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. 2024 Feb 14;14(2):e11014.
doi: 10.1002/ece3.11014. eCollection 2024 Feb.

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

Kara K S Layton  1   2 , Nerida G Wilson  3   4
Affiliations

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

Kara K S Layton et al. Ecol Evol. .

Abstract

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 the 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 multigene 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.

Keywords: Nudibranchia; divergence rate; geminate pairs; molecular clock.

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Conflict of interest statement

The authors declare that there are no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
Sampling locations for 25 Chromolaichma specimens collected from either side of the Isthmus of Panama. Sample numbers appear next to locations. Images derived from iNaturalist (on left, photographer: S. & J. Johnson) and Museum of Comparative Zoology (on right, photographer: G. Giribet).
FIGURE 2
FIGURE 2
(a) Maximum‐likelihood phylogeny (COI + 16S + ANT) for Chromolaichma ‘sedna’ where triangles represent collapsed clades, hash marks indicate the branch was shortened by 50%, and bootstrap support is presented at nodes. Individual bars represent congruent clades from the ABGD and bPTP delimitation analyses. (b) COI haplotype network where color corresponds to geographic location and the size of circles corresponds to sample size. Roman numerals are assigned to unique haplotypes and black circles denote mutational steps. Images derive from iNaturalist and represent C. sedna and Chromolaichma hemera sp. nov. lineages from Baja California and Chetumal, Mexico (photographers: on left, A. Velasco and on right, María Ecléctica Photography).
FIGURE 3
FIGURE 3
Chromolaichma hemera sp. nov. holotype collected from Smithsonian Tropical Research Station (STRI) (MCZ:M:381343) (photographer: G. Giribet).
FIGURE 4
FIGURE 4
Images from iNaturalist representing Chromolaichma sedna (a–d) and Chromolaichma hemera sp. nov. (e–h) illustrating key differences in the color and crenulation of the mantle edge. C. sedna from (a, b) Mexico (photographers: V. Mas and M. Krampf), (c) El Salvador (photographer: A. Trejo), and (d) Costa Rica (photographer: K. Soto Venegas). C. hemera sp. nov. from (e) Florida (photographer: A. Shure), (f) Bahamas (photographer: M. Bokach), and (g, h) Belize (photographers: T. Heusse and A. McKinlay).
See this image and copyright information in PMC

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