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Aeolidida is one of the largest clades of nudibranchs with at least 560 known species. However, its systematics has not been studied in a comprehensive manner. Phylogenetic analyses of larger clades such as Nudibranchia or Cladobranchia have usually included a poor sample of aeolids. Furthermore, phylogenetic studies at the family or generic level in Aeolidida are a few and far between. The first molecular phylogeny of the aeolid family Aeolidiidae is presented here. This study, the most comprehensive for Aeolidida to date, uses new sequences of two mitochondrial (COI and 16S) genes and one nuclear gene (H3). 251 specimens from members of seven families of Aeolidida, including 39 species of Aeolidiidae were studied. Excluding Pleurolidia juliae, Aeolidiidae is monophyletic. Our results resolve the systematic relationships within the Aeolidiidae at a generic level, requiring changes in the systematics of this family. Spurilla, Anteaeolidiella, Limenandra and Aeolidia are well-supported and monophyletic clades. Aeolidiella stephanieae is transferred to Berghia and Aeolidiopsis ransoni and Spurilla salaamica to Baeolidia, to maintain the monophyletic lineages reflected in this study. The systematics of Cerberilla remains unclear. Some species earlier attributed to Aeolidiella are now grouped in a previously unnamed clade that we designate as Bulbaeolidia gen. nov.

In recent decades, thanks to the use of integrated taxonomy, the traditional recognition of a nudibranch species based on observation and colour pattern variation has become increasingly questioned, mainly due to the presence of cryptic and pseudocryptic species complexes. Individuals with the same colour pattern can be genetically identical, but individuals with different colour patterns may also be genetically identical and this variation may instead represent different life stages. But things can get even more complicated. What happens when the same species changes its colour pattern radically as it ages? Here we present two extraordinary examples in species of the genus Nembrotha based on laboratory observation. Specimens of Nembrotha livingstonei Allan, 1933 and Nembrotha yonowae Goethel and Debelius, 1992 were collected in Mozambique and kept in captivity as long as feeding was possible. The results showed that colour patterns in both species changed over time and that this change was linked to diet. Furthermore, species delimitation analysis and comparison of the uncorrected COI pairwise distances of examined specimens from Mozambique and others downloaded from GenBank confirmed that N. yonowae Goether and Debelius, 1992 is a junior synonym of N. cristata Bergh, 1877. Similar studies with laboratory observations are needed on other species of the genus, as they were described on the basis of different colouration, but integrated taxonomy may show different results.

The subfamily Polycerinae includes eight genera, from the monospecific Lamellana and Lecithophorus to the diverse Polycera and Gymnodoris, with 33 and 26 valid species, respectively. The monophyly of the subfamily has been tested by molecular data although not all genera were included. To date, relationships within the subfamily are not supported. In the present paper, three new species of polycerid nudibranchs are fully described based on specimens collected in Marshall Islands and Australia: one Palio species (Palio gaeli sp. nov.), one Polycera species (Polycera nimbsi sp. nov.) and a new genus (Paliota galactica gen. and sp. nov.). The new genus is described based on its peculiar radular teeth and genetic divergence. The internal anatomy was studied by dissections and scanning electron microscope photographs. Partial sequences of mitochondrial cytochrome oxidase c subunit I (COI) and 16S ribosomal RNA (16S) as well as nuclear histone H3 (H3) were also obtained. A phylogenetic framework for two of these three species is proposed, also including for the first time the species Paliolla templadoi and Polycera melanosticta.

Vayssierea is an understudied nudibranch genus characterized by its orange colouration and small size (up to 5 mm in length). To date, there are four described species, distributed in the Indo-Pacific Ocean. Here, individuals of Vayssierea were recorded for the first time in the North Atlantic Ocean on the Canary Islands (Spain). This study aims to evaluate the systematic and taxonomic status and distribution of the genus through multilocus phylogenetic, morphological, and radular analyses. Phylogenetic results show the monophyly of Vayssierea and evidence indicating that the genus is included in the new subfamily Okadaiinae stat. nov. within Polyceridae. According to species delimitation tests, four different species have been sequenced from Russia to Australia, in addition to our new records in the Atlantic Ocean, but more information is needed to identify the species. Nevertheless, our specimens from the Canary Islands belong to two different species, one of which is identical to the Australian species. Bearing in mind that they lack a planktonic larval stage; we hypothesize that they arrived by shipping transportation or aquarium releases, becoming a non-indigenous species of the Atlantic Ocean.

Species of Kaloplocamus Bergh, 1880 are enigmatic Nudibranchia sea slugs, and only two valid species are reported in the northwestern Pacific. Kaloplocamus japonicus (Bergh, 1880) was initially described based on alcohol-fixed specimens. In the latest revision of Kaloplocamus , it was synonymized with Kaloplocamus ramosus (Cantraine, 1835). Recently, several nudibranchs were collected from Tianheng, Shandong Province, China, and one of them is identified as an undescribed species described here as Kaloplocamus albopunctatus sp. nov. based on integrated approaches incorporating morphological observations, internal anatomy, and phylogenetic analyses of two mitochondrial (COI, 16S rRNA) genes. The other species is identified as K. japonicus Bergh, 1880 based on the anatomy of the reproductive system. The new species K. albopunctatus sp. nov. is similar to K. ramosus in having a bright orange-red color pattern but differs significantly in the structure of appendages and reproductive system. Kaloplocamus japonicus can be easily distinguished from other Kaloplocamus species by its translucent, white-pink coloration and unique features of the female reproductive organ. Both species are supported as distinct species in all molecular analyses. The phylogenetic analyses propose a new estimate of the relationship between Kaloplocamus and Plocamopherus , and the evolution of bioluminescence within Triophinae is discussed. Our results also suggest cryptic biodiversity within the K. ramosus species complex.

The subfamily Polycerinae includes four genera with around 46 species described to date. This subfamily is characterized by a limaciform body, which may have simple tentacular processes on the margin of the oral veil. Phylogenetic relationships between the genera of the subfamily Polycerinae (Polyceridae) have not yet been studied, and therefore, the only available information is based on morphological descriptions. The present study reports the first phylogenetic analysis of Polycerinae based on the mitochondrial genes cytochrome oxidase subunit I and the large ribosomal subunit (16S rRNA) using maximum likelihood and Bayesian methods. Our results showed that Polycerinae is monophyletic, but the relationships within the subfamily as well as within Polycera remain unresolved. A key finding of this study is that there are clearly two sympatric species of Polycera present in South Africa: Polycera capensis Quoy and Gaimard, 1824 also found in Australia and an undescribed Polycera sp. On the other hand, the studied specimens of the genus Gymnodoris were clustered within Polycerinae, reopening the problem of the systematic position of this genus. Additional genes and species of Polycerinae and Gymnodoris would provide more information and probably fully resolve this situation.

In this study, the complete mitogenome sequence of sea slug, Nembrotha kubaryana (Mollusca: Polyceridae), has been decoded for the first time by low coverage whole genome sequencing method. The overall base composition of N. kubaryana mitogenome is 31.5% for A, 14.0% for C, 16.4% for G and 38.0% for T, and has low GC content of 30.5%. The assembled mitogenome, consisting of 14,598 bp, has unique 13 protein coding genes (PCGs), 22 transfer RNAs and two ribosomal RNAs genes. The N. kubaryana has the common mitogenome gene organization and feature of Nudipleura (a clade of sea slugs and sea snails). The complete mitogenome of N. kubaryana provides essential and important DNA molecular data for further phylogenetic and evolutionary analysis for sea slugs and sea snails.

A new species of polycerid nudibranchs of the genus Tambja is described from Mallorca Island (Spain) and Malta. So far, only two species of Tambja had been recorded in the Mediterranean Sea with a distribution limited to southern Spain. With Tambja mediterranea sp. nov., the distribution of the genus in the Mediterranean Sea is extended, and the new species represents the first occurrence of Tambja at the Balearic Islands and Malta. Externally, the new species is mainly characterized by having ground orange-red colour, dorsum covered with rounded whitish tubercles, rhinophores red with whitish tips and three gill branches with orange-reddish rachis and whitish branches. In the present paper, external and internal features of T. mediterranea are described and compared with other species of the genus, especially with its most similar species, T. limaciformis . Additionally, phylogenetic analyses (Bayesian and maximum likelihood) based on mitochondrial sequences (COI) show that T. mediterranea sp. nov. is sister to T. divae and that both species cluster together with T. limaciformis and T. amakusana with the maximum support.

On two separate occasions during fieldwork in Semporna (eastern Sabah, Malaysia), sea anemones of the family Edwardsiidae were observed attempting to feed on the nudibranch species Nembrotha lineolata and Phyllidia ocellata . These are the first in situ observations of nudibranch predation by sea anemones. This new record is compared with known information on sea slug predators.

The use of morphological characters as the basis for species recognition and identification has permitted the development of a consistent taxonomy. However, limitations are evident when dealing with cryptic speciation or when intra-specific variability matches the total inter-species variation. Molecular techniques complement or enhance morphological inference by providing sets of data directly applicable to the taxonomic problem. Cases in which molecular techniques are particularly relevant are those involving larval or juvenile identification for which taxonomic characters are based on adult organisms and also those in which the original taxon description leads to uncertainty over the applicability of the species name. In this paper we report the use of mitochondrial DNA sequence data in a group of nudibranchs to exemplify the two cases mentioned above. The first issue is the longstanding debate on the taxonomic status of Tambja abdere and Tambja fusca, and the second issue is the identification of two juvenile specimens previously considered to represent two different undescribed species of the genus Tambja from the scarcely explored waters of Costa Rica. We also present a preliminary molecular phylogeny of the subfamily Nembrothinae.