Abstract
‘Cryptic’ species are an emerging biological problem that is broadly discussed in the present study. Recently, a cryptic species definition was suggested for those species which manifest low morphological, but considerable genetic, disparity. As a case study we present unique material from a charismatic group of nudibranch molluscs of the genus Trinchesia from European waters to reveal three new species and demonstrate that they show a dual nature: on one hand, they can be considered a ‘cryptic’ species complex due to their overall similarity, but on the other hand, stable morphological differences as well as molecular differences are demonstrated for every species in that complex. Thus, this species complex can equally be named ‘cryptic’, ‘pseudocryptic’ or ‘non-cryptic’. We also present evidence for an extremely rapid speciation rate in this species complex and link the species problem with epigenetics. Available metazoan-wide data, which are broadly discussed in the present study, show the unsuitability of a ‘cryptic’ species concept because the degree of crypticity represents a continuum when a finer multilevel morphological and molecular scale is applied to uncover more narrowly defined species making the ‘cryptic’ addition to ‘species’ redundant. Morphological and molecular methods should be applied in concordance to form a fine-scale multilevel taxonomic framework, and not necessarily implying only an a posteriori transformation of exclusively molecular-based ‘cryptic’ species into morphologically-defined ‘pseudocryptic’ ones. Implications of the present study have importance for many fields, including conservation biology and fine-scale biodiversity assessments.
Highlights
The difficulty of delineating the ‘cryptic species’ concept from the basic biological species definition was highlighted, and it was suggested that this concept should be used with care and only as a temporary formalization for taxonomic complexes for which a robust morphological framework is not yet established[8]
The strong trend to distinguish ‘cryptic’ and ‘non-cryptic’ species, and the explosive growth of exploration and description of ‘cryptic’ diversity, without a clearly defined terminological ground, is already resulting in recent outstanding controversies in important fields such as conservation biology - regarding suggestions for special regulations of species names, that will affect the core of taxonomy[14,15]
The specimens were deposited in the Zoological Museum of Lomonosov Moscow State University (ZMMU), in the Gothenburg Natural History Museum (GNM), National Museums Northern Ireland (BELUM.Mn), and the Department of Science of the Roma Tre University (RM3)
Summary
The difficulty of delineating the ‘cryptic species’ concept from the basic biological species definition was highlighted, and it was suggested that this concept should be used with care and only as a temporary formalization for taxonomic complexes for which a robust morphological framework is not yet established[8]. According to the present study, these species are robustly distinguished by their molecular data and were never taxonomically separated previously and fit within the ‘cryptic species’ concept and its updated definition including “low morphological disparity9”. The apparent cryptic morphology for that complex had previously been noted and questioned[16,17] and in the present study those morphological distinctions have been supported by novel molecular data These ‘cryptic’ species can be distinguished morphologically (and can be defined as ‘pseudo-cryptic’ species) a posteriori after molecular study, as was commonly suggested and has become a notable modern tendency[18,19], but by using multilevel morphological information as a primary source. This adds a new perspective to the understanding of the core biological species problem, which should not just be to adhere to a putative cryptic notion, but methods should rather follow the idea “from cryptic to obvious species” with the rapid progress of various molecular analyses and aids to morphological differentiation[20,21] in order to reveal an immense hidden multilevel biological diversity
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