The family Caecidae comprises approximately 260 species of micro snails, distributed in shallow tropical and temperate marine waters. Most of them carry tube-like shells originating from an unusual straightening process during development, leading to intraspecific variety. Ambiguities arising from this morphological diversity and small size, render purely shell-based taxonomy highly problematic. This work is the first molecular study of Caecidae and aims to understand this enigmatic shell evolution, to critically assess the existing classificatory system, and to test the monophyly of the established genera. A worldwide sampling, comprising the genera Caecum, Meioceras and Parastrophia, was used to generate multi-gene (nuclear 28S rRNA, mitochondrial 16S rRNA and COI) phylogenies by maximum likelihood and Bayesian inference analyses. The results reject the independent evolutionary origins of Meioceras and Parastrophia and place these taxa within Caecum. The existence of cryptic species indicates that morphological characters may be insufficient to define species boundaries. Molecular clock analysis suggests that major diversification within Caecum occurred 45 to 25 million years ago in the Indo-Pacific region. Character tracing on the phylogeny identified feature-poor shells as the ancestral state of Caecum, suggesting an evolution from a benthic progenitor via a decrease in size and adaptation to an infaunal habitat.
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