The Cape Blue-eye was scientifically described by Allen & Ivantsoff in 1984. Its scientific name is Pseudomugil majusculus. It was first collected for scientific purposes in 1979 by Collette, an American ichthyologist. Pseudomugil majusculus is easily confused with Pseudomugil signifer.

Description
The Cape Blue-eye can reach a length of 5 cm (2 inches). The body has pale yellowish shade and is decorated with dusky scale outlines. The lower edge of the breast is yellow. White dots form a horizontal row along the middle of the body. The anal and second dorsal fins have white margins.  

Geographical distribution, habitat and conservation
The Cape Blue-eye lives in eastern Papua Guinea, including the Fergusson Island in the D'Entrecasteaux. (The D'Entrecasteaux Islands are located near the eastern tip of New Guinea in Milne Bay Province of Papua New Guinea.) The Cape Blue-eye has been found in a salty mangrove swamp and in a freshwater creek near the ocean. The Cape Blue-eye is listed as “Lower Risk: least concern” at the IUCN Red List of Threatened Species.

Keeping Cape Blue-eye in aquariums
The Cape Blue-eye is not a common aquarium species but it adapts well to a life in captivity and can be kept in species aquariums or be combined with peaceful species of similar size in a community set up. A 30 litre (8 gallon) aquarium is large enough for a small group of Cape Blue-eyes, as long as you know how to keep the water quality up in such as small aquarium. The recommended water temperature is 20-28 degrees C (68-82 degrees F). Keep the water neutral or slightly alkaline, from pH 7.0 to 7.8.

The Cape Blue-eye has been found in both a salty mangrove environment and in a freshwater creek. It is therefore probably an adaptable species that can be kept in freshwater, brackish and marine aquariums. Different populations might favour different conditions, and it is therefore always a good idea to find out from which environment your particular fishes hail. Generally speaking, the Cape Blue-eye seems to appreciate 0.5 grams of salt per litre water in the aquarium. 

Feed your Cape Blue-eye a varied diet to prevent malnutrition in the aquarium. You can for instance combine high-quality flake food with live food such as brine shrimp, daphnia and insect larvae. 

Breeding Cape Blue-eye
The Cape Blue-eye becomes sexually mature when it’s roughly 6 months old. Since it is such a rare fish in the aquarium hobby, little is known about its exact breeding habits. If you want to breed Cape Blue-eye, you can follow the guidelines for breeding Pacific Blue-eye (Pseudomugil signifier) since these two species are very similar. This means breeding your Cape Blue-eye in groups consisting of at least 2-3 females for each male. It is important to provide the females with suitable hiding spots in the aquarium, since the males are highly aggressive throughout the courting period. You should be prepared to evacuate females that aren’t interested in spawning.

Ample servings of live food are known to coax Pacific Blue-eye and many other species of blue-eye into spawning. It is also important to provide them with a suitable spawning medium in the aquarium, e.g. spawning mops or densely grown java moss. The Pacific Blue-eye seems to prefer to deposit the eggs no higher than 10 cm (4 inches) from the bottom.

The female Pacific Blue-eye can deposit up to 12 eggs each day. The safest course of action is to move the spawning medium with the eggs to a separate container, since the adult fish might otherwise devour eggs and fry. Even if you keep the water temperature in the higher end of the recommended range, the incubation period will be fairly long – up to 18 days. At a lower temperature, it can take up to three weeks before the eggs hatch. You can feed the fry infusoria and finely powdered flake food until they are big enough to eat newly hatched brine shrimp.   

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Other Blue-eyes

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Popondetta Blue-eye
Blueback Blue-eye
Forktail Blue-eye
Spotted Blue-eye
Inconspicuous Blue-eye
Honey Blue-eye
New Guinea Blue-eye
Swamp Blue-eye
Paska’s Blue-eye
Vogelkop Blue-eye
Pacific Blue-eye
Delicate Blue-eye
Red-finned Blue-eye