Coexistence of triploid and diploid forms of spined loach, Cobitis taenia: a model-based approach

A tritrophic model of Lotka-Volterra type was developed to describe the dynamics of a unisexual-bisexual complex in fish. The model assumes two genetically isolated forms, bisexual (diploid) and unisexual (triploid), competing for a common resource. The efficiency of food conversion into offspring for the unisexual form is dependent on the availability of the diploid males. By means of isoplane analysis the conditions of a non-trivial equilibrium existence are determined. These conditions imply that the unisexual form should be more fit than the bisexual one in terms of lesser mortality and/or more efficient resource utilization. During field work in the summer of 2001, a unisexual-bisexual complex of spined loach, Cobitis taenia, was found in the Sura River (Mordovia). Analysis of demographic and tropho-dynamic parameters of diploid and triploid forms in this complex suggests that the triploid form, though it does not have the presumed advantage in population fecundity, does have an advantage in survival and consumption rate. The overall ratio between population parameters of the two loach forms indicates that the theoretical condition derived from the model equations is met. Thus, ecological traits of the unisexual and the bisexual forms of spined loach make it possible for them to coexist even under conditions of fully overlapping niches.