Validating a molecular clock for nudibranchs: No fossils to the rescue

Time-calibrated phylogenies are typically reconstructed with fossil
information but for soft-bodied marine invertebrates that lack hard parts,
a fossil record is lacking. In these cases, biogeographic calibrations or
rates of divergence for related taxa are often used. Although nudibranch
phylogenies have advanced with the input of molecular data, no study has
derived a divergence rate for this diverse group of invertebrates. Here,
we use an updated closure date for the Isthmus of Panama (2.8 ma) to
derive the first divergence rates for chromodorid nudibranchs using
multi-gene data from a geminate pair with broad phylogeographic sampling.
Examining the species Chromolaichma sedna (Marcus & Marcus, 1967),
we uncover deep divergences among eastern Pacific and western Atlantic
clades and we erect a new species designation for the latter
(Chromolaichma hemera sp. nov.). Next, we discover extensive
phylogeographic structure within C. hemera sp. nov. sensu lato, thereby
refuting the hypothesis of a recent introduction. Lastly, we derive
divergence rates for mitochondrial and nuclear loci that exceed known
rates for other gastropods and we highlight significant rate heterogeneity
both among markers and taxa. Together, these findings improve
understanding of nudibranch systematics and provide rates useful to apply
to divergence scenarios in this diverse group.