Re-evaluating the case for poecilogony in the gastropod Planaxis sulcatus (Cerithioidea, Planaxidae)

Abstract

Abstract Background Planaxis sulcatus has been touted as a textbook example of poecilogony, with members of this wide-ranging Indo-Pacific marine gastropod said to produce free-swimming veligers as well as brooded juveniles. A recent paper by Wiggering et al. (BMC Evol Biol 20:76, 2020) assessed a mitochondrial gene phylogeny based on partial COI and 16S rRNA sequences for 31 individuals supplemented by observations from the brood pouch of 64 mostly unsequenced individuals. ABGD and bGYMC supported three reciprocally monophyletic clades, with two distributed in the Indo-Pacific, and one restricted to the northern Indian Ocean and Red Sea. Given an apparent lack of correlation between clade membership and morphological differentiation or mode of development, the reported 3.08% maximum K2P model-corrected genetic divergence in COI among all specimens was concluded to represent population structuring. Hence, the hypothesis that phylogenetic structure is evidence of cryptic species was rejected and P. sulcatus was concluded to represent a case of geographic poecilogony. Results Our goal was to reassess the case for poecilogony in Planaxis sulcatus with a larger molecular dataset and expanded geographic coverage. We sequenced an additional 55 individuals and included published and unpublished sequence data from other sources, including from Wiggering et al. Our dataset comprised 108 individuals (88 COI, 81 16S rRNA) and included nine countries unrepresented in the previous study. The expanded molecular dataset yielded a maximum K2P model-corrected genetic divergence among all sequenced specimens of 12.09%. The value of 3.08% erroneously reported by Wiggering et al. is the prior maximal distance value that yields a single-species partition in ABGD, and not the maximum K2P intraspecific divergence that can be calculated for the dataset. The bGMYC analysis recognized between two and six subdivisions, while the best-scoring ASAP partitions recognized two, four, or five subdivisions, not all of which were robustly supported in Bayesian and maximum likelihood phylogenetic analyses of the concatenated and single gene datasets. These hypotheses yielded maximum intra-clade genetic distances in COI of 2.56–6.19%, which are more consistent with hypothesized species-level thresholds for marine caenogastropods. Conclusions Based on our analyses of a more comprehensive dataset, we conclude that the evidence marshalled by Wiggering et al. in support of Planaxis sulcatus comprising a single widespread, highly variable species with geographic poecilogony is unconvincing and requires further investigation in an integrative taxonomic framework.