Morphology, molecules, and Monogenean parasites; A new approach to cichlid taxonomy

On April 29th, 2015, one of the most influential papers regarding cichlid taxonomy was published in PLOS One: Van Steenberge et al. (2015) "Morphology, molecules, and Monogenean parasites: An example of an integrative approach to cichlid biodiversity". For the first time a cichlid taxonomic classification was presented that is based on a combination of morphology, molecular analyses (DNA signatures), and parasite characterization.

Morphology has historically been the main tool for determining the systematics and taxonomy of fish, and much of the past work on cichlids, for example, that of Boulenger, Regan, Trewavas, Greenwood, and Poll, has been based solely on morphometrics (counts and measurements). While morphology continues to be important as a method of describing the physical characteristics of cichlids, it has sometimes proved inadequate for determining their taxonomic status and systematic relationships because of their ability to rapidly adapt their physiology to new opportunities, especially feeding opportunities. Thus one of the main morphological tools in the past, dentition, has proved unreliable for determining phylogeny, and phylogeny is essential for determining generic placement.

During the last two decades a large body of research on cichlids has been performed using their genetic material (DNA) to unravel the scenario of how the individual genera and species evolved. This research has proved a useful additional tool in determining their correct classification, but up to now, no taxonomic acts have been proposed for cichlids based on such DNA analyses.

A male of Pseudosimochromis curvifrons at Cape Mpimbwe, Lake Tanganyika [Tanzania]. Photo by Juan Miguel Artigas Azas. (2010-09-17). determiner Patrick Tawil

Recently Van Steenberge and colleagues (2015) revised the Tanganyika genera Simochromis and Pseudosimochromis in a most elegant manner. The Haplochromini, the tribe to which those two genera belong, pose enormous taxonomic challenges if only morphological data are considered, and even when such studies are combined with molecular analyses the resulting phylogenies are rarely congruent and often pose more questions than answers. Van Steenberge et al. added a third dimension to the question of relationships: gill flukes! These minuscule flatworms turn out to be very host-specific, so much so that almost every cichlid species in Lake Tanganyika has its own fauna of flatworm species. They may be tiny but they represent a completely new set of characters for distinguishing the various cichlid species. Combining morphology, DNA analyses, and flatworm identification, Van Steenberge et al. convincingly demonstrated that the type species of the genus Simochromis, S. diagramma, is not closely related to the other species previously included in the genus, S. babaulti, S. marginatus, S. pleurospilus, and S. margaretae. This had already been suggested by DNA analyses but now additionally with flatworms. As relatedness between gill fluke species usually reflects relatedness between hosts, Van Steenberge et al. considered Simochromis monotypic because the three gill fluke species found on S. diagramma belonged to a different morphotype than those found on the other Simochromis. The transfer of S. babaulti, S. marginatus, S. pleurospilus and S. margaretae to Pseudosimochromis was again justified by the similarity of their gill fluke fauna.

The parasite data also supported the synonymy between S. pleurospilus and S. babaulti, a species that contains a large amount of geographical morphological variation. Using morphology, DNA signature patterns, and parasite characterization Van Steenberge et al. were unable to distinguish between P. babaulti and P. pleurospilus and hence regarded the latter as a junior synonym.

This paper sets the standard of how the “next generation” taxonomy would need to be performed. The addition of host-specific parasites adds a completely new dimension to phylogenetic studies and I’m certain that cichlid taxonomists all over the world are now anxiously searching for similar parasites in their study subjects. It almost speaks for itself to investigate Malawi cichlids for such flatworms although that cichlid fauna may still be too young to have evolved such a rich flatworm diversity as that found in Tanganyika cichlids.

The publication is freely downloadable.