Family Eleotridae
sleepers

The family Eleotridae contains about 35 genera and 150 species, and of these most are found in the tropical Indo-Pacific. They are commonly known as sleepers, or gudgeons in Australia and New Guinea, and in New Zealand they are called bullies. The family is similar to Gobiidae but generally lacks the pelvic fin fusion that creates a “sucking disc” in gobies. The majority of eleotrids lives in brackish or fresh water. Only a few species are truly marine, but many fresh water species have a marine larval stage and return inland as juveniles. They are carnivorous, and in turn are eaten by humans in many parts of their range. Sixteen species of eleotrids are listed as near threatened or vulnerable to extinction. (Berra, 2001; Helfman, Collete, and Facey, 1997; Moyle and Cech, 2000; Nelson, 1994; The World Conservation Union, 2002; Wheeler, 1985)

Eleotrids can be found worldwide between the 40th parallels (tropical and subtropical regions), reaching farther south in New Zealand. They occur on five continents and are common in the islands of the Indo-Pacific. (Allen and Robertson, 1994; Berra, 2001; Nelson, 1994)

Eleotrids occupy mostly fresh and brackish water. While only a few species are marine, many freshwater eleotrids spend their larval stage in the ocean and return inland as juveniles. They live in tropical and subtropical areas, and a few occur in warm temperate waters. They are common residents of mangrove and other estuarine environments. Eleotrids can be found near rocky reefs in bays, in intertidal areas, and in streams or ponds. Some prefer still water among aquatic vegetation. Many live on muddy substrates (bottoms)—most eleotrids are benthic (bottom-dwelling)—but a few are free-swimming. (Allen and Robertson, 1994; Berra, 2001; Graham, 1997; Kuiter, 1993; Nelson, 1994; Wheeler, 1985)

The family Eleotridae contains about 35 genera and 150 species. Some estimates range as high as 300 species. The variability in estimates may be due in part to lack of clarity about where to place members of the family; eleotrids are very similar to gobies. Hoese and Gill (1993) created the family Odontobutidae and placed in it three genera formerly contained in Eleotridae (Nelson 1994). Some forms are still undescribed. (Kuiter, 1993; Nelson, 1994)

Eleotrids are small, most measuring between three and 20 cm, although one species, Oxyeleotris marmorata, reaches 66 cm. They are similar to members of the family Gobiidae, with similar head shape, an elongate body, no lateral line, and two separate dorsal fins. They are generally distinguished from Gobiidae on the basis of their separated pelvic fins that do not form a sucking disc. The degree of pelvic fin separation varies, however, and cannot reliably be the only characteristic used to identify eleotrids. Eleotrids can have cycloid or ctenoid (rough-edged) scales. They lack sensory pores, and have canals only on the head. Their mouths, filled with several rows of conical teeth, can be upturned or terminal, but never inferior. The first dorsal fin contains two to eight flexible spines, and a single spine heads the second. Many eleotrids have a well-developed swim bladder, although they are generally benthic (bottom-dwelling). Some have dull, brownish or dark coloration, while others are colorful. One species that lives in wells and sinkholes, Milyeringa veritas, is white or pinkish and has no eyes. Some eleotrids may be permanently sexually monomorphic (males and females alike), as is the case with most reef-dwelling gobies, but males of some species develop distinctive coloring for courtship, or when excited by the presence of a competitive male. During the breeding season a hump on the head behind the eyes appears on males in the species Hypseleotris galii. (Click here to see a fish diagram). (Allen and Robertson, 1994; Berra, 2001; Graham, 1997; Helfman, Collete, and Facey, 1997; Kuiter, 1993; Nelson, 1994; Thresher, 1984)

Many freshwater eleotrids are amphidromous: after hatching they float downstream to brackish or marine waters where they pass through a planktonic larval stage, growing and feeding for a few months before they migrate back to fresh water as juveniles. This marine stage is thought to indicate that Eleotridae originated as a marine family. Some freshwater gobies develop without a planktonic larval stage, becoming a benthic juvenile directly after hatching, and this may be the case for some eleotrids as well. (Berra, 2001; Helfman, Collete, and Facey, 1997; Moyle and Cech, 2000; Nelson, 1994; Thresher, 1984)

No information was found regarding the lifespan in Eleotridae. However, some small gobies (similar in many respects to eleotrids) may mature quickly and live only one or two years. (Thresher, 1984)

Eleotrids earned their common name, “sleeper,” for their habit of remaining still on the bottom or positioned near an object in the water while they wait for prey. When chasing prey, however, they can perform very quick movements. Some, such as Hypseleotris klunzingeri, form dense schools in midwater. Many eleotrids occupy habitats that can become low in oxygen (hypoxic), and some can breathe air through the skin on the front of the head. When Dormitator latifrons encounters hypoxic waters, oxygen begins to fill the gas bladder and the top surface of the head begins to fill with a thick bed of capillaries. In order to attain positive buoyancy and keep the head exposed to air, the gas bladder must fill completely, but this may take up to six hours. Until the gas bladder fully inflates, these fish use submerged objects to prop themselves up. The top of the head does not become fully vascularized for 10 to 20 days. (Berra, 2001; Graham, 1997)

Eleotrids (except, presumably, for one eyeless species, Milyeringa veritas) recognize food and potential mates by sight. During breeding season males of some species change color, providing a visual message to potential mates and competitors. Other modes of communication likely exist, but no information was found regarding these or non-visual perception channels. (Allen and Robertson, 1994; Berra, 2001; Helfman, Collete, and Facey, 1997; Moyle and Cech, 2000)

Eleotrids are carnivores that feed on crustaceans and other benthic invertebrates, small fishes, and insects. Many species pass through a marine larval stage during which they feed on plankton. (Allen and Robertson, 1994; Berra, 2001; Graham, 1997; Helfman, Collete, and Facey, 1997)

Some eleotrids have dull coloring that may help them hide from predators, and some form dense schools, which protect individual fishes from predation. (Berra, 2001; Kuiter, 1993)

Often the major predators in island stream systems, eleotrids form an important element of the freshwater fauna in the regions in which they are found. Many impact not only the crustaceans, fishes, and insects on which they feed as adults, but also join the marine planktonic ecosystem as larvae. Eleotrids are able to occupy various habitats, including brackish and hypoxic (low-oxygen) areas. (Allen and Robertson, 1994; Berra, 2001; Graham, 1997; Helfman, Collete, and Facey, 1997)

No specific information was found concerning any negative impacts to humans.

Eleotrids are used for food in many regions. The freshwater species Dormitator maculates (fat sleeper) is considered a delicacy in Thailand, Sumatra, Borneo, and the Malay Peninsula. Eleotrids are easy to breed in captivity and are used as aquarium fish. (Berra, 2001; Wheeler, 1985)

Based on information gathered in 1994, 16 species within Eleotridae are near threatened or vulnerable. Their populations are either small in terms of adult individuals or in terms of total area in which they are found, rendering them vulnerable to human exploitation, pollution, hybridization, competitors, parasites, or disease. (The World Conservation Union, 2002)