recognise that there were some slight differences. This, however, is inconsistent with recent genetic research that indicated there are extensive differences among the various populations.
The research provided a good reason for suggesting that Pseudomugil signifer may represent at least two distinct species. Support for this contention is not only consistent with some of the previous taxonomic designations, but is also in keeping with recent behavioural studies. Breeding experiments showed that fish from opposite ends of the species range would not interbreed. In Queensland, there are two major geographical populations, those north and south of the Herbert River, with an intermediate form in the Townsville area. These results suggest that further work is warranted on the taxonomic status of all Pseudomugil signifer populations.
It has long been the belief of native fish hobbyists in Australia that the various forms of Pseudomugil signifer are different, particularly the northern and southern populations. Also that the variety found in the Townsville area is different. I maintained and bred populations from Harvey and Lacey creeks in north Queensland, the Ross River variety from Townsville, and a number of different populations from south-east Queensland over a period of 20 years. However, you only have to maintain these species for a very short time before you realise that they not only look different, but that they behave differently as well. Therefore, they should be maintained and bred within their own localised groups and it is wise not to interbreed the various geographical varieties.
Pseudomugil signifer is one of the most readily identifiable and ubiquitous members of the Australian fish fauna. They have a moderately compressed and elongated semi-transparent body that can vary in colour from pale olive, yellow to bluish, with fine, dark coloured scale outlines on the upper body. They have two dorsal fins, very close together, the first much smaller than the second. As the common name suggests, the iris is blue. The operculum and belly region are silvery. There is often a mid-lateral row of 10~12 vertically-elongated white or reflective spots along the side of the body. The males are larger and more colourful than females. The males display spectacular fin embellishments that are rapidly raised and lowered during courtship and agonistic encounters with other males. Due to the wide geographical range of this species the colours and markings on the body and fins can be variable. There is also substantial inter-population variation in male body size and fin length. Male specimens from northern populations can reach 90 mm and females 65 mm, although they can be considerably smaller over much of their range. Southern populations rarely exceed 40 mm.
Male specimens from north Queensland populations (especially specimens from Harvey and Laceys Creeks) have extremely long extended filaments on both the dorsal and anal fins. These filaments are usually shed during capture and if not, once placed in the confines of an aquarium, are nipped off by the other fish. From my experience, these fin extensions never re-grow or appear in captive populations. They have a silvery to yellowish body, with the elongated parts of fins blackish, also the margins of the second dorsal and anal fins. Edges of upper scales dark, and often a dark stripe along the side. The young are yellowish-brown with dark spots on the dorsal and caudal fins. A black band along the middle of sides with a similar but shorter dark band above and below it on the caudal peduncle. About six rows of dark spots along the scale rows, fading out posteriorly. Spawning males display a coppery-gold body colouration, particularly along the lower jaw region.
Pseudomugil signifer [Bowen, Qld] – photo© Günther Schmida
The first thing that is evident about the northern variety at Pseudomugil signifer is the size of these fish in comparison to the southern forms. The specimens I received from Laceys Creek in 1980 were about 60 mm in size and a number of the males had long extended filaments on both the dorsal and anal fins, extending past the tail. Another obvious difference is that northern males are also highly territorial and aggressive and will often kill sub-dominant males. Outside of breeding, aggression can be suppressed by maintaining them in reasonably sized aquaria in company with a small group of rainbowfishes.
The Ross River variety is a large, deep-bodied species with huge fins that are wide and long, but less intense in colouration. They usually have less body colour, but have a row of brilliant blue or purple reflective scales along the posterior section of the lateral line, often merging to form a continuous band, that is flashed on and off like a neon sign as they display to passing females. They are generally found in brackish reaches of the Ross River and surrounding streams. A similar form extends south, down at least as far as Eurimbula Creek just north of the Town of 1770.
Southern populations have a body colouration of translucent to olive-greenish above, canary yellow below; the caudal peduncle is tinged with red. There is sometimes a broad dark band from pectoral to tail. The first and second dorsal spines are long and white though blackish at the base, while the rest of the fin is translucent. The second dorsal has the front and exterior margins black while the rest of the fin is yellow or orange. The anal fin is similar to the second dorsal. The caudal fin can be clear to orange or yellow with the outer rays, tips of lobes, and sometimes the central rays blackish. The pectoral fins are clear to opaque, with upper rays black. The ventral fins are usually bright yellow or orange. As the name indicates, the eyes are a beautiful blue. Females are less colourful; have smaller rounded clear dorsal and anal fins with a dark line on the anterior edge of the second dorsal fin. Specimens collected from freshwater habitats generally have deep orange coloured dorsal and anal fins whereas specimens from saltwater or brackish water habitats have yellow fins. Fish from acidic, tannin strained streams in south-eastern Queensland also show deep orange on the dorsal and anal fins. However, colour is extremely variable and will depend upon the mood of the fish, water conditions and diet.
Distribution & Habitat
Pseudomugil signifer is the most widely distributed blue-eye in Australia. They have extensive distribution from Merimbula Lake just north of Eden on the southern coast of New South Wales to Cape York Peninsula including islands in the Torres Strait. They have also been found in the Embley and Mission rivers near Weipa on the west coast of Cape York Peninsula. They are abundant in freshwater habitats, and inhabit rainforest streams, riverine habitats and freshwater swamps, but do not usually penetrate far inland. They can tolerate brackish to fully marine conditions, being found in tidal mangrove creeks, estuaries and saltmarshes, and on several offshore islands, including Moreton Bay in south-east Queensland. They are frequently found in the waters of canal housing estate developments on the Gold Coast in south-east Queensland. Diadromous migration is not an essential requirement of the species. Rather, the species is characteristic of the estuarine-freshwater interface. They inhabit waters with a temperature range of 15-28° Celsius, and pH 5.5-8.3.
Pseudomugil signifer is often recommended as a biological agent for mosquito control for frog breeding habitats such as backyard ponds and water features. However, if you want frogs and tadpoles in your garden and pond, never introduce fish into your pond as they will eat both the frog eggs and tadpoles. This is even more important if you have rare or endangered frog species in your area. All native fish will eat tadpoles, although the smaller native fishes such as Pseudomugil signifer will only eat small tadpoles. As a general rule, an absence of fish will result in a larger frog population, which also reduces mosquito numbers to some extent. Many tadpoles will eat mosquito larvae during their normal feeding activities. Mosquito developmental rates may be reduced if frog ponds contain dense populations of large tadpoles.