Eleotridae

Diversity

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.

Geographic Range

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.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• oriental oriental
  • native native
• ethiopian ethiopian
  • native native
• neotropical neotropical
  • native native
• australian australian
  • native native
• oceanic islands oceanic islands
  • native native
• indian ocean
  • native native
• pacific ocean pacific ocean
  • native native
• mediterranean sea
  • native native

• Other Geographic Terms
• island endemic island endemic

Habitat

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.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine
• freshwater freshwater

• Aquatic Biomes
• benthic benthic
• reef reef
• lakes and ponds
• rivers and streams
• coastal coastal
• brackish water brackish water

• Other Habitat Features
• estuarine estuarine
• intertidal or littoral intertidal or littoral

Systematic and Taxonomic History

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.

Physical Description

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 ).

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike
• male larger
• sexes colored or patterned differently
• ornamentation ornamentation

Development

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.

Reproduction

No information was found that addresses mating systems specifically in eleotrids, but Thresher (1984) includes Eleotridae in his general account of reproduction in the suborder Gobioidei . Gobies exhibit a wide variety of mating systems but most seem to be promiscuous, either organized into a hierarchical social system or small territories maintained by individuals. At least one species of eleotrid ( Thalasseleotris adela ) is usually found in pairs. In gobies, a typical mating sequence begins with nest preparation by the male, which involves clearing and cleaning the area where eggs will be deposited. The female’s readiness for spawning is evidenced by her swollen ventral area. The male swims back and forth between the female and the nest site and in some cases he will nudge her with his snout. Eleotrid courtship behavior probably follows a similar pattern, with some males assuming intense courtship colors and leading females to the nest.

• Mating System
• monogamous monogamous
• polygynous polygynous
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Eleotrids attach their eggs to vegetation or a substrate (bottom surface). Females in Hypseleotris compressa deposit up to 3000 eggs. No other information was found that addresses reproduction specifically in eleotrids, but Thresher (1984) includes Eleotridae in his general account of reproduction in the suborder Gobioidei . Most gobies have extended spawning seasons with peak spawning depending on the species, but in colder regions breeding may only occur once or twice a year. Females may deposit from five to several hundred eggs, which the male then fertilizes. In estuarine species the lunar cycle is thought to play a role in spawning behavior as well as larval recruitment.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • external external
• oviparous oviparous

Males in Hypseleotris compressa guard the nest. No other information was found that addresses parental care specifically in eleotrids, but Thresher (1984) includes Eleotridae in his general account of reproduction in the suborder Gobioidei . In most cases, male gobies guard the eggs after they are fertilized, and even if females are permanently paired they rarely take part in parental care. The young probably stay close to adults for a period of time after hatching. In some freshwater island species parental care is not practiced at all. Many eleotrids may fall into this category, since the larvae are carried downstream to the ocean where they feed and grow before ascending the freshwater streams.

• Parental Investment
• no parental involvement
• male parental care male parental care

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• natatorial natatorial
• motile motile
• social social

Communication and Perception

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.

• Communication Channels
• visual visual

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

Food Habits

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.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • insectivore insectivore
  • eats non-insect arthropods

Predation

Some eleotrids have dull coloring that may help them hide from predators, and some form dense schools, which protect individual fishes from predation.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

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.

Economic Importance for Humans: Positive

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.

• Positive Impacts
• pet trade pet trade
• food food

Economic Importance for Humans: Negative

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

Conservation Status

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.

Additional Links

Encyclopedia of Life

Contributors

Monica Weinheimer (author), Animal Diversity Web.

R. Jamil Jonna (author), Animal Diversity Web.

References

Allen, G., D. Robertson. 1994. Fishes of the Tropical Eastern Pacific . Honolulu, HI: University of Hawaii Press.

Berra, T. 2001. Freshwater Fish Distribution . San Diego, CA: Academic Press.

Böhlke, J., C. Chaplin. 1968. Fishes of the Bahamas and Adjacent Tropical Waters . Wynnewood, PA: Published for the Academy of Natural Sciences of Philadelphia by Livingston.

Graham, J. 1997. Air-Breathing Fishes: Evolution, Diversity, and Adaptation. . San Diego, CA: Academic Press.

Helfman, G., B. Collete, D. Facey. 1997. The Diversity of Fishes . Malden, MA: Blackwell.

Kuiter, R. 1993. Coastal Fishes of South-Eastern Australia . Honolulu, HI: University of Hawaii Press.

Moyle, P., J. Cech. 2000. Fishes: An introduction to ichthyology – fourth edition . Upper Saddle River, NJ: Prentice-Hall.

Nelson, J. 1994. Fishes of the World – third edition . New York, NY: John Wiley and Sons.

The World Conservation Union, 2002. "IUCN 2002" (On-line). 2002 IUCN Red List of Threatened Species. Accessed August 23, 2003 at http://www.iucnredlist.org/ .

Thresher, R. 1984. Reproduction in reef fishes . Neptune City, NJ: T.F.H. Publications.

Wheeler, A. 1985. The World Encyclopedia of Fishes . London: Macdonald.