Synodontis species flocks in Lake Tanganyika

Regular readers may remember that I have a softspot for catfish and earlier this year purchased a lace catfish (Synodontis nigrita), a species native to many African countries. The genus Synodontis (Cuvier 1816) is interesting for a number
of reasons. For example, S. multipunctatus (the gorgeous fish pictured above) is the only fish known to practice brood parasitism: it manages to mix it eggs with those of mouthbrooding cichlids in Lake Tanganyika, its larvae grow faster than those of the host and feed on them.

Lake Tanganyika is, of course, famous for the cichlids which have been studied as an example of a rapid, recent radiation which was caused by environmental change (in this case, fluctuations in water level). It is also home to to other endemic fauna, including ten species of Synodontis. A recent study has used mitochondrial DNA to study the history of the genus in the region.

Day and Wilkinson [1] used mtDNA to examination the relationships between central and east African species of Synodontis. All methods of analysis recovered the same phylogeny.

They note that "the origin and initial divergence of the East African Synodontis are compatible with isolation through vicariance during the major rifting events that took place between 25 and 10 Myr ago", an observation seen in many terrestrial, but few aquatic, groups. Subsequent to this East/West split, the Lake Tanganyika endemic species diversified [node D above] due to aridification as lake levels dropped. This pattern is similar to that seen in other East African organisms such as some cichlids
and platythelphusid crabs.

The authors note that the placement of S. victoriiae (a species endemic to Lake Victoria) is problematic and suggest that the species may have originated with Tanganyika endemics recolonizing rivers and then Victoria. Such an hypothesis can only be tested with additional data.

[1] J.J. Day & M. Wilkinson (2006) "On the origin of the Synodontis catfish species flock from Lake Tanganyika" Biology Letters 2(4):548 - 552. DOI: 10.1098/rsbl.2006.0532