Fine-scale species delimitation: speciation in process and periodic patterns in nudibranch diversity

Abstract

Using the nudibranch genus Amphorina as a model, ongoing speciation is demonstrated, as well as how periodic-like patterns in colouration can be included in an integrated method of fine-scale species delimitation. By combining several methods, including BPP analysis and the study of molecular, morphological, and ecological data from a large number of specimens within a broad geographic range from northern Europe to the Mediterranean, five species are recognised within the genus Amphorina, reviewed here for the first time. Two new species from the southwestern coast of Sweden are described, A. viriolasp. nov. and A. andrasp. nov. Evidence is provided of a recent speciation process between the two closely related, yet separate, species which inhabit the same geographic localities, but demonstrate strict water depth differentiation, with one species inhabiting the shallow brackish top layer above the halocline and the other species inhabiting the underlying saltier water. The results presented here are of relevance for currently debated issues such as conservation in relation to speciation, fine species delimitation, and integration of molecular, morphological and ecological information in biodiversity studies. The periodic approach to taxonomy has considerable practical potential for various organismal groups.

A synoptic review of the family Dendronotidae (Mollusca: Nudibranchia): a multilevel organismal diversity approach

“…A major modern feature of this is the general denial of huge polytypic and pan-geographic species in favour of fine-scale interspecific morphological differences in combination with limited geographic ranges. Within restricted "narrow species" a polymorphism (a basis for the polytypic concept) is often manifested in the occurrence of parallel morphs, which may obscure species diversity (see e.g., Korshunova et al, 2020b). Though Mayr was among the major developers of the population-based, variable, "non-typological" species concept, he also admitted the possible existence of a hardly detectable diversity of "sibling species" (Mayr, 1942;Yoder et al, 2005).…”

Young but distinct: description of Eubranchus malakhovi sp.n. a new, recently diverged nudibranch species (Gastropoda: Heterobranchia) from the Sea of Japan

“…At the same time, modern taxonomical revisions and dedicated phylogenetic studies have not paid enough attention to this genus. During the last two decades only two contributions performed integration of morphological and molecular data within this group: (1) a recent revision of the family Fionidae (Cella et al, 2016) that evaluated Eubranchus as a member of this family and suggested that the amphiboreal Eubranchus rupium (Møller, 1842) represented a species complex of at least three distinct species; and (2) revision of the Eubranchus farrani (Alder et Hancock, 1844) -E. pallidus (Alder et Hancock, 1842) species complex (Korshunova et al, 2020) completed with a reinstatement of the generic name Amphorina for these molluscs and the description of two new species from the North-East Atlantic: Amphorina andra Korshunova et al, 2020 andA. viriola Korshunova et al, 2020.…”

The multilevel organismal diversity approach deciphers difficult to distinguish nudibranch species complex

“…There were attempts to describe chromatic variants within another nudibranch family Chromodorididae under the term “colour groups” 58 or as different colour morphs in frames of a phylogeny 59 , 60 , but not in a periodic framework. However, when similar morphs of different closely related species are mapped in the same horizontal sections, the partial periodicity can be clearly revealed 18 .…”

“…chromatic variants) and also potentially not yet discovered morphs of the closely related species. Currently we have no information about which particular genes underlie any common genetic basis in the polychromatic nudibranchs 61 , but it must inevitably imply similar developmental genes basis and it was recently used for delineation of a very difficult nudibranch species complex of the genus Amphorina 18 . A North Pacific species, Polycera atra MacFarland, 1905, provides evidence for the existence of the underlying similar genomic basis, that appears in parallel in various phylogenetic lineages.…”

“…However, despite that, there are continuous recent attempts to putatively distinguish the terms “cryptic” and “non-cryptic” species, commonly with the presentation of some morphological distinguishing characters that, by definition, mean that the newly separated species cannot be considered as “cryptic” 13 – 15 . Hidden diversity therefore is an important problem, especially in the context of the polymorphism 16 , 17 , which can appear as a parallel-like pattern of characters in two or more closely related species 18 , 19 . Because within already fine-scale delineated species, it is still possible to uncover more diversity 12 , the commonest European nudibranchs Polycera are investigated in this study in order to further test the reliability of the currently widely discussed “cryptic species” concept .…”