Marble cat fish
Clarias batrachus (Linnaeus, 1758)
Common names: Marble cat fish, Walking catfish (English), Magura (Sinhala)
Synonyms: Clarias assamensis Day, 1877; Clarias punctatus Valenciennes, 1840; Silurus batrachus Linnaeus, 1758;
This species complex includes true C. batrachus, confined to Indonesian island of Java, C. magur found in northeast India and Bangladesh and three closely related undescribed species from Indochina, the Thai-Malay Peninsula, Sumatra, Borneo and the Philippines. The undescribed species have been referred to as Clarias cf. batrachus. It is likely that all species in this complex are utilized in the tropical fish aquarium trade. As a result, it is possible that information on the behaviour and ecology of introduced populations of C. batrachus may actually be for the closely related undescribed species that have been confused with true C. batrachus (Ng & Kottelat, 2008).
Identification characters: D 63-74; A 47-58; P I.8; V I5; C i,7,7,i
It has a posterior compressed body that tapers towards the tail. The head is flat and wide with small eyes. There are four pairs of barbels on the upper and lower jaws. The mouth is broad with a series of small pointed teeth. The dorsal and an anal fin are long and terminate in a lobe near the caudal fin. The body is scale-less and the colour is typically a uniform shade of grey or grey-brown with many small white spots along their sides (Froese & Pauly, 2014). However, albinos and calico morphs are also present. It has a large accessory breathing organ that has given it the ability to breathe atmospheric oxygen.
Total length: Generally 26 cm, but can grow up to 47 cm
Morphologically similar species:
Clarias batrachus resembles the native endemic species of catfish, C. brachysoma and Heteropneustes fossilis. However, C. brachysoma can be distinguished from C. batrachus by the presence of D 70; A 53-60; P I.8; V I.5; C 21, and from H. fossilis which has a short small dorsal fin.
History of introduction:
This fish has been taught to be accidentlly introduced to natural habitats in Sri Lanka from tropical fish aquaria (Silva & Kurukulasuriya, 2010).
Clarias batrachus is native to the island of Java, Indonesia. It has been introduced widely within Asian countries such as Japan, Guam, Sulawesi-Indonesia, Papua New Guinea, Philippines and Taiwan, as well as Florida, in the United States (Froese & Pauly, 2014).
In Sri Lanka, this species has established itself in the rivers, streams and canals of the Western and the North-western Provinces (Silva & Kurukulasuriya, 2010).
Dispersal and reproduction (breeding and dispersal):
Females lay a stream of adhesive eggs in excavated nests in submerged mud banks and dykes of flooded rice fields. In its native range in Java, C. batrachus spawns during the rainy season when rivers rise (Knud-Hansen et al., 1990).
Impact on native species and habitats:
The impact on native species in Sri Lanka or elsewhere is not reported. However, due to their ecological similarity, C. batrachus could compete with the native catfish species, C. brachysoma and H. fossilis.
Direct exploitation/destruction of native species:
The direct impact of this species on the native fauna is not reported in Sri Lanka or elsewhere. It has been reported to compete with native catfish species in Florida, USA (Shafland, 1996) and prey on native tadpoles in small ponds (Baber & Babbitt, 2003). Its diet consists of insect larvae, earthworms, shells, shrimps and small fish. In Florida, walking catfish are known to invade aquaculture farms and enter ponds to prey on fish stocks (IUCN SSC Invasive Species Specialist Group, 2010).
In Thailand walking catfish are heavily utilized as food fish. It is highly sought after in the aquarium trade in Sri Lanka.
Natural threats (predators):
It is probable that otters and predatory birds such as storks and herons may feed on them, but no such observations are reported.
Prevention and control:
No control efforts are recorded in Sri lanka to date. Thus containment, to stop further spread in to other areas, is the only available method.