A deceptive variety: integrative systematics and phylogeography suggest Adalaria proxima and Adalaria loveni represent a single species (Gastropoda: Nudibranchia)

In this paper we provide a thorough morphological re-examination of the type species of the genus Adalaria (Gastropoda: Onchidorididae), Adalaria loveni, and its closely related species Adalaria proxima in boreal and subarctic seas, together with a molecular phylogeny, and an analysis of the population structure of these lineages. The external morphology, spicule composition, and features of the buccal armature and the reproductive system were studied using light microscopy, scanning electron microscopy and micro-CT scan technology. The molecular study included the inference of phylogenetic relationships within the family Onchidorididae based on two mitochondrial and three nuclear markers, as well as a COI-based haplotype network reconstruction. Our results show that Adalaria proxima is a trans-Arctic species and conspecific with A. loveni, with a wide geographic range and a high degree of morphological variability, both in colour and morphology of the notal tubercules. Our analysis does not support the monophyly of the genus Adalaria, but we consider it to be premature to introduce new generic names or redefine the concept of Adalaria until a comprehensive phylogeny at species level is available. Two distinct haplogroups were identified in A. proxima; one with individuals from the Pacific, Canadian Arctic, and northwestern Atlantic, and the other with individuals from the northeastern Atlantic and adjacent subarctic regions. This suggests restricted gene flow between the North-East and the North-West Atlantic, with potential genetic exchange between the North-West Atlantic and Pacific populations. One notable characteristic of Adalaria proxima populations is their significant variability in both external and internal morphology.