Matt Clarke on the Dusky sleeper goby, Eleotris fusca, which has recently turned up in shipments labelled Oxyeleotris marmoratus.
Common name: Dusky sleeper goby
Scientific name: Eleotris fusca
Origin: This is an extremely widespread amphidromous species found in freshwater, brackish water and the oceans from Polynesia to East Africa. Museum specimens have been recorded from Indonesia, Hawaii, the USA, Tanzania, China, the Comoro Islands, Amer Somoa, Fiji, the Cook Islands, Guam, French Polynesia, Madagascar, Malaysia, the Maldives and India! As a result, this species could turn up in imports from almost anywhere.
Size: Adults can reach sizes of around 25cm/10", and are fairly stocky.
Diet: Stomach analyses of wild fish show that E. fusca is a predator that feeds on crustaceans and small fishes.
Water: These were in freshwater, but would probably be best in brackish water. According to Mauge (1986), juveniles, such as this one, are usually found in saline lagoons and estuaries. Jarayam (1981) reports that E. fusca is very common in freshwater hundreds of miles inland in India, so freshwater should be fine in most cases.
Aquarium: These predatory fish have quite big mouths and should only be kept alongside fish that wouldn't be considered food. Adults will need a spacious tank of 120cm/48" or more, furnished with bogwood and rocks to provide shelter.
Breeding: According to Pethiyagoda (1991), this species spawns on small-leaved plants and guards the brood.
Similar species: Quite a few eleotrids look like this one, and these were originally imported as Oxyeleotris marmoratus, which do look quite similar. Eleotrids in general are tough to identify, and there are hundreds of them... I had to resort to expert help from taxonomist Dr Gerald Allen. He told me: "I'm not 100% certain, but I'm fairly positive this is Eleotris fusca, which is widely distributed on high islands of the Indo-Pacific."
Identification: To double check the ID, I did some ray counts, and it appears that this fish does have the 6-7 dorsal spines, and 7-9 branched dorsal rays seen in this species, although I wasn't able to check the anal fin counts. There should be 57-65 scales in the lateral series.
Availability: You'll be lucky to find these in the shops. These were imported by Maidenhead Aquatics in Harlestone Heath as Marbled sleeper gobies, O. marmoratus. Unlikely to be listed on import availability lists, or in shops, under the correct name.
Price: Expect to pay around 5-10.00.