Adaptive radiation of Lake Tana's (Ethiopia) Labeobarbus species flock (Pisces, Cyprinidae)

Abstract

Studying species flocks (e.g. Darwin's finches, Caribbean anoline lizards, East African cichlid fishes) has proven to be highly successful in understanding the forces driving speciation. The only known, intact species flock of cyprinid fishes, the 15 Labeobarbus species in Lake Tana (Ethiopia), includes eight piscivorous species. Piscivory is a rare specialisation among the highly successful (>2000 species) but mostly benthivorous Cyprinidae. The extent and mechanisms of diversification of this remarkable Labeobarbus species flock, particularly among the unexpected piscivorous species, are still largely unknown. In the present study we demonstrate that all 15 Labeobarbus species are segregated to a great extent along spatial, trophic and/or temporal dimensions. The spatial distribution, diet (prey species but not prey size), time of active feeding and predation techniques differed significantly among the eight piscivores. Lake Tana's cyprinids displayed their retained potential for ecological diversification and speciation, including the uncommon specialisation of piscivory. The latter is probably a result of the absence of common African specialist piscivores in Lake Tana. We suggest that the evolution of Lake Tana's Labeobarbus species flock at this stage is predominantly structured by ecological selection models. The labeobarbs most likely underwent sequential stages of radiation and speciation: habitat divergence followed by trophic divergence.

Growth, biomass, and production of two small barbs (Barbus humilis and B. tanapelagius, Cyprinidae) and their role in the food web of Lake Tana (Ethiopia)

“…Nagelkerke et al, 1994;Sibbing & Nagelkerke, 2001). Surprisingly, eight of these are piscivores (de Graaf et al, 2008). Studies on the three species of small barbs, B. humilis, B. pleurogramma and B. tanapelagius started only recently (e.g.…”

Selection and constraint underlie irreversibility of tooth loss in cypriniform fishes

“…Several lineages have adopted piscivory, which in noncypriniform fishes is usually associated with oral teeth for gripping and upper pharyngeal teeth for transporting prey (11,14,15). These piscivorous cypriniforms have been suggested to be less-efficient predators than other taxa with more extensive dentition (11,16,17). In addition, the cypriniform species Danionella dracula has evolved dramatic fangs, not by regaining oral teeth but by sculpting the shape of bones of the jaw margin (18).…”

Diversification of prey capture techniques among the piscivores in Lake Tana’s (Ethiopia)Labeobarbusspecies flock (Cyprinidae)

“…The four benthic predators L. dainellii, L. gorguari, L. acutirostris and L. platydorsus are characterized by their downward protrusion (large protrusion angles, 10-44°; Table 2) and the lack of the surfacedwelling B. tanapelagius in their diets (de Graaf et al 2008). Labeobarbus dainellii and L. gorguari both occur in habitats with high structural complexity, rocky shores (both species) and between the stems of submerged vegetation (L. gorguari) at the edge of the lake (de Graaf et al 2008). Both environments are unfavourable for pursuit hunting using overswimming (Muller & Osse 1984).…”

“…Prey fish were fed commercial flake food (Trouvit, Trouw, Putten). Barbus humilis was used as prey in all feeding performance experiments as it is by far the most common prey species in Lake Tana and forms a significant part of the diet of all piscivorous Labeobarbus species (de Graaf et al 2008). The 16 indoor 250 l tanks were part of a semi-closed system; c. 10% of the water was replaced every day by water pumped directly from the lake.…”

“…For example, de Graaf et al (2008) showed that average prey size of Lake Tana's piscivorous Labeobarbus is small (predator-to-prey length ratio (PPR) mean 0.15, max 0.25) compared to non-cyprinid freshwater piscivores (mean 0.25-0.40 PPR, max 0.40-0.70 PPR;van Ooijen 1982;Winemiller & Kelso-Winemiller 1994;Mittelbach & Persson 1998).…”