Long distance dispersal and pseudo-cryptic species in Gastrotricha: first description of a new species (Chaetonotida, Chaetonotidae, Polymerurus) from an oceanic island with volcanic rocks

Abstract

The majority of meiofaunal organisms have limited abilities to disperse over long distances, yet they may still have disjointed distributions. Many studies have found evidence of long distance meiofauna dispersal due to passive transport by wind and/or animals that serve as vectors for these widespread distributions. Our research on an archipelago in northeast Brazil uncovered a species of freshwater gastrotrich that at first sight appeared to be a ‘cosmopolitanʼ species that had surpassed the connectivity constraint to occupy an island more than 350 km from the mainland. However, through an integrative approach using molecular sequences and morphology, we have uncovered evidence of a pseudo-cryptic species in this freshwater gastrotrich. Polymerurus insularis sp. nov. closely resembles its congeners and can easily be mistaken for similar species such as P. nodicaudus, a cosmopolitan gastrotrich. Unique to P. insularis sp. nov. are (1) a cuticular armature composed of simple spined scales with polygonal shape (Type 1 scales), (2) a single, spineless dorsal scale with a triangular shape located terminally next to the furca base (Type 2 scale), (3) a spineless zone composed by a patch lacking cuticular ornamentation and flat, rounded or polygonal scales without spines (Type 4 scales) (4) particular sets of terminal spined or keeled scales located both dorsally and ventrally around the furca base (Types 3, 5, 6, 7 and 8 scales). The presence of this species on a volcanic island is discussed, as is the relationship between pseudocryptism and dispersal in gastrotrichs and other meiofauna.

Similar Ones Are Not Related and Vice Versa—New Dendronotus Taxa (Nudibranchia: Dendronotidae) from the North Atlantic Ocean Provide a Platform for Discussion of Global Marine Biodiversity Patterns

“…Our study, therefore, reveals the complex combinations of morphological and molecular similarities and differences (Figures 1-6), which contradict the simplistic division into 'cryptic' or 'non-cryptic' species groups, which only superficially appear indistinguishable until detailed integration of niche, molecular data, or other lines of evidence reveal clearly different and distinguishable species lineages [79]. This contrast led to a misleading distinction between 'cryptic', 'pseudocryptic' and just 'ordinary' species [80][81][82][83][84][85][86][87]; see detailed discussion in [16,18,63,88,89]. This consideration is of crucial importance for practical taxonomy.…”

A huge undescribed diversity of the subgenus Hystricochaetonotus (Gastrotricha, Chaetonotidae, Chaetonotus) in Central Europe

“…Dispersal abilities have also a significant impact on the diversification rate and alfa diversity detected in individual study plots. Dispersion of meiofaunal organisms is quite limited due to their small size, absence of a larval phase, and low movement capacity (Giere 2009;Cerca et al 2018;Magpali et al 2021). 'Cosmopolitan' species could comprise cryptic, near-cryptic (subtle, easily overlooked differences, or only statistically supported differences; Lücking et al 2021), or pseudo-cryptic species (overlooked or briefly mentioned and not effectively characterized morphological characters; Magpali et al 2021) that could significantly affect the diversity estimates.…”

“…Dispersion of meiofaunal organisms is quite limited due to their small size, absence of a larval phase, and low movement capacity (Giere 2009;Cerca et al 2018;Magpali et al 2021). 'Cosmopolitan' species could comprise cryptic, near-cryptic (subtle, easily overlooked differences, or only statistically supported differences; Lücking et al 2021), or pseudo-cryptic species (overlooked or briefly mentioned and not effectively characterized morphological characters; Magpali et al 2021) that could significantly affect the diversity estimates. To properly address the distribution patterns and species identities of gastrotrichs, broad sampling and sound taxonomy are indispensable.…”

“…Morphology-based identification of gastrotrichs is, however, hampered by (1) the lack of standardized methods for species descriptions in the past, (2) a great disparity of species descriptions among authors, (3) the briefness and generality of 'old' descriptions, (4) the over-schematization and simplification of illustrations, and (5) the necessity of detailed examination of living specimens Balsamo et al 2008;Garraffoni & Melchior 2015;Kieneke & Nikoukar 2017;Magpali et al 2021 and references cited therein). Nowadays, an integrative approach combining detailed morphological observations (e.g., differential interference contrast optics, confocal laser scanning microscopy, SEM) with multi-gene data is the preferred way for the description of new gastrotrich species (e.g., Kånneby 2011;Todaro et al 2012;Garraffoni et al 2017Garraffoni et al , 2019aKolicka et al 2016Kolicka et al , 2018Kolicka , 2019bBosco et al 2020;Magpali et al 2021). Molecular data are, indeed, crucial for the recognition of cryptic and near-cryptic species, as it is the very definition of cryptic species that it is difficult, if not impossible, to find morphological characters that distinguish among them.…”