Abstract
Caecidae is a species-rich family of microsnails with a worldwide distribution. Typical for many groups of gastropods, caecid taxonomy is largely based on overt shell characters. However, identification of species using shell characteristics is problematic due to their rather uniform, tubular shells, the presence of different growth stages, and a high degree of intraspecific variability. In the present study, a first integrative approach to caecid taxonomy is provided using light-microscopic investigation with microsculptural analyses and multi-marker barcoding, in conjunction with molecular species delineation analyses (ABGD, haplotype networks, GMYC, and bPTP). In total 132 specimens of Caecum and Meioceras collected during several sampling trips to Central America were analyzed and delineated into a minimum of 19 species to discuss putative synonyms, and supplement the original descriptions. Molecular phylogenetic analyses suggest Meioceras nitidum and M. cubitatum should be reclassified as Caecum, and the genus Meioceras might present a junior synonym of Caecum. Meiofaunal caecids morphologically resembling C. glabrum from the Northeast Atlantic are a complex of cryptic species with independent evolutionary origins, likely associated with multiple habitat shifts to the mesopsammic environment. Caecum invisibile Egger & Jörger, sp. nov. is formally described based on molecular diagnostic characters. This first integrative approach towards the taxonomy of Caecidae increases the known diversity, reveals the need for a reclassification of the genus Caecum and serves as a starting point for a barcoding library of the family, thereby enabling further reliable identifications of these taxonomically challenging microsnails in future studies.
Highlights
In the past fifteen years molecular barcoding and molecular species delineation have revolutionized the assessment of species diversity and traditional taxonomy, allowing for fast and reproducible species identification and delimitation, and adding to objectivity and reliability in species diagnoses (Leasi et al 2013; Fontaneto et al 2015; Scarpa et al 2016; Martínez-Arce et al 2020)
The two established genera Caecum and Meioceras, are not recovered as reciprocally monophyletic but instead species of Meioceras group among Caecum species in different parts of the tree (Fig. 3, taxa highlighted in yellow): M. nitidum sister to C. heptagonum (0.96 Posterior probability (PP)), and M. cubitatum sister to C. cf. semilaeve
Inconspicuous specimens with smooth shells and few characters that were morphologically ascribable to C. glabrum or the American Pacific look-alikes like C. glabriforme are polyphyletic and form four lineages separated by branches of comparable length to morphologically distinct species (Fig. 3, highlighted in blue)
Summary
In the past fifteen years molecular barcoding and molecular species delineation have revolutionized the assessment of species diversity and traditional taxonomy, allowing for fast and reproducible species identification and delimitation, and adding to objectivity and reliability in species diagnoses (Leasi et al 2013; Fontaneto et al 2015; Scarpa et al 2016; Martínez-Arce et al 2020). The taxonomy of Gastropoda, one of the most species-rich and betterknown clades of invertebrates in the marine environment, is largely based on shell characteristics (Bouchet and Strong 2010) This approach is generally problematic as several studies have revealed species exhibiting phenotypic plasticity in shell form due to environmental factors or predation (Trussell 2000; Weigand et al 2011), and uncovered cryptic species with the aid of molecular data (Haase et al 2007; Puillandre et al 2010; Jörger et al 2012). These studies question evolutionary hypotheses based on species delimited by shell characteristics alone and point to the need for an integrative approach using both molecular and morphological data in future research
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