Steatocranus gibbiceps, the Rapid River Bumphead

The cichlid fishes of the Great Lakes of Africa are rightly renowned as one of the world's most spectacular species radiations. Hundreds of species, occupying a wide range of ecological niches, have evolved in what is, geologically speaking, a short period of time. However, cichlids in Africa are not a phenomenon of the Great Lakes alone and many interesting species may be found in other parts of the continent, some of them belonging to local radiations of their own. Consider, for instance, the Congo River rapids endemic Steatocranus gibbiceps.

Male Steatocranus gibbiceps, copyright Polypterus.

The Congo is one of the largest African rivers with a drainage basin covering one-eighth of the continent (Schwarzer et al. 2011). Downstream of Kinshasa, the river gets funneled into an intermittently deep, narrow channel for a distance of some 300 km before broadening as it approaches the sea. The result is the world's longest stretch of river rapids. Many fish species are found only in this unique region of fast-flowing waters, among them multiple species of the cichlid genus Steatocranus including S. gibbiceps. The genus as a whole is restricted to the Congo basin; a single species previously recognised from the Volta River has since been transferred to its own genus (Weiss et al. 2019). The names Steatocranus and gibbiceps both basically mean the same thing: 'fat head', in reference to a fleshy swelling atop the fish's noggin. The exact size of this swelling varies between individuals, being most prominent in large males. Vernacular names given to Steatocranus species generally reflect this feature, such as bumphead cichlid or buffalo-head cichlid. Half a dozen species have been named within Steatocranus with several more being recognised but not yet formally described, most of them belonging to the radiation within the rapids. Schwarzer et al. (2012) found evidence for extensive historical cross-breeding between species and suggested that hybridisation may have been a significant factor in the genus' diversification.

Steatocranus gibbiceps is the largest species in this genus of moderately-sized fishes, growing up to about nine centimetres in length (Roberts & Stewart 1976). Its fast-current habitat is reflected in its slender body form. It is olive brown in coloration with the scales being light in colour at the centre and darker around the margins. Steatocranus gibbiceps is most clearly distinguished from other described species in its genus by its teeth: the front teeth of both the upper and lower jaws are conspicuously large and truncate. It also has a shorter gut than its congeners. This species appears to be specialised in feeding on freshwater snails which it scoops up and swallows whole, though it will take a broader range of food in captivity. Other species of Steatocranus mostly feed on algae.

Steatocranus species are not buoyant and tend to sit at the bottom of the water (Chase Klinesteker describes their behaviour as 'hopping around the bottom like a goby'). They escape the current by spending time in the hollows and crevices among rocks. Breeding happens within such hollows with dedicated pairs forming and females affixing their eggs to the rocks. Like many other cichlids, Steatocranus gibbiceps are dedicated parents after the eggs hatch. Tending of the fry is mostly the responsibility of the female while the male patrols the territory on the watch for danger. In this way, the baby bumpheads are given the best possible start at life.

REFERENCES

Roberts, T. R., & D. J. Stewart. 1976. An ecological and systematic survey of fishes in the rapids of the lower Zaïre or Congo River. Bulletin of the Museum of Comparative Zoology 147 (6): 239–317.

Schwarzer, J., B. Misof, S. N. Ifuta & U. K. Schliewen. 2011. Time and origin of cichlid colonization of the lower Congo rapids. PLoS One 6 (7): e22380.

Schwarzer, J., B. Misof & U. K. Schliewen. 2012. Speciation within genomic networks: a case study based on Steatocranus cichlids of the lower Congo rapids. Journal of Evolutionary Biology 25: 138–148.

Weiss, J. D., F. D. B. Schedel, A. I. Zamba, E. J. W. M. N. Vreven & U. K. Schliewen. 2019. Paragobiocichla, a new genus name for Gobiochromis irvinei Trewavas, 1943 (Teleostei, Cichlidae). Spixiana 42 (1): 133–139.