Tilapiine cichlid

The Tilapiini are a tribe within the family Cichlidae commonly known as tilapiine cichlids. Most of the taxa herein are called "tilapias", a diverse and economically important group containing the genera Oreochromis, Sarotherodon, and Tilapia. A number of smaller genera, such as Alcolapia, Danakilia, Iranocichla, and Steatocranus are also placed herein. They are now placed in the subfamily Pseudocrenilabrinae of African and Middle Eastern cichlids; formerly, these were often regarded as a distinct subfamily Tilapiinae.

Systematics

The tilapiines were recognised by the ichthyologist Ethylwynn Trewavas.

Recent DNA sequence analyses, however, suggest that the Tilapiini as presently delimited are not monophyletic. mtDNA studies[1] found that Tilapia, in which formerly all tilapias were united, still seems to constitute a paraphyletic assemblage, composed of a basal grade, T. bemini which seems closer to haplochromines, and a group containing for example T. buttikoferi and T. ruweti of presently unresolved relationships. The first group, as well as Iranocichla, Oreochromis, Sarotherodon and Tristramella, are fairly close to tribes such as the Lamprologini and Tropheini. Unfortunately, the type species of Tilapia (and hence, the tilapiines) does not seem to be among them.

mtDNA-based phylogenies of tilapiines must be evaluated with caution, however, as they are usually close to, but do not represent the true evolutionary relationships of these fishes. The reason is that hybridization within any one of these major lineages is known to usually produce fertile offspring, and might also do so between the lineages. Gene pools in these fishes have been kept (largely) separate by behavioral cues for millions of years, but reproductive incompatibility has been far slower to evolve, like in many Pseudocrenilabrinae (African cichlids).[2]

A small sample size—one to a mere handful of specimens per taxon—as is often used in molecular studies further acerbates the problem. As discussed below for the example of mouthbreeding, nonmolecular data such as morphology or behavior have also turned out to be extremely prone to homoplasies, not the least due to the small but ongoing gene flow between evolutionarily quite distant gene pools.

Essentially, most traditional and mtDNA-based phylogenetic hypothesis for tilapiines must be considered with a high degree of caution. This problem could be alleviated to some extent by using nDNA sequences. Comparing these with the mtDNA data, hybridization effects could be discerned. Also, resolution of nDNA likely is still good enough to delimit the clades that apparently exist in the "tilapiines" if numerous taxa and specimens are sampled. Researchers could then reanalyze morphological data to discover actual autapomorphies.

Evolution seems to run quickly in this group. Even the fast-evolving mtDNA sequences often are incapable of properly resolving interspecies relationships.[3] The precise evolutionary history of some tilapiines may not be properly resolved with presently available methods, for the reasons discussed above.

Diversity of breeding behaviour

Like other cichlids, tilapiines exhibit complex reproductive behaviours and guard their eggs and fry. Broadly speaking, the plesiomorphic trait is substratum-spawning behavior, meaning that the fish form pairs, lay the eggs on a rock or into a depression made in the substrate, and then both parents guard the eggs and fry.

Oreochromis and Sarotherodon are mouthbrooders, carrying the eggs and fry in their mouths instead of placing them in a nest. The numerous Oreochromis species are maternal mouthbrooders, meaning only the female looks after the eggs and fry, with the male offering no protection or help at all. Instead, they form leks where they compete with one another for further opportunities to mate with females.

By contrast, most of the far fewer Sarotherodon species are biparental mouthbrooders, with both parents protecting the eggs and fry. A few, such as Sarotherodon melanotheron, are paternal mouthbrooders, with only the male caring for the eggs and fry. In addition, some species usually placed in Oreochromis might belong into Sarotherodon.[2] Either this is incorrect due to hybridization effects, or they are not maternal mouthbrooders, or the diversity of brood care is even higher among the smallest group of tilapias than it was hitherto believed.

References

  1. ↑ Nagl et al. (2001), Klett & Meyer (2002)
  2. 1 2 Nagl et al. (2001)
  3. ↑ Compare support values and proposed relationships in Nagl et al. (2001) to Klett & Meyer (2002)

Further reading

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