In the present contribution we report, for the first time, records of the nudibranch Zelentia ninel from Norway. The species is previously known only from the Barents Sea coast in northern Russia where it was described for the first time in 2017. Records of Z. ninel from six different localities in northern Norway from 2019–2023 are presented and discussed. The identity of the specimens was determined by investigation of external and internal morphology as well as molecular analysis of preserved specimens. Photographs of live specimens and radulae of preserved specimens are presented, and species delimitation is confirmed by molecular phylogenetic analysis. In order to assess the biogeographic distribution of Z. ninel based on all available information, recent unanticipated genetic data supporting a putative occurrence of Z. ninel at the Aleutian Islands, Alaska is also discussed where we provide arguments in support of occasional anthropogenic transportation outside its natural range. The present records significantly expand the known natural range of distribution for Z. ninel and demonstrate that the species is not restricted to the Russian Barents Sea coast but that the range encompasses also at least northern Norway. These data can provide important insights to the understanding of natural ranges of marine invertebrates across biogeographical regions affected by rapid ongoing climatic change.

This work aimed to test whether the colour variability featured by the European nudibranch Polycera quadrilineata is consistent with the concept of a single polychromatic species or may hide multiple lineages. Samples from across the geographic range of P. quadrilineata together with representatives from worldwide species with a focus on Atlantic diversity, were gathered and studied using an integrative taxonomic approach. Morpho-anatomical characters were investigated by light and scanning electron microscopy. Bayesian molecular phylogenetics using MrBayes, the Automatic Barcode Gap Discovery species delimitation method, and haplotype network analysis using the PopArt software were employed to help delimit species using the mitochondrial gene cytochrome c oxidase subunit I (COI). The results supported the existence of a second species, here described and named Polycera norvegica sp. nov., only known from Norway where it is sympatric with P. quadrilineata. The COI uncorrected p-genetic distance between the two species was estimated at 9.6–12.4%. Polycera norvegica sp. nov. differs by exhibiting a black dotted or patchy dotted pattern occasionally with more or less defined orange/brown patches, but never black continuous or dashed stripes as in P. quadrilineata. The two species share a common colouration with a whitish base and yellow/orange tubercles. Anatomically, P. norvegica sp. nov. has a weaker labial cuticle, a smaller radula with fewer rows, and only four marginal teeth, a reproductive system with a single lobed bursa copulatrix, shorter reproductive ducts, and a penis armed with two kinds of spines: needle-like and hook-shaped penile spines.

A new Scyllaeidae of the genus Notobryon is described from Guadeloupe, in the Lesser Antilles. Notobryon caribbaeus sp. nov. is characterized by having the anterior pair of body lobes remarkably bigger than the posterior pair, a stomach with eight triangular plates, a black and very wide ampulla, a lemon-shaped bursa copulatrix and a complex and well-differentiated sponge-like prostate. The first Caribbean records of Notobryon were provisionally assigned to the Australian species Notobryon cf. wardi and later transferred to Notobryon panamica. However, the structure of the male genital system is one of the main morphological characters to discriminate species in the genus and the presence of a prostate in N. caribbaeus sp. nov. distinguishes it from N. panamica, which remains confined to the eastern Pacific. Of the remaining four species in the world, only Notobryon bijecurum shares this character, but its external anatomy is different: it lacks a bursa copulatrix and the deferent duct is much shorter. Notobryon caribbaeus sp. nov. was captured in the context of an intensive expedition (‘Karubenthos’) organized by the Muséum National d'Histoire Naturelle, Paris and its description raises the total inventory of sea slugs in Guadeloupe to 150.

The small volcanic island of Ascension is situated in the middle of the South Atlantic Ocean, more than 1500 km from the coast of Africa, its nearest continental area. To date, eight ‘opisthobranch’ species were reported from the island. As a result of a recent survey, 10 species were found. Seven species are new records from Ascension: Platydoris angustipes (Mörch, 1863), Diaulula sp., Dolabrifera dolabrifera (Rang, 1828), Aplysia parvula Guilding in Mörch, 1863 and Caliphylla mediterranea A. Costa, 1867, and two new species: Phidiana mimica sp. nov.; and Felimida atlantica sp. nov. Half of the species found have a wide geographical distribution, being not restricted to the Atlantic Ocean. However, traditional taxonomy based on few characters is probably masking complexes of species.

Preliminary molecular data obtained from one mitochondrial (COI) and one nuclear marker (H3) from the available species of the genus Okenia have revealed a new cryptic species of this genus. The new species (Okenia harastii sp. nov.), from New South Wales (NSW) eastern Australia, has a light brown body with scattered dark brown and white spots of different sizes, and six pairs of translucent mantle processes. Okenia harastii sp. nov. is compared with other Okenia species from NSW and with other morphologically similar species.

Polycerella emertoni and Favorinus ghanensis were originally described from Connecticut and Ghana respectively. While the last species has been found only twice since it was described, P. emertoni has already been reported in the eastern Atlantic and Mediterranean (Portugal, Gulf of Cadiz and Gulf of Naples). In this paper, we provide new records of both species from the region of Agadir (Morocco). Since the specimens studied in this paper were collected within harbour areas, we suggest shipping activities as the most plausible vector of introduction.

Three specimens of the genus Corambe were found in the intertidal zone of Santos, São Paulo (Brazil); two of them were identified as C. evelinae and one as C. carambola. Both species are reported for the first time since more than about 40 years ago.

A new species of the family Tritoniidae is described from the tropical South Atlantic Ocean. The animal was found off north-east Brazil. Tritonia khaleesi sp. nov. is up to 12 mm long, with a slender white body, of which the notum is covered with one broad white band extending from between the eyes and veil to the tail; veil with 4 velar appendages; retractable white rhinophores; rhinophoral sheath with fleshy extension; seven pairs of branchial plumes; the anus is located between the 3rd and 4th gills on the right side, and the genital opening is under the 2nd gill. Internally, T. khaleesi sp. nov. is distinguished from other tritoniids by jaws with 10–14 rows of denticles on the inner lips, absence of stomach plates and the radular formula 32 × 2–5.1.1.1.2–5 teeth. Tritonia khaleesi sp. nov. is the only Tritonia that possesses a unicuspid rachidian tooth as an adult.

A new species of the family Tritoniidae is described for the tropical South Atlantic Ocean. The animal was found off north-east Brazil. Marionia limceana sp. nov. is up to 31 mm long, with a sturdy white body, of which the notum is covered with two rows of red polygons running from the rhinophores to the tail; notum with tubers; bi-lobed veil with 16 velar papillae; retractable white rhinophores; 11–14 pairs of branchial plumes; the anus is located below the 4th gill on the right side, and the genital opening is under the 3rd gill. Internally, M. limceana sp. nov. is distinguished from other tritoniids by jaws with three or four rows of denticles on the inner lips, a belt of 18 stomach plates; and the radular formula 26 × 26–32.1.1.1.26–32 teeth. The animal was found feeding on octocorals of a species of the new genus Stragulum, and is the first reported nudibranch to feed on this genus.

The present study reports the first record of Herviella mietta from the Andaman Sea, Indian Ocean, more precisely from the Andaman and Nicobar Islands, India. The species is identified by colour pattern, morphological characters and microscopic investigation of the radula and the lateral denticles. The important diagnostic characters are the carrot-shaped cerata that are predominantly black in colour, black pigment on the body, and 8–9 denticles on each side of the small central cusp of each tooth.

The occurrence of the opisthobranchs Diaphana globosa, Hermaeopsis variopicta, Doris bertheloti, Rostanga rubra, Janolus cristatus and Flabellina bulbosa in the Azores is reported for the first time. Our findings expand the known geographical distribution of these species in the north-east Atlantic Ocean and increase the known diversity of opisthobranchs in the archipelago to 150 species. These results provide further support for the well-documented affinity of the Azorean marine biota to the eastern Atlantic and Mediterranean coasts.

The Brazilian Northeastern coast includes diverse ecosystems, particularly mangroves, coral and sandstone reefs. Although apparently high, the biodiversity of marine invertebrates from the State of Alagoas is still poorly documented. This study presents 28 new records of opisthobranch molluscs from Alagoas, 11 of which are also new records from the Brazilian Northeastern coast. In addition to comprising an important addition to knowledge about Alagoas reef biodiversity, the new records are useful data for future investigations such as biogeographical studies. Therefore, a list of all opisthobranch species known to date from Alagoas, as well as colour photographs of 32 species is provided.

A new opisthobranch gastropod is reported from the Azores resulting from fieldwork undertaken in the island of São Miguel (Western Group, Azores) during October–November 2009 and March–April 2010. Intertidal and subtidal areas down to 25 m deep were sampled. Thirteen species were collected one of which, Crimora papillata is recorded for the first time in the Azores, increasing the number of opisthobranchs in these islands to 142 (108 littoral species and 34 deep-water species).

Armina is the most species-rich genus of the Arminidae family with over 50 nominal species. Material of the genus Armina from the western Atlantic and the eastern Pacific was revised. Six species have been documented from the western Atlantic; however, we have determined that only four of them are valid: Armina muelleri, A. wattla, A. juliana and A. elongata. Also, only three out of seven species previously registered in the eastern Pacific were recognized in the present study: A. californica, A. cordellensis and Armina sp., an unnamed species. The phylogenetic analysis of 13 taxa and 17 characters was performed using the program PAUP (Phylogenetic Analysis Using Parsimony). The Branch-and-Bound algorithm generated a 29-step tree with the following relations: (Histiomena convolvula, (Dermatobranchus sp., ((((A. californica , (A. maculata, A. muelleri)), ((A. loveni, A. neapolitana), A. wattla)), (A. cordellensis, (Armina sp., A. juliana))), (A. tigrina, A. elongata)))). The monophyletic evidence for Armina is discussed and compared to possible speciation processes similar to those found in other Opisthobranchia groups.

The present paper reports the first record of Piseinotecus gabinierei (Mollusca: Piseinotecidae) in the Aegean Sea; the nudibranchs were found in December 2008 in two different sites on the island of Crete (South Greece). In the first location it was feeding on Eudendrium racemosum, a hydroid very common in this area while in the second location it was found crawling among algae.

Polycera hedgpethi and Godiva quadricolor are two species of nudibranchs widely spread along the Indo-Pacific by shipping. The first of these two species was originally described from Californian coasts, while the second was described from temperate South African coasts. Records of both species outside the Indo-Pacific are scarce, but have already been recorded in the Mediterranean (Gulf of Naples and Adriatic, Italy; French coasts). Moreover, G. quadricolor in the Mediterranean has been misidentified and its significance misinterpreted. In this paper, we provide new records of a significant number of specimens of both species from the region of Agadir (Morocco, Atlantic), as well as that of an additional specimen of G. quadricolor from the Bay of Algeciras (Strait of Gibraltar). This last species is recorded for the first time for the Moroccan and Spanish fauna and P. hedgpethi is also new for the Moroccan fauna. The shipping activities carried out in the areas where the specimens reported in this paper have been collected, as well as their habitual eutrophic conditions, lead us to the expectation of the settlement in the near future, if not yet, of well-established populations of both species.