Evolution of Squeaker Catfishes in Africa's Lake Tanganyika

tags: evolutionary biology, speciation, species flocks, molecular phylogeny, behavioral ecology, Synodontis species, squeaker catfish, cuckoo catfish, Lake Tanganyika, peer-reviewed paper

One of the groups of fishes that I worked with as an aquarist for nearly my entire life are the synodontids, often known as "squeaker catfish" for their ability to make high-pitched sounds. These medium- to large-sized African catfishes are attractive, long-lived and intelligent, and many species in this genus exhibit a variety of distinctive breeding behaviors. For example, S. multipunctatus (pictured above) is the only fish documented to practice brood parasitism: it sneaks its eggs in with those of mouthbrooding cichlids in Lake Tanganyika, and its larvae grow faster than those of the host and feed on them. Add that to a fascinating evolutionary history, which is still being deciphered, and you have a very interesting group of fishes.

The synodontid catfish are endemic throughout many of the lakes and rivers of sub-Saharan Africa, including Lake Tanganyika. Located in the famous Great Rift Valley of central Africa (see map, below), Lake Tanganyika is one of the largest and deepest freshwater lakes in the world, and is home to at least 250 species of cichlid fish and 150 non-cichlid species, nearly all of them unique.

Because of its isolation, Lake Tanganyika is one of several aquatic islands in the Great Rift Valley, and is home to numerous groups of closely related species, known as "species flocks". These species flocks evolved rapidly and in genetic isolation from their close relatives through a process known as adaptive radiation and thus, they are important for helping scientists further understand the process of speciation. Some examples of especially famous species flocks include Darwin's Finches, the Hawaiian Honeycreepers, and of course, the cichlid (SICK lid) fishes found in Lake Tanganyika and several other Great Rift Valley lakes.

Synodontis catfishes share several similarities with the cichlids, including their geographic distribution and their overall species richness, so it is logical to conclude that the synodontids are a useful comparative model system for studying speciation in cichlids. But surprisingly, the evolution of Synodontis catfish remains controversial and poorly known. For example, two independent pilot studies published in 2006 generated conflicting data regarding the colonization history of Lake Tanganyika and molecular relationships among synodontids (Figure 1);

To address these inconsistencies, it is necessary to conduct further research by adding more DNA regions from more taxa to the analysis. So a research team, headed by JJ Day, investigated the relationships between these species and tested whether these fish are monophyletic -- that is, if they descended from one common ancestor -- by analyzing the nuclear (nc)DNA and mitochondrial DNA (mtDNA) from 65 individuals of roughly 40 species (some species are cryptic) to reconstruct a molecular phylogeny for all the Tanganyika synodontids as well as those most major African drainage basins (Figure 2);

Despite using two different analytic methods, the team recovered the same phylogenetic tree with only minor differences. For example, the two main clades, A and B, are consistently recovered, and the Lake Tanganyika synodontids are consistently recovered as a single clade (K), while species that are endemic to Lake Malawi cluster together with riverine species from the Zambezi and Okavango drainage basins (clade J). Most -- but not all -- south African species form a distinct lineage (clade F) that evolved from Congolese ancestors that hybridize under laboratory conditions, suggesting that this a very young species flock. The 'East African' lineages (clade I) lack strong statistical support and are poorly resolved, probably due to rapid speciation in this group. Further, even though these data reveal that the Synodontis catfish are monophyletic, several geographically clumped species clades are not monophyletic, indicating a more complex evolutionary history than previously thought.

The team then investigated the evolutionary history of Synodontis species by calibrating their maximum and minimum dates from the Burdigalian period (16.3 - 21.5 Myr), from a fossil Synodontis (Figure 2);

According to the above chronogram, the inferred timing of the Synodontis radiation corresponds to the deepening of Lake Tanganyika, which occurred around 5-6 Myr, with the two endemic clades diversifying during a time when Lake Tanganyika experienced lower lake levels due to climate change. Similarly, all the major cichlid tribes were estimated to have radiated at roughly the same time.

However, several questions remain to be addressed:

1. Since it appears that Synodontis evolved in the now-extinct lake basin in the Great Rift Valley, why are there no Synodontis species flocks in either Lakes Malawi or Victoria?
2. Why don't Synodontis have multiple species flocks in Lake Tanganyika, as seen in cichlids?
3. Even though they apparently arose at similar times, why do cichlids show such tremendous morpholical diversity while Synodontis do not?

By investigating these and more questions for Synodontis and the cichlids, scientists will gain a clearer idea of how geography influences speciation differently in two lineages that share a similar evolutionary history.

Source:

DAY, J., BILLS, R., & FRIEL, J. (2009). Lacustrine radiations in African Synodontis catfish. Journal of Evolutionary Biology, 22 (4), 805-817 DOI: 10.1111/j.1420-9101.2009.01691.x.