Pseudomugil signifer

Pacific blue-eye
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Atheriniformes
Family:	Pseudomugilidae
Genus:	Pseudomugil
Species:	P. signifer
Binomial name
Pseudomugil signifer Kner, 1865
Synonyms
Atherina signata Günther, 1867 Pseudomugil signata (Günther, 1867) Atherinosoma jamesonii Macleay, 1884

Pseudomugil signifer, the Pacific blue-eye, is a species of fish in the family Pseudomugilidae. It is a common fish of rivers and estuaries of eastern Australia, where it forms loose schools of hundreds to thousands of individuals. It eats water-borne insects, as well as flying insects that land on the water surface, foraging for them by sight. A small silvery fish averaging around 3–3.5 cm (1 ¹⁄₈–1 ³⁄₈ in) in total length, it is recognisable by its blue eye ring and two dorsal fins. It adapts readily to captivity.

Taxonomy

Austrian naturalist Rudolf Kner described the species in 1866,^[1] from a specimen collected in Sydney and taken to Vienna by the SMS Novara in 1858.^[2] Albert Günther described Atherina signata from collections in Cape York in 1867.^[3] William Sharp Macleay named a "curious little fish", collected from the Bremer River, a tributary of the Brisbane River, by one Mr Jameson of Ipswich, Atherinosoma jamesonii in 1884, which was later classified as the same species by James Douglas Ogilby in 1908.^[2] Variable across its range, the Pacific blue-eye is considered to be a single species, though has been split by some into northern signata and southern signifer, with the former found from Ross River northwards and the southern from the Calliope River south. The division occurs at a biogeographic dividing point known as the Burdekin Gap.^[3] Gilbert Whitley examined material from the Low Isles off Cairns and split the taxon into P. signifer and P. signata in 1935. In 1979, Hadfield and colleagues analysed the variations described and felt both species were more highly variable than different to each other, and that no characteristics enabled people to distinguish either species. Hence they recommended combining the species again.^[4] However, a 2004 molecular study showed the two populations were genetically distinct and suggested that they may be once again reclassified as species.^[5]

Within the northern population, five subclades have been identified: one from Ross River and Herbert River, a second from Johnstone, Barron and Tully Rivers, a third from Mulgrave/Russell River and Trinity Inlet, a fourth from Daintree and Mossman Rivers and a fifth from Low Isles and Cape Melville. Four subclades have been identified in the southern population: the first from the Don, Calliope, Pioneer and Kolan Rivers, the second from Burnett and Mary Rivers, the third from Pine River and the fourth from Clarence River southwards.^[3]

Alternate names include southern blue-eye and northern blue-eye.^[6] Species from the northern and southern extremes of the range do not appear to interbreed in captivity, suggesting that there may be two separate species within the current concept of the species.^[7]

Description

The Pacific blue-eye generally reaches a total length of around 3–3.5 cm (1 ¹⁄₈–1 ³⁄₈ in) long, males can reach 8.8 cm (3 ¹⁄₂ in) and females 6.3 cm (2 ¹⁄₂ in). The elongate body is partly transparent and pale yellow or olive with a silver operculum and belly. The scales are relatively large and longer vertically than they are horizontally. There are two dorsal fins, the first arising in line with or just posterior to the longest pectoral fin ray. The forked tail fin has rounded tips. The eye is large and has a blue iris. The bottom and top edges of the tail fin are edged with white.^[3] The male has extended filaments on its dorsal, anal and pelvic fins. There are black markings at the base of the anterior rays of the anal and rear dorsal fins, and the front (anterior edge) is sometimes white and the rear (posterior) edge greyish in colour. The male's fins may turn orange during breeding season.^[3] Preserved specimens generally discolour to yellow or tan.^[3]

The Pacific blue-eye can be distinguished from the noxious introduced eastern mosquitofish (Gambusia holbrooki) by its forked tail fin.^[8]

Distribution and habitat

The Pacific blue-eye is found from Narooma in southern New South Wales north to the Rocky River in Cape York, though is uncommon in eastern Cape York. It lives in small, generally slow moving, streams to estuaries, as well as dune lagoons and salt marshes. It is also found in brackish and marine waters on some Queensland offshore islands such as Hinchinbrook Island, Lizard Island, Low Island and Dunk Island. It has been recorded as far as 300 km (185 mi) upstream in the Mary and Dawson Rivers in Queensland. Numbers can be prolific in some locations, such as the Mary River. Conversely it is uncommon in the Elliott and Kolan Rivers. Fish species it is commonly found with include Marjorie's hardyhead (Craterocephalus marjoriae), crimson-spotted rainbowfish (Melanotaenia duboulayi), Australian smelt (Retropinna semoni) and western carp gudgeon (Hypseleotris klunzingeri).^[3]

In the wet tropics, the Pacific blue-eye is mostly found in streams flowing up to 30 cm (12 in) per second, or rarely 90 cm (35 in) per second. Within areas of fast, it shelters in areas of slower-moving water (less than 20 cm per second), sometimes in the lower half of the water column or in the lee of underwater rocks. Further south in southeastern Queensland it is mostly found in water flowing more slowly than 10 cm (4 in) per second.^[3]

Behaviour

The Pacific blue-eye is found in loose schools of hundreds to thousands of fish. They are generally found in the mid- to upper water column within 1 metre of the riverbank and often close to underwater cover.^[3]

The Pacific blue-eye is euryhaline—it can survive in a wide range of water salinities from fresh-water to marine environments. It responds to changes in salinity (and resulting change in buoyancy) by changing the volume of its swim bladder, which takes up to 6 hours and 40 minutes when salinity is reduced and around 5 hours when it is increased. In the meantime, the fish can swim with a head-up or head-down posture, which either increases or decreases buoyancy respectively. This adaptation helps the fish in the range of salinities it encounters in its estuarine environment.^[9]

Feeding

The diet is made up of water-based and terrestrial insects, flying insects, such as various types of fly, and, to a lesser extent, tiny crustaceans and algae.^[3] Field work on Narrabeen Lakes showed that Pacific blue-eyes spent time near the surface looking for dead flying insects, consuming anything below their mouth gape size. This varied from around 2.5 to 3.5 mm, and was proportional to the length of the fish. The Pacific blue-eye forages using vision, and the turbidity of the water impacted on its ability to find food.^[10]

Breeding

Female Pacific blue-eyes are sexually mature at six months of age or when they have reached 2.3 cm (⁷⁄₈ in) in standard length. Males are mature at 2.8 cm (1 ¹⁄₈ in) standard length.^[3] A study published in 2003 showed that males will preferentially choose larger females (who are more fecund as a rule) unless more energy is required to do so (such as swimming further against a current).^[11] Fish can breed and fresh and salt water. The life span of the species is around 1–2 years in the wild, and around 2–3 years in aquariums, though some males may reach 4 years of age.^[3] In an aquarium, Pacific blue-eyes spawn in gravel or moss at the base of aquatic plants.^[3]

An experiment housing Pacific blue-eyes and mosquitofish together showed that growth and breeding of the former fish were severely affected by the presence of the latter. The mechanism was unclear—there were some signs of direct aggression (bite marks on fins of Pacific blue-eyes) but stress from contact was thought to be a major factor.^[12]

References