Young but distinct: description of Eubranchus malakhovi sp.n. a new, recently diverged nudibranch species (Gastropoda: Heterobranchia) from the Sea of Japan

Abstract

In this paper we perform an integrative investigation of the Eubranchus odhneri -Eubranchus sanjuanensis species complex. Several specimens morphologically similar to E. sanjuanensis were collected in the Sea of Japan. However, preliminary molecular data indicated that they were closer to the Arctic species E. odhneri. For our analysis we obtained new data on mitochondrial COI and 16S rRNA, and nuclear histone H3 and 18S rRNA molecular markers to reconstruct their phylogeny (Bayesian Inference and Maximum Likelihood approaches), delimit species with ASAP, bPTP and GMYC analyses and estimate divergence times of putative species. We also studied morphological and ecological features including external morphology, colouration, jaw and radular morphology, configuration of the reproductive system, ultrastructure of cnidosacs, and determination of possible diet preferences. Our results show that specimens from the Sea of Japan represent a new species described herein, Eubranchus malakhovi sp.n. It differs both morphologically and genetically from its closely related species, the Arctic E. odhneri and the North-East Pacific E. sanjuanensis. The new species is closely related to E. odhneri with low mtDNA divergence, and our divergence time estimations suggest these species split ~1.5 Mya.

Nematocyst sequestration within the family Fionidae (Gastropoda: Nudibranchia) considering ecological properties and evolution

“…Full description of cnidosac morphology in these two species given in Ekimova et al [53] (present study using CLSM and TEM confirms these data). Cnidophages large, voluminous cells containing numerous NCs (mastigophores), enclosed into vacuoles.…”

“…S6), while in other species, cnidophages are very large cells with a dozen of NCs per cell, like in E. odhneri (Fig. 3 D, I, see also [ 53 ]). In Tergipes tergipes, cnidophages are also large and contain numerous NCs, but many more intact NCs are located in the cnidosac lumen (Additional file 14 : Fig.…”

“…However, this taxonomical scheme indicates a paraphyly of the Trinchesiidae (because of the position of the genus Rubramoena ) and some genera like Trinchesia, Catriona and Cuthona. It also shows very poor node support and weak synapomorphies for other groups (i.e., Zelentia to other Trinchesiidae, Amphorina to other Eubranchidae) [ 40 , 43 , 52 , 53 ] . All this indicates a necessity for further revision of the group.…”

“…8 E). Eubranchus odhneri in the White Sea is often found on Sertulariella gigantea, and E. malakhovi occurs on different sertulariid hydrozoans [ 53 ]. At the same time, E. pallidus occurs in the same community as E. rupium in the Barents Sea.…”

Hidden beauty of the north: a description of Eubranchus scintillans sp.n. (Gastropoda: Nudibranchia) from the Barents Sea and North-East Atlantic

“…Moreover, two new species of the latter genus were described (Korshunova et al, 2020). However, some researchers highlighted poor phylogenetic resolution at the generic level and therefore did not support the validity of the genus Amphorina and the two newly described species because the genus is paraphyletic and new species do not form two monophyletic groups (Ekimova et al, 2021). Poor taxon sampling and high levels of undescribed biodiversity with-in the genus Eubranchus make it more difficult to resolve the phylogenetic relationships and further studies of this group are required.…”