How do Lake Victoria cichlids coexist?

Posted on July 2, 2014

The search for explanations as to how closely related, ecologically similar, species manage to coexist has fascinated generations of biologists. In cases where coexisting species vary in colouration but seemingly little else, it’s been suggested that there’s been selection on less obvious traits that are correlated with body colour. The highly diverse cichlid fish of Lake Victoria, which have undergone an explosive species radiation, offer an ideal model system for studies of natural selection. Male Pundamilia nyererei have crimson backs while males of the sister species P. pundamilia have blue backs. Both types are aggressively territorial in the presence of other males. Red-backed P. nyererei tend to win contests with P. pundamilia, but interestingly, this dominance advantage doesn’t eliminate the blue-backed form. So do P. pundamilia have a compensating non-colour advantage over P. nyererei ? For example, the metabolism of P. pundamilia might be more energy-efficient. To examine this possibility, rates of oxygen consumption and swimming activity of males of the two species were measured in experimental chambers. When a fish was allowed to see another male of the same colour in an adjacent tank, it immediately intensified its colour and switched to territorial behaviour. The two species showed similar rates of activity, and when they were visually isolated their metabolic rates were also similar. However, when they could see the other species, P. pundamilia males used significantly more oxygen for a given level of aggressive behaviour than did P. nyererei males. So, contrary to the energy-efficiency hypothesis, P. nyererei males used oxygen more efficiently when being territorial, and therefore had a metabolic as well as a territorial advantage. As a result, more research is needed before we can explain how the two species manage to coexist. Another suggested explanation is that P. nyererei may incur a greater physiological cost by producing or displaying the red colouration, and a further idea is that P. nyererei males might be more easily detected by visual predators.

Reference: Dijkstra, P.D., Seehausen, O. & Metcalfe, N.B. 2013. Metabolic divergence between sibling species of cichlids Pundamilia nyererei and Pundamilia pundamilia. Journal of Fish Biology 82, 1975–1989.