Sympatric morphotypes of the restricted-range Tashan Cave Garra: distinct species or a case of phenotypic plasticity?

Among Tashan cave barb Garra tashanensis inhabiting a small cave in southwest Iran, two mental disc (sucking mouth disc) forms were observed. To assess their phylogenetic relationships, disc-less and disc-bearing individuals were analyzed using mitochondrial cytochrome oxidase subunit I ( COI ) partial DNA sequences. Both mental disc forms nested within one clade with absolute bootstrap support ( BS = 100), and the genetic distances between the disc-bearing and disc-less individuals (0.3–0.8%) were considerably lower than inter-species mtDNA sequence distances reported among members of the genus Garra ; further, most species delimitation algorithms used here showed disc-bearing and disc-less Tashan cave barbs to be members of a single taxonomic unit. Hence, the observed mental disc variation was not inferred to be a taxonomic feature or a consequence of character displacement. Instead, it was inferred to be a case of character release to diversify among ecological niches in the limited subterranean habitat or a case of relaxed selection, which should be clarified in detail in follow-up ecological and population genetic studies.