Mitogenomic Phylogeny Of Mud Snails Of The Mostly Atlantic/Mediterranean Genus Tritia (Gastropoda: Nassariidae)

The mud snails endemic to the East Atlantic/Mediterranean region (genus Tritia; subfamily Nassariinae) account for the second highest diversity within the family Nassariidae (Gastropoda: Buccinoidea). In order to understand how the diversity of species, shell morphologies and ecological traits evolved within this genus, a robust phylogenetic framework is needed, yet still unavailable due to high levels of homoplasy in shell morphology, the main trait used for their taxonomic classification. Here, the near-complete mitogenomes of 20 species representing more than half of the diversity of Tritia were sequenced. All mitogenomes of Tritia shared the same gene order, which is identical to the consensus reported for caenogastropods. The reconstructed phylogeny indicates that all analysed Tritia species formed a natural group except Tritia vaucheri, which was sister to an early diverging clade within subfamily Nassariinae that includes species of genus Reticunassa sister to Nassarius jacksonianus and Nassarius sp. Within Tritia, the North-west Atlantic species Tritia obsoleta was placed as the sister group of three mostly East Atlantic/Mediterranean clades (I-III), prompting the reinstatement of the genus Ilyanassa. The latter three clades corresponded to different shell features (I, shell mostly with marked sculpture; II, shell with strong nodules and small size; and III, smooth shell). For Tritia incrassata, the analysed specimens from Norway and from the Spanish Mediterranean coasts showed notable genetic divergence, which may indicate the existence of cryptic species. The ancestral character state reconstruction of protoconch inferred that the ancestor of Tritia had planktotrophic larvae and that a transition to lecithotrophy occurred independently at least three times within Nassariinae. The reconstructed chronogram dated the origin of Tritia in the Oligocene and main diversification events during the Miocene to Pleistocene, correlated with drastic shifts in local paleoecosystems caused by cooling events, eustatic sea level changes and the Messinian Salinity Crisis that favoured temperate taxa.