On the brink of extinction: a new freshwater amphipod Jesogammarus acalceolus (Anisogammaridae) from Japan

Abstract

Freshwater habitats, especially cold springs, are environments in which the risk of extinction faced by organisms remains high due to human activities. To conserve endangered species, it is important to describe and name them. Here, a new, endangered freshwater anisogammarid amphipod species, Jesogammarus (Jesogammarus) acalceolus sp. nov., found in a spring in Aomori Prefecture, Japan, is described which is potentially the sole remaining habitat of this species. Both morphological and molecular phylogenetic results strongly support the nesting of the new species within Jesogammarus. Jesogammarus (J.) acalceolus sp. nov. is the first species of genus Jesogammarus that was found to lack a calceolus, a sensory organ located on male antenna 2. Thus, the diagnostic criteria for this genus required amendment. A reconstruction of ancestral calceoli, based on a molecular phylogenetic tree, revealed that the common ancestor of Jesogammarus possessed calceoli, which were secondarily lost in J. (J.) acalceolus sp. nov. Our results indicate that this new species, which is key to clarifying the evolution of the calceolus, is of high conservation significance.