Losing cichlid fish biodiversity: genetic and morphological homogenization of tilapia following colonization by introduced species

Asilatu Shechonge,Rashid Tamatamah,George F Turner,Martin J Genner,Jack Harrington,Antonia G P Ford,Stephanie J Bradbeer,Benjamin P Ngatunga

doi:10.1007/s10592-018-1088-1

Abstract

Among the many negative impacts of invasive species, hybridization with indigenous species has increasingly become recognized as a major issue. However, relatively few studies have characterized the phenotypic outcomes of hybridization following biological invasions. Here we investigate the genetic and morphological consequences of stocking invasive tilapia species in two water bodies in central Tanzania. We sampled individuals from the Mindu Reservoir on the Ruvu river system, and at Kidatu on the Great Ruaha–Rufiji river system. We screened individuals at 16 microsatellite loci, and quantified morphology using geometric morphometrics and linear measurements. In both the Mindu and Kidatu systems, we identified evidence of hybridization between indigenous Wami tilapia (Oreochromis urolepis) and the introduced Nile tilapia (Oreochromis niloticus) or blue-spotted tilapia (Oreochromis leucostictus). At both sites, purebred individuals could largely be separated using geometric morphometric variables, with hybrids occupying a broad morphospace among the parental species. Our data demonstrate that the gene pools and phenotypic identity of the indigenous O. urolepis have been severely impacted by the stocking of the invasive species. Given the lack of evidence for clear commercial benefits from stocking invasive tilapia species in waters already populated by indigenous congenerics, we suggest further spread of introduced species should be undertaken with considerable caution.

Highlights

Hybridization is a widespread phenomenon in nature (Olden et al 2004)
Our study demonstrated that purebred species were most readily separable using geometric morphometric characters, related to body depth and eye size, but linear measurements of head and body characters, and measurements of lower pharyngeal jaw morphology, showed significance differences among the species
Since our results indicate that assignment of individuals based on morphology alone may lead to misassignment, we suggest that such classification should be coupled with evidence from genetic markers

Summary

Introduction

In the field of invasion biology, hybridization is generally considered a negative process for biodiversity, as it can lead to the erosion of unique genetic diversity (Todesco et al 2016). Hybrids may possess novel traits that enhance their potential to have deleterious impacts on indigenous populations (Gaskin and Schaal 2002; Facon et al 2005). Genetic or demographic swamping during hybridization is considered a major driver of biodiversity loss, alongside habitat loss and pollution (Scribner et al 2000; Perry et al 2002). In Africa, freshwater ecosystems are critically important for both biodiversity and food security, supporting capture fisheries of major significance for inland human populations (Vörösmarty et al 2010; McIntrye et al 2016; Lynch et al 2016; Winemiller et al 2016). Many major capture fisheries in Africa are overexploited, leaving little capacity

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Discussion

Conclusion