The biological reality of a phantom abalone “Haliotis madaka”

Abstract

The two large abalone species caught in the Japanese archipelago, Haliotis discus discus and H. madaka, are genetically close, and although their distribution areas overlap, the two species exhibit unique morphological and ecological characteristics. On the other hand, these two species can be crossbred under laboratory conditions, and the first generation of hybridization is reproductive. These facts suggest that the two species are reproductively isolated by ecological factors in natural environments (i.e., ecological isolation). Of these two species, the landings of H. madaka have declined dramatically in recent years, therefore, an effective conservation strategy for this species is needed. However, two problems make the conservation of this species complex. The first is the morphological characteristics used to identify H. discus discus and H. madaka. The two species are usually identified based on several qualitative traits, but their morphological differences have not been adequately tested quantitatively and remain unclear. The second is about hybridization between the two species in natural environments. While previous population genomics analysis showed that the two species that caught in Tokushima and Kanagawa prefectures are genetically distinct, this study also suggested hybridization between the two species that caught in Niigata prefecture. These results indicate that the strength of reproductive isolation between the two species may vary by region. This “blurring” of biological species boundaries is an issue should be resolved for the conservation of these abalones. Here, we clarify the biological reality of H. discus discus and H. madaka based on genetic and morphological studies, with a brief introduction to abalone speciation processes.