Introgression Threatens the Survival of the Critically Endangered Freshwater Crayfish Cherax tenuimanus (Decapoda: Parastacidae) in the Wild

Abstract

Hybridization and genetic introgression following the introduction of exotic species can pose a significant threat to the survival of geographically restricted species. A remnant population of the critically endangered freshwater crayfish Cherax tenuimanus in the upper reaches of the Margaret River in southwestern Australia is under threat following the introduction and spread of its congener Cherax cainii. Here, we examine the extent of hybridization and introgression between the two species using twelve polymorphic microsatellite loci. Our study reveals there are three times more C. cainii than C. tenuimanus at our study site in the upper reaches of the Margaret River. There is also evidence of hybridization and introgression between C. tenuimanus and C. cainii at this site, with F1, F2 and backcrossed individuals identified. While interbreeding was confirmed in this study, our simulations suggest that the levels of introgression are much lower than would be expected under random mating, indicating partial reproductive barriers exist. Nevertheless, it is apparent that hybridization and introgression with C. cainii pose a serious threat to C. tenuimanus and their survival in the wild will require active adaptive management and continued genetic monitoring to evaluate management effectiveness.

Detecting Sex-Related Changes to the Metabolome of a Critically Endangered Freshwater Crayfish During the Mating Season

“…Hybridisation between these congeneric marron species has been confirmed (Austin and Ryan, 2002;Bunn et al, 2008;Kennington et al, 2014;Guildea et al, 2015), but it has been suggested that there are partial reproductive barriers present as levels of introgression are lower than would be predicted under random mating (Guildea et al, 2015). The partial reproductive barriers could be explained by the difference in timing of breeding seasons: although the mating seasons of both species overlap, C. cainii mate and reproduce earlier in the season than C. tenuimanus.…”

“…Only recently recognised as separate species, the highly restricted C. tenuimanus and its widespread sister-species C. cainii have experienced introgressive hybridisation, with hybrids producing fertile offspring (Austin and Ryan, 2002;Duffy et al, 2014;Guildea et al, 2015); hence, it was assumed that they require similar conditions for successful breeding. C. cainii has been a popular aquaculture species with a long history of captive breeding in Western Australia (Morrissy, 1970(Morrissy, , 1979Morrissy et al, 1990;Huner, 1994;Lawrence, 2007), and requirements for captive breeding (such as food, water quality, environmental parameters, stocking densities, etc.)…”

“…C. cainii males therefore may be larger, due to earlier emergence of juveniles and access to resources, than C. tenuimanus. This, with the earlier breeding season of C. cainii is likely to prevent C. tenuimanus males access to potential mates and furthering the decline of the wild C. tenuimanus population (Guildea et al, 2015).…”

“…For the period where the reproductive receptiveness of both species' females overlap, C. cainii will continue to interfere with attempts by the male C. tenuimanus to mate and some hybrids would be produced. Particularly if C. tenuimanus densities (in the wild) are so low that finding females is problematic, this pseudocompetition may also be taking place indirectly, resulting in fewer conspecific encounters and mating between C. tenuimanus individuals (Guildea et al, 2015). By the time the C. cainii females have finished their reproductive cycle, and C. tenuimanus females have exclusive access to the males, the males may have reached the end of their reproductive cycle.…”