# Demographic Models and Behavioral Assessments Uncover Distinct Species Histories in the Pseudocryptic Nudibranch Genus Hermissenda

## TL;DR

This study uses genetic and behavioral data to show that what was once thought to be a single nudibranch species is actually three distinct species with old divergence times and reproductive isolation.

## Contribution

The study provides new demographic and behavioral evidence supporting the split of Hermissenda crassicornis into three pseudocryptic species.

## Key findings

- The divergence times between Hermissenda species are estimated to be 0.55 and 1.29 million years ago.
- There is no recent or current gene flow between sympatric Hermissenda species.
- Assortative mating suggests a prezygotic reproductive barrier between H. crassicornis and H. opalescens.

## Abstract

The Pacific nudibranch
Hermissenda crassicornis
(sensu lato) is a well‐known model organism in neuroscience. This species was recently split into three pseudocryptic species based on differences in genetics, morphology, and behavior. We used ddRADSeq data from 33 individuals (2354 loci) and coalescent isolation‐with‐migration models alongside forward simulations to estimate and evaluate the demographic history of the clade. We inferred (1) a novel phylogenetic tree topology with the North American species as sisters, (2) relatively old divergence times (0.55 and 1.29 mya), (3) a much larger population size in the southern species
H. opalescens
, and (4) no recent or current gene flow between the sympatric species
H. crassicornis
and
H. opalescens
. Then we examined behavioral differences between sympatric
H. crassicornis
and
H. opalescens
to characterize possible mechanisms promoting their reproductive isolation. Mating experiments showed that both
H. crassicornis
and
H. opalescens
display assortative mating, consistent with the existence of a prezygotic reproductive barrier. Our results support the splitting of
H. crassicornis
(sensu lato) and reinforce the need to reassess previous studies that used the species complex as a model organism.

We reconstructed the speciation history of the recently split pseudocryptic nudibranch genus Hermissenda, finding divergence times in the Pleistocene (1.29–0.55 mya). Demographic modeling and behavioral experiments show no gene flow and assortative mating between sympatric species
H. crassicornis
(s.s.) and
H. opalescens
, consistent with prezygotic isolation, and show large differences in effective population size. Our results support the splitting of
H. crassicornis
(s.l.) and reinforce the need to reassess previous studies that used the species complex as a model organism.