Home

Main Index of Articles

Fish Breeding,Keeping Index

Please read the 'Agreement' section on the View Articles page before downloading this article.

African Siluriforms of the family Mochokidae

By Mitchell Mckenzie
An original for Aquarticles.com

Synodontis eupterus (Photograph by Mitchell McKenzie)

Introduction

Found exclusively in Africa, the Mochokidae family consists of 7 genera (depending on the taxonomic method used) and can be found in all water systems (Burgess, 1989, 182), ranging from the still lakes of Malawi and Tanganyika to the fast moving rivers of the Niger and Chobe. Mochokidae represents a diverse group of catfish which differ greatly in both shape and size, from the small Mochokiella paynei of just 35mm to the relatively large Synodontis acanthomias which may attain 600mm.

All catfish within this family are scaleless and there is generally a cephalo-nuchal shield which may form a sharp projection which extends to the pectoral fins (Burgess, 1989), in some species the first pectoral ray forms a spike. The dorsal fins are large with a strong primary ray. The dorsal fins of adults may become filamentous, as with S. eupterus. This is evident in the superb specimen depicted in the above photograph. The adipose fin is also fairly large and is characteristic of the family. The mandibular barbels are often branched and the maxillary barbels may have branchlets (Burgess, 1989, 184).

Most genera contain only a single species; however the Synodontis genus has some 119 species identified to date (Burgess, 1989). Sexing of Synodontis species may be problematic due to the constant changes of both colouration and fin structure which a juvenile undergoes before it reaches full maturity at approximately 3 years old

Synodontis robbianus - Photograph by Mitchell McKenzie

Many species have adopted an 'upside down' posture. This has lead to their common name of upside down catfish. This enables the fish to graze on the underside of rocks and fallen branches. In the Synodontis nigraventris and Brachysynodontis batensoda this is permanent and when viewed ventrally counter shading can be observed. Presumably the counter shading is to avoid predation. Species which do not readily invert tend to have the classic light underside and darker backs. The exact mechanism which controls this posture is not yet fully understood, but the otolith organs are thought to play an important role, as observed by Ohnishi et al, 2004.

Mochokids are fairly hardy and adaptable catfish, as their native habitats are constantly altered by fluctuations in both temperature and pH. This is readily observed during the rainy season when the heavy rains may alter the pH in either direction by as much as 2.50. Similarly temperatures may fall by 50C in only a few hours. These environmental changes may trigger certain species to spawn as it does with Central American Doradids, however this is purely speculation as little information has been gathered on breeding behavior

The following descriptions are based on the most well known species of each genus. The descriptions are sadly brief and some what limited, due in part to the space restrictions of this paper, but also to our limited knowledge of this fascinating family.

The 7 genera of the Mochokidae family - Brachysynodontis, Synodontis, Mochokiella, Mochokus, Hemisynodontis, Acanthocleithron and Microsynodontis.

Discussed genera - Synodontis, Hemisynodontis, Brachysynodontis and Mochokiella

Synodontis eupterus - Photograph by Mitchell McKenzie

Probably the most well known of all Mochokids, Synodontis represents the largest genus, containing over 100 individual species (Burgess, 1989). Species differentiation within the genus is narrow, often differing only in the number of teeth and fin rays, although skin colouration and pattern can be extremely varied. Relatively few species have been thoroughly examined in their natural habitat, but an 8 month survey of the Upper Zambezi river flood plain carried out by K.O. Winemiller and L.C. Kelso-Winemiller (1995) concluded that of the 5 species they examined all appeared to be omnivorous, their stomachs were found to contain insects, crustaceans, scales, seeds and general detritus. Ofori et al 1992, in a similar survey of the Kpong head pond in Ghana, discovered that Synodontis schall displays distinct sexual dimorphism, as the adult males have a visible short urino genital papilla during the spawning season, while mature females may contain between 2000 and 209,000 eggs.

All Synodontis species possess the weberian apparatus, a collection of 4 small bones which connect the swim bladder to the ear (Burgess, 1989), according to Ladich et al, 1996, this is actually composed of 2 structures, a pectoral spine stridulatory apparatus and a swim bladder which can vibrate with the aid of adjacent muscles. These organs enable the catfish to produce audible noises. Many species are territorial and may use sound as a warning to other individuals. The weberian apparatus may also be part of a defensive strategy to reduce predation, Burgess (1989, 182) believes that the weberian apparatus and the noise it produces has lead to the local name for Synodontis - squeaker.

Synodontis gallinae? - Photograph by Mitchell McKenzie

One interesting member of the Synodontis genus is Synodontis multipunctatus. Endemic to Lake Tanganyika (Axelrod, 2001), S. multipunctatus is a nocturnal omnivorous species which will devour larvae, insects and crustaceans, as well as algae and plant matter. It may be found in shoals of several hundred in shallow waters and may attain a size of up to 300mm (Axelrod, 2001, 675). Wisenden, 1999, describes a curious form of brood parasitism in which S. multipunctatus spawns along side Cichlids, the young catfish hatch with the fry of the Cichlid and being mouth brooders the Cichlids raise the young catfish as their own, Wisenden, 1999, believes that this is an extreme form of allo parental care. This remarkable behavior has earned the catfish the common name of cuckoo catfish.

Hemisynodontis

Hemisynodontis membranaceus is the only representative of this genus. Many authors consider H. membranaceus to be a synonym of the genus Synodontis, while others believe it to be a separate genus. A cephalo nuchal shield is present (Burgess, 1989) and is granular in appearance. The maxillary barbels are covered by a membrane and the mandibular barbels are branched. A large adipose fin is present, which extends from the relatively small dorsal fin to the forked caudal fin (Burgess, 1989). The colour is a uniform blue/black. The gill operculum contains gill rakers which form a unique palatine organ (1989, 184). This enables the catfish to filter plankton/zooplankton. H. membranaceus is found in the river Volta and its 4 tributaries. Ofori et al completed another survey of the river Volta in 2001, in which he discovered an alarming decline in the population (2001), due to several factors including over fishing and habitat destruction. He concluded that the species requires immediate assistance by implementing conservation measures. Brachysynodontis

Brachysynodontis batensoda is the only representative of this genus and is very similar to the previously discussed species, however there is no palatine organ, the maxillary barbels are not membranous and there is a sharp serrated dorsal spike (Burgess, 1989, 184). B. batensoda is a true 'upside down catfish, spending at least 90% of its time in an upside down position (1989, 184). It is usually found in swampy regions in large schools. Thompson et al, 1996 describes the species as extremely common throughout its native Nigeria and is a commercially important species although no conservation measures are in place despite heavy exploitation.

Mochokiella

Mochokiella again consists of only one species - Mochokiella paynei and is one of the smallest Mochokids, fully grown at 35mm (Burgess, 1989, 192). It originates from the former Sierra Leone and is instantly recognizable as a Mochokid. Aquarium observations have shown it to be a light sensitive species with nocturnal habits. As with many Mochokids the adipose fin is relatively large and a strong dorsal spike is present. The colouration is varied, alternating from light brown to cream with the addition of darker patches. The species is of no commercial value (Crespi, 1998) but interest has been shown in the aquarium trade where it commands a high price, possibly due to the high transportation costs involved

Conclusion

The Nile perch is an important food fish and has been introduced to many regions of Africa where it has become very successful, to the detriment of other native species. Witte et al, 1997 states that since its introduction to Lake Victoria in the 1980's species of Mochokid such as S. victoriae and S. afrofischeri have suffered heavy predation. Young Synodontis which have not yet developed locking pectoral spikes appear to be particularly susceptible. All immature Synodontis exhibit diurnal behavior (Baensch, 1997, 502) until they become nocturnal in adulthood. This leaves them prone to the attention of the Nile perch, which has diurnal habits. However Witte also states that some refuge can be sought in shallower waters as the Nile perch is largely pelagic (1997)

Arguably the most attractive member of the family is S. angelicus, which has suffered greatly at the hands of the aquarium industry, purely because of its beauty and rarity. Many attempts have been made to breed the species in captivity; most have failed (Burgess, 1989). Some success has been shown using hormone injections in a similar method used in the UK fish farming industry. The convention for the international trade in endangered species (CITES) has banned all exports of the 'Angel catfish' until its numbers have recovered (1989, 186). However many still pass through customs under the guise of another legal species. It is a sad fact that demand for the 'Angel catfish' is still rising despite its critically endangered status

Bibliography

Comments are welcome and should be directed to Mitchellmckenzie@aol.com

Electronic journals -

Wisenden BD. 1999, Allo parental care in fishes, Reviews in fish biology and fisheries, vol.9. (Available online at http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1023/A:1008865801329), [Accessed - 1/12/05]

Ladich F, Bass AH, 1996, Sonic/vocal-acoustic lateralis pathways in teleost fishes: A transneuronal biocytin study in Mochokid catfish, Journal of comparative neurology, vol.347, (4), pp.493-505. (Available online at http://dx.doi.org/10.1002/(SICI)1096-9861(19961028)374:4<493::AID-CNE2>3.0.CO;2-X [Accessed - 1/12/05]

Ohnishi K et al, 2004, Comparative study of otolith organs between two species of upside-down swimming catfish, Synodontis nigraventris showing converse swimming postures, Acta Zoologica, vol.85, (3) pp.201-209. (Available online at - http://www.blackwell-synergy.com/doi/abs/10.1111/j.0001-7272.2004.00171.x) [Accessed - 2/12/05]

de Graaf GJ, 2001, Growth and mortality of the catfish Hemisynodontis membranaceus (Geoffroy St. Hilaire), in the northern arm of Lake Volta, Ghana, Fisheries Management and Ecology, vol.8 (1), pp.37-45. (Available online at - http://www.blackwell-synergy.com/doi/abs/10.1046/j.1365-2400.2001.00214.x) [Accessed - 30/11/05] Ofori D, 1992, Ecology of some species of catfish in the Kpong Head pond in Ghana, Environmental Biology of Fishes (Historical Archive), vol.35 (1), pp.49. (Available online at - http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1007/BF00001157) [Accessed - 29/1/05]

Ofori D et al 2001.Growth and mortality of the catfish Hemisynodontis membranaceus (Geoffroy St. Hilaire) in the northern arm of Lake Volta, Ghana Fisheries Management and Ecology, vol. 8, (1) pp.37-45 (Available online at - http://www.blackwell-synergy.com/doi/abs/10.1046/j.1365-2400.2001.00214.x) [Accessed - 28/11/05]

Thompson A.B, et al, 1996, Distribution and breeding biology of offshore pelagic cyprinids and catfish in Lake Malawi/Niassa. Environmental Biology of Fishes (Historical Archive), vol. 47, (Available online at - http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1007/BF00002377) [Accessed - 27/11/05]

Witte F, 1997, the catfish fauna of Lake Victoria after the Nile perch upsurge. Environmental Biology of Fishes, vol.49. (Available online at - http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1023/A:1007311708377) [Accessed - 17/4/06]

Books -

Burgess WE, 1989, An atlas of freshwater and marine catfishes, A preliminary study of the Siluriforms, NJ, United states, TFH publications inc.

Axelrod R, 2001, Aquarium fishes of the world, NJ, United states, TFH publications inc

Baensch H.A & Reihl R, 1997, Aquarium atlas, 6th ed, Melle, Germany, Hans A Baensch