Fishes of the genus Eleotris are a group of gobioids commonly known as the spinycheek sleepers. I haven't found a definite statement as to why they're called sleepers, but presumably it's because, as sit-and-wait ambush predators, they spend a lot of time lying around on the bottom. Eleotris species are found in tropical and subtropical waters around the world, mostly in estuaries and freshwater. They are smallish fish, with most species seeming to be in the ten to twenty centimetre size range. The 'spinycheek' part of the vernacular name refers to the presence of a hook-like spine on the lower corner of the preoperculum (the bone running between the cheek and the gill cover on the side of the head). This spine may be covered with tissue and so not always readily visible, but Pusey et al. (2004) note that it 'can be easily detected by running a thumbnail lightly, and carefully, along the preoperculum margin'. Carefully, I think, is the operative word here.
The species of Eleotris are mostly a conservative bunch appearance-wise, and the genus seems to have gotten a reputation for being difficult to work with taxonomically (it doesn't help matters that for a long time 'Eleotris' was something of a dumping ground for generalised gobioids). The Japanese species were revised in 1967 by Akihito (yes, that Akihito), the West African species have been revised by Miller (1998), and the North and South American species by Pezold & Cage (2002), but species from the remainder of the Indo-Pacific remain unrevised. There has been some disagreement over the status of a group of New World species classified in the genus Erotelis, which resemble Eleotris species but are generally more elongate and have higher numbers of fin rays (Pezold & Cage 2002). Miller (1998) felt that this genus should be synonymised with Eleotris, but Pezold & Cage (2002) argued that its members were distinct enough to be kept separate. A molecular phylogenetic analysis of the gobioids by Thacker & Hardman (2005) suggested that 'Erotelis' is nested within Eleotris, which may support their synonymisation.
The sleepers are amphidromous, meaning they spend part of their life in the sea. Sleepers enter the sea as larvae, returning to fresher waters as they mature. As a result of this marine stage in the life cycle, individual species of Eleotris may be widespread and can often be found in places such as oceanic islands that lack populations of permanently freshwater species. It has even been suggested they may cross oceans: Miller (1998), noting similarities between species on either side of the Atlantic, suggested that this may be the result of trans-Atlantic dispersal. Among the evidence cited in favour of this possibility was the record in 1987 of a specimen of the northern South American species Eleotris pisonis from the island of St Helena in the mid-Atlantic. However, Miller also noted that the amount of time it would take to disperse across the Atlantic is greater that the time it would take for the larva to develop to maturity (and mature Eleotris are not known from the open sea). Pezold and Cage (2002) were more skeptical about the possibility of trans-Atlantic dispersal, even though they admitted to being unable to identify any characters distinguishing the Caribbean E. amblyopsis from the West African E. daganensis. They queried whether the St Helena record may have been an individual transported in ship ballast water, rather than an unaided dispersal.
Miller, P. J. 1998. The West African species of Eleotris and their systematic affinities (Teleostei: Gobioidei). Journal of Natural History 32 (2): 273-296.
Pezold, F., & B. Cage. 2002. A review of the spinycheek sleepers, genus Eleotris (Teleostei: Eleotridae), of the western hemisphere, with comparison to the West African species. Tulane Studies in Zoology and Botany 31: 19–63.
Pusey, B., M. Kennard & A. Arthington. 2004. Freshwater Fishes of North-eastern Australia. CSIRO Publishing: Collingwood.
Thacker, C. E., & M. A. Hardman. 2005. Molecular phylogeny of basal gobioid fishes: Rhyacichthyidae, Odontobutidae, Xenisthmidae, Eleotridae (Teleostei: Perciformes: Gobioidei). Molecular Phylogenetics and Evolution 37: 858-871.