Phylogeography of the genus Epiplatys (Aplocheiloidea: Cyprinodontiformes)

Calcified gill filaments increase respiratory function in fishes

The morphology of fish gills is closely linked to aerobic capacity and tolerance of environmental stressors such as hypoxia. The importance of gill surface area is well studied, but little is known about how the mechanical properties of gill tissues determine function. In some fishes, the bases of the gill filaments are surrounded by a calcified 'sheath' of unknown function. We tested two non-exclusive hypotheses: (i) calcified gill filaments enhance water flow through the gill basket, improving aquatic respiratory function, and (ii) in amphibious fishes, calcification provides support for gills out of water. In a survey of more than 100 species of killifishes and related orders, we found filament calcification was widespread and thus probably arose before the evolution of amphibious lifestyles in killifishes. Calcification also did not differ between amphibious and fully aquatic species, but terrestrial acclimation caused calcium deposition on the filaments of the killifish Kryptolebias marmoratus, suggesting a possible structural role when out of water. We found strong evidence supporting a role for filament calcification in enhancing aquatic respiratory function. First, acclimation to increased respiratory demands (hypoxia, elevated temperatures) induced calcium deposition on the filaments of K. marmoratus. Next, gentle removal of filament calcification decreased branchial resistance to water flow, indicating disruption of gill basket positioning. Thus, the mechanical properties of the gill filaments appear to play an important and previously unappreciated role in determining fish respiratory function.

†Kenyaichthyidae fam. nov. and †Kenyaichthys gen. nov. - First Record of a Fossil Aplocheiloid Killifish (Teleostei, Cyprinodontiformes)

The extant Cyprinodontiformes (killifishes) with their two suborders Cyprinodontoidei and Aplocheiloidei represent a diverse and well-studied group of fishes. However, their fossil record is comparatively sparse and has so far yielded members of the Cyprinodontoidei only. Here we report on cyprinodontiform fossils from the upper Miocene Lukeino Formation in the Tugen Hills of the Central Rift Valley of Kenya, which represent the first fossil record of an aplocheiloid killifish. A total of 169 specimens - mostly extraordinarily well preserved - and a sample of ten extant cyprinodontiform species were studied on the basis of morphometrics, meristics and osteology. A phylogenetic analysis using PAUP was also conducted for the fossils. Both the osteological data and the phylogenetic analysis provide strong evidence for the assignment of the fossils to the Aplocheiloidei, and justify the definition of the new family †Kenyaichthyidae, the new genus †Kenyaichthys and the new species †K. kipkechi sp. nov. The phylogenetic analysis unexpectedly places †Kenyaichthys gen. nov. in a sister relationship to the Rivulidae (a purely Neotropical group), a probable explanation might be lack of available synapomorphies for the Rivulidae, Nothobranchiidae and Aplocheilidae. The specimens of †K. kipkechi sp. nov. show several polymorphic characters and large overlap in meristic traits, which justifies their interpretation as a species flock in statu nascendi. Patterns of variation in neural and haemal spine dimensions in the caudal vertebrae of †Kenyaichthys gen. nov. and the extant species studied indicate that some previously suggested synapomorphies of the Cyprinodontoidei and Aplocheiloidei need to be revised.

The root of the East African cichlid radiations

BACKGROUND

For decades cichlid fishes (Perciformes: Cichlidae) of the East African cichlid radiations (Teleostei: Cichlidae) have served as natural experimental subjects for the study of speciation processes and the search for potential speciation key factors. Despite numerous phylogenetic studies dealing with their intragroup relationships, surprisingly little is known about the phylogenetic placement and time of origin of this enigmatic group. We used multilocus DNA-sequence data from five nuclear and four mitochondrial genes and refined divergence time estimates to fill this knowledge gap.

RESULTS

In concordance with previous studies, the root of the East African cichlid radiations is nested within the so called "Tilapias", which is a paraphyletic assemblage. For the first time, we clarified tilapiine intragroup relationships and established three new monophyletic groups:"Oreochromini", "Boreotilapiini" and a group with a distribution center in East/Central Africa, the "Austrotilapiini". The latter is the founder lineage of the East African radiations and emerged at the Miocene/Oligocene boundary at about 14 to 26 mya.

CONCLUSION

Our results provide the first resolved hypothesis for the phylogenetic placement of the megadiverse East African cichlid radiations as well as for the world's second most important aquaculture species, the Nile Tilapia, Oreochromis niloticus. Our analyses constitute not only a robust basis for African cichlid phylogenetics and systematics, but provide a valid and necessary framework for upcoming comparative phylogenomic studies in evolutionary biology and aquaculture.