Clinostomids are a group of digeneans in which substantial diversity has been recently discovered, with some metacercariae specific to their fish hosts. Genetic analysis has been instrumental in elucidating species diversity within this genus. Recently, seven COI lineages were reported in Argentina, along with three metacercarial morphotypes lacking molecular data. Two of these were found parasitising Rivulidae fishes. The discovery of Clinostomum metacercariae in Trigonectes aplocheiloides and Titanolebias monstrosus from temporary ponds in the western Chacoan region allowed us to redescribe the metacercariae previously identified by other authors and provide the first sequences of this lineage. The morphology of the metacercariae in both hosts matched previously reported descriptions. Genetic analysis clustered the new lineage with Clinostomum detruncatum, Clinostomum sp. 7, Clinostomum L1, and Clinostomum CRA. This discovery leaves only two morphological records of metacercariae to be characterised using DNA sequencing: one in another Rivulidae (Neofundulus paraguayensis) and one in a Loricaridae (Hypostomus sp.). The present results represent the eighth clinostomid lineage sequenced from Argentina, highlighting the extensive diversity in South America and the many lineages yet to be described or identified, considering that only one of these lineages is formally described based on adult specimens found in the heron Ardea cocoi.

Eleonora's falcon (Falco eleonorae Géné, 1839) is a well-known long-distance migrant of the Afro-Palaearctic flyway, a summer breeder of the Mediterranean region and North-west Africa and a winter resident of Madagascar and surrounding areas, thus characterized as a double endemic. Within the context of a long-term monitoring and conservation programme on Antikythira Island, Greece, which accommodates one of the largest concentrations of breeding pairs of Eleonora's falcons globally, birds were subjected to regular inspections for the presence of ticks from 2017 to 2023. In total, 104 adults and 149 nymphs (all belonging to Haemaphysalis genus) were collected. All ticks, apart from 2 nymphs, exhibited broadly salient palpi and did not possess the pronounced palpal segment 2 spurs or spur-like angles that are characteristic of adults, nymphs and most larvae of Rhipistoma, thus placed them in the Ornithophysalis subgenus. Following comprehensive morphological assessment and genetic analysis of the mitochondrial genome by means of next-generation sequencing of both adult and nymphal stages of the ticks, our empirical findings substantiate the delineation of a previously unclassified species. This taxonomic assignment situates the newly described species within the Ornithophysalis subgenus and the Haemaphysalis doenitzi group, marking its presence for the first time within the Western Palaearctic region.

Egg masses from an unknown mollusc have been found in South-West Iceland since 2020, but it was not until September 2023 that the adult organism was collected. Morphological analysis of both adults and egg masses pointed towards the identification of the species as Melanochlamys diomedea. This was further confirmed through DNA analyses using COI, H3, and 16S rRNA markers, which established the presence of a new non-indigenous species in the North Atlantic. Members of the genus Melanochlamys have predominantly been found in the Indo-Pacific basin and the Pacific Ocean, with only one species known to exist across the Madeira Islands, Canary Islands, and Cape Verde in the Atlantic. The known distribution range of M. diomedea extends from Alaska to California on the Pacific side of North America, where it typically inhabits sandy-muddy areas of the littoral in the tidal zone and below. It is not known how the species arrived in Iceland. However, maritime transport through either ballast water or biofouling is being considered as the most likely mode of dispersal.

Taxonomic intricacies and high interspecific similarity have hampered the identification of scyllarid phyllosoma larvae to the species level. The pygmy locust lobster, Scyllarus pygmaeus, is distributed across the Mediterranean Sea and in the eastern Atlantic; however, its phyllosoma larvae were previously recorded only from the western Mediterranean. We employed DNA barcoding using the mitochondrial COI gene to identify S. pygmaeus phyllosoma collected from the offshore waters of the southeastern Mediterranean Sea and described its morphology. We further discuss the lack of genetic structure in S. pygmaeus with potential implications for species connectivity and conservation.

Here we describe a new species of the genus Saccocoelioides found parasitizing Astyanax dissimilis Garavello & Sampaio, Psalidodon bifasciatus (Garavello and Sampaio) and Bryconamericus ikaa Casciotta, Almirón & Azpelicueta from the Iguazu National Park, Misiones province, Argentina. Saccocoelioides miguelmontesi n. sp. was studied based on morphological and molecular (28S rDNA and COI mtDNA sequences) data. The COI mtDNA tree indicated that the specimens collected from the three fish hosts are conspecific, with an intragroup p-distance of 0%. The new species shows an intermediate morphological configuration between the diminutive and robust forms described for Saccocoelioides by Curran (2018). Although, in the 28S rDNA tree, it is placed in a well-supported clade with the two robust species analysed (S. elongatus and S. magnus; p-distance of 1 and 2%, respectively), it differs from the robust group by the range of body size, mature egg size, oral and ventral sucker size, sucker ratio, oral sucker to pharynx ratio, and post-cecal or post-testis/body length percentage. Our results led us to redefine the robust group as having eggs shorter or equal in length to the pharynx. Saccocoelioides miguelmontesi n. sp. the 10th species reported from Argentina and the 7th species within the robust group.

Dermoergasilus madagascarensis n. sp. is described from the gills of Paretroplus polyactis, an endemic cichlid fish in Madagascar, using a combined morphological (light microscopy and SEM) and molecular approach (partial 18S rDNA, 28S rDNA, and COI sequences). The new species is characterized mainly by possessing: (i) roughly pentagonal cephalosome; (ii) antennal endopodal segments covered with slightly inflated membrane; (iii) maxillule bearing 2 equally long outer setae and a minute inner seta; (iv) interpodal sternites of swimming legs ornamented with 3–4 rows of spinules; (v) genital segment and first abdominal somite both barrel-shaped; and (vi) a caudal ramus projecting into a digitiform process with inconspicuous terminal seta and bearing 3 terminal setae. The obtained DNA sequences of Malagasy species represent the first molecular data for species of Dermoergasilus. The 28S rDNA phylogeny showed the affiliation of D. madagascarensis n. sp. to Ergasilidae and its sister relationship with cosmopolitan Ergasilus sieboldi von Nordmann, 1832. The first checklist for all species of Dermoergasilus is provided.

Clinostomidae is a diverse family of digenean parasitizing fish-eating birds as adults and fishes as metacercariae. The species composition, within the genus Clinostomum has been steadily increasing in recent years. In Argentina, four named species of Clinostomum have been documented, accompanied by four metacercariae representing distinct genetic lineages whose adults have not been identified. This study focused on examining clinostomids in three fish species – Australoheros scitulus (ASI), Cichlasoma dimerus (CDIM), and Pimelodella laticeps (PLA) – at various localities in Argentina. We conducted both morphological and molecular characterizations of the Clinostomum metacercariae collected from these fish species. Molecular phylogenetic analyses using COI mtDNA were performed to determine the placement of these metacercariae within the clinostomid phylogenetic tree. Clinostomum ASC represents a distinct lineage, morphologically distinguishable from other sequenced metacercariae due to its body shape (widest anteriorly and becoming slender towards the posterior end); this lineage was found to be closely related to C. caffarae. While Clinostomum CDIM and Clinostomum PLA exhibited morphological differences, they clustered together genetically with metacercariae reported in previous studies as Clinostomum L3 and Clinostomum CVI. This outcome, coupled with a low genetic distance (0 to 3%), suggests that they are conspecific with metacercariae found in fish across Mexico, Costa Rica, and Argentina. In light of the extensive diversity of fish species in Argentine freshwater ecosystems (over 500 species), and considering the relatively constrained extent of prior investigations, the anticipation of unearthing additional Clinostomum species or lineages is plausible.

Multiple parasites can infect a single host, creating a dynamic environment where each parasite must compete over host resources. Such interactions can cause greater harm to the host than single infections and can also have negative consequences for the parasites themselves. In their first intermediate hosts, trematodes multiply asexually and can eventually reach up to 20% of the host's biomass. In most species, it is unclear whether this biomass results from a single infection or co-infection by 2 or more infective stages (miracidia), the latter being more likely a priori in areas where prevalence of infection is high. Using as model system the trematode Bucephalus minimus and its first intermediate host cockles, we examined the genetic diversity of the cytochrome c oxidase subunit I region in B. minimus from 3 distinct geographical areas and performed a phylogeographic study of B. minimus populations along the Northeast Atlantic coast. Within localities, the high genetic variability found across trematodes infecting different individual cockles, compared to the absence of variability within the same host, suggests that infections could be generally originating from a single miracidium. On a large spatial scale, we uncovered significant population structure of B. minimus, specifically between the north and south of Bay of Biscay. Although other explanations are possible, we suggest this pattern may be driven by the population structure of the final host.

This is the first simultaneous morphological and barcoding characterization with the cytochrome c oxidase subunit I (COI) of the bramble shark Echinorhinus from the coast of Oman. The morphology of the specimen was consistent with previous records of Echinorhinus from the Gulf of Oman and the Arabian Sea (Northwestern Indian Ocean). However, the new COI haplotype clustered together with homologous sequences of specimens from India. The specimen from Oman distinguished morphologically and genetically from an E. brucus from the Western Atlantic Ocean on the shape and size of the dermal denticles, the proportions of twelve morphometric measurements (differences ⩾3%) and the genetic p-distance = 3.8% of the COI fragment. The haplotype reported here increases the genetic diversity in genus Echinorhinus in the Northwest Indian Ocean, demonstrates conspecificity between specimens from Oman and Echinorhinus cf. E. brucus distributed in India and extends its range of distribution. The limited morphological and molecular data available constrained assigning our specimen to other than Echinorhinus cf. E. brucus (Bonnaterrez, 1788). Our findings highlight the urgent need of morphological review, redescription and the assignment of a neotype in order to guarantee accurate species identification and thus effective conservation measures for these deep-sea sharks. The existence of a third living species in the genus is briefly discussed.

The bearded fireworm Hermodice carunculata is a conspicuous omnivorous benthic invertebrate distributed in the wider Atlantic Ocean, Mediterranean Sea and the Red Sea. Questions were raised in the past regarding the taxonomic status of amphiatlantic fireworm specimens but subsequent analysis of morphological data with new molecular data reconfirmed the presence of only one species, H. carunculata with amphiatlantic distribution. Hermodice carunculata has been the subject of several taxonomic and genetic studies but there are still questions regarding the genetic diversity and population structure of the species throughout its range. We contributed to the genetic studies of H. carunculata with ~200 new samples from over 20 locations in the Caribbean, Eastern Atlantic Ocean and the Mediterranean Sea. We sequenced the mitochondrial markers Cytochrome c oxidase Subunit I (COI) and Cytochrome b (Cytb) from each polychaete to examine the patterns of genetic diversity of H. carunculata. Our data revealed a significant population structure between the Caribbean H. carunculata including those from Brazil compared with those from the eastern Atlantic (Tenerife, Canary Islands) and eastern Mediterranean (Greece and Malta). We inferred rates of gene flow between eastern and western Atlantic populations by estimating the bidirectional effective migration rate (Nem). We corroborate previous studies indicating the existence of one species with genetically distinct populations. However, the biological significance of the observed population divergence should be evaluated with cross-breeding experiments.

Pomatomus saltatrix is a high-value marine pelagic coastal fish, that is fished throughout subtropical and temperate coastal waters around the world. Despite its large economic potential, there are no global data on its genetic diversity, which could compromise the conservation of the species. The aim of this study was to analyse the genetic-evolutionary structuring of the species, with the intention of evaluating different genetic P. saltatrix stocks that may indicate potential species. Based on 157 Cytochrome C Oxidase Subunit 1 sequences, the molecular delimitation analyses of species (distance and coalescence methods), as well as the haplotype network, found profound geographic structuring related to five distinct units with high and significant FST pairwise values. The divergence of these molecular units is mostly related to the Pleistocene glacial and interglacial cycles of climatic oscillations. It is hypothesized that one ancestral lineage, adapted to cold water environments, diversified into two lineages, with one more adapted to warmer environments. The high values of global genetic diversity (π = 0.016; h = 0.96) may be related to the existing profound genetic differentiation. Due to the presence of five Molecular Operational Taxonomic Units (MOTUs) within the species it is necessary to employ different regional management strategies. Traits of low haplotype richness and shallow population contraction were identified in the MOTUs V (Venezuela and Brazil) and III (Turkey and Australia), respectively, representing conservation priorities. Other molecular markers, as well as morphological data, should be explored with the aim of defining the taxonomic status of P. saltatrix stocks.

Bopyrid isopods of the genus Probopyrus are well-known parasites of freshwater prawns of the genus Macrobrachium. The parasitism of coastal populations of Macrobrachium amazonicum by Probopyrus bithynis, for example, has been documented since the late 1980s. Despite this, molecular data on different populations are not available for any Probopyrus species. The present study is the first to describe Probopyrus populations from distinct regions of the Amazon basin based on sequences of two genes, the mitochondrial cytochrome oxidase C subunit I (COI) and the nuclear 18S ribosomal DNA (18S rDNA) gene. The analyses indicated the presence of two Probopyrus species, each parasitizing either the coastal or the inland populations of M. amazonicum. The results indicated the potential use of the COI barcode for the identification of Probopyrus species. We discuss the potential implications of the findings for the taxonomy of Probopyrus bithynis and other species of the genus Probopyrus.

Of the five nominal species in the genus Caranx Lacepède 1801 distributed throughout the Eastern Central Pacific, Caranx caballus and Caranx sexfasciatus are the only two that have formal fish larval descriptions based on diagnostic characteristics (morphology, meristics and pigmentation). In this study, the diagnostic characteristics of three Caranx species larvae were validated using DNA barcoding analysis cytochrome c oxidase subunit I (COI; 651 bp). For the first time, the morphological taxonomic assignation of C. caballus fish larvae was confirmed using COI gene partial sequences of adults, with a genetic similarity between 99.8–100%. However, molecular evidence demonstrated that fish larvae previously described as C. sexfasciatus had high genetic similarity (99.7–100%) and low genetic distance (<1%) to Caranx caninus adults. An undescribed larval morphotype collected in the present study genetically matched (100%) with COI sequences of C. sexfasciatus adults. The diagnostic characteristics of this new morphotype were a lack of pigmentation in the supraoccipital crest, over the gut, and at the terminal region of the gut. The combination of diagnostic characteristics and DNA barcoding evidence allowed the discrimination and validation of C. caballus, C. caninus and C. sexfasciatus larvae. The diagnostic characteristics and COI sequences of Caranx lugubris and Caranx melampygus larvae, which are also distributed in the Eastern Central Pacific, remain to be investigated.

Evolution and population genetic structure of marine species across the Persian Gulf and Gulf of Oman are shaped by two complex factors: the geological history and the present pattern of marine currents. Characterizing and comparing the genetic structures of co-existing species, such as host–parasite associations, allow the discrimination of the relative importance of environmental factors and life history traits that influenced gene flow and demographic events. In this study, we compare two associated Clausidium copepod species (C. persiaensis Sepahvand & Kihara 2017 and C. iranensis Sepahvand, Kihara & Boxshall 2019) along the south coast of Iran in two geological settings. We analysed partial nucleotide sequences of the mtCOI gene using divergence estimates (FST) and haplotype networks to infer intraspecific population connectivity in the two Gulfs. The results obtained demonstrate the existence of two genetically and geographically distinct clades, corresponding to the Persian Gulf and Gulf of Oman populations. Our results also indicate high diversity, population expansion and high connectivity among populations of C. persiaensis and C. iranensis in the Persian Gulf and Gulf of Oman.

Members of Clibanarius virescens show considerable intraspecific colour variation, including colouration of the second/third pereopods (green/white) and the dactyls of the second/third pereopods (with or without dark bands/patches). However, factors inducing these colour variations have not yet been elucidated. Here, we investigated the occurrence of colour variation in this species with particular emphasis on change of colouration associated with growth stage and region in specimens from tropical/subtropical to warm temperate areas in the Indo–West Pacific, including evidence from molecular phylogeny based on the cytochrome c oxidase subunit I (COI). We have, then, clarified that the colouration on the pereopod dactyls gradually changed from solid colour (yellow/white) to having dark-coloured area(s) or transverse band(s) as a result of the growth stage. The frequency of occurrence of the solid colour dactyls was higher than those of other colour types in tropical regions. Our results also indicated that the white ambulatory leg type was the colouration type that was frequently seen in juvenile stages. However, significant genetic differences were not detected between each colouration determined by molecular analysis of samples from 14 localities in the Indo-West Pacific region; in contrast, two genetically differentiated regional populations (North Australia; Phuket, Thailand; and Lombok, Indonesia) were detected. The present study, therefore, emphasizes the necessity for further study on the colour variation of marine animals focusing on growth stages and regional differences, with molecular data to facilitate the research on adaptation and/or speciation, especially in geographically widely distributed species.

Horizontal and vertical distribution of cephalopod paralarvae (PL) from the Mesoamerican Barrier Reef System (MBRS) in the Western Caribbean was studied during two oceanographic cruises in 2006 and 2007. A total of 1034 PL belonging to 12 families, 22 genera, 24 species, 5 morphotypes and a species complex were identified. Abralia redfieldi, Onychoteuthis banksii and Ornithoteuthis antillarum were the most abundant taxa. The taxonomic identification from these three species was corroborated with DNA barcoding (99.8–100% of similarity). Paralarvae of Octopus insularis were reported for the first time in the wild. Most PL occupied the Caribbean Surface Water mass in the 0–25 m depth stratum. Largest paralarval abundances were related to local oceanographic features favouring retention such as the Honduras Gyre and Cozumel eddy. No day-night differences were found in PL abundance, although Abralia redfieldi showed evidence of diel vertical migration. Distribution of PL in epipelagic waters of the MBRS was probably related to ontogenetic migration, hydrographic features of meso and subscale, and to the circulation regimes dominated by the Yucatan Current. The MBRS represents an important dispersion area for PL, potentially connecting a species-rich Caribbean community with the Gulf of Mexico and Florida waters.

In the Laboratory of Parasites of Fishes, Crustaceans and Mollusks (CEPAVE), we undertook a parasitological study on three species of fish from the Espinal and Esteros del Iberá ecoregions of Argentina. Clinostomid metacercariae were found parasitizing Characidium rachovii, Crenicichla vittata and Gymnogeophagus balzanii. In this study, we analysed the damage that these parasites inflict on their hosts through the evaluation of histological sections. In addition, Clinostomum metacercariae were identified using morphological characters and DNA barcoding. In the pathological analysis, we observed that muscle tissue was the most affected. The inflammatory response showed vascular congestion areas and infiltration of numerous inflammatory cells, mainly lymphocytes. The molecular and morphological approach supports the presence of three new lineages of clinostomid metacercariae in Argentina. This could lead to the discovery of a high number of lineages or species of Clinostomum from South America.

The mojarras (Eucinostomus) are a widespread group of coastal fishes of controversial taxonomy because of similarities in their external morphology. In the present study, we assessed the genetic diversity of species and populations of Eucinostomus using DNA barcodes using a systematic and phylogeographic context. In total, 416 COI sequences of all valid Eucinostomus representatives were analysed based on public databases and collected specimens from the north-eastern coast of Brazil (Western South Atlantic). Several cases of misidentification were detected in the barcode dataset (E. argenteus, E. harengulus, E. gula, E. dowii and E. jonesii) that could account for the taxonomic issues in this genus. In contrast, we identified four molecular operational taxonomic units (MOTUs), with divergence above 2% in the Western Atlantic, that correspond to cryptic forms within E. argenteus, E. harengulus, E. gula and E. melanopterus. These data suggest that Plio-Pleistocene events (rise of the Panama isthmus, Amazonas outflow and sea-level fluctuations) played a major role in the diversification of mojarras. While subtle morphological differences have been used as proxies to discriminate Eucinostomus species, the genetic data proved to be efficient in differentiating them and revealing potentially undescribed taxa. Therefore, we recommend that further taxonomic studies in mojarras should incorporate DNA-based evidence.

A new species of ampithoid amphipod, Sunamphitoe gigantea sp. nov., is described from Katsurakoi, Hokkaido, Japan. The new species most closely resembles S. eoa, but can be distinguished from it by the shorter flagellum of antenna 1, the maxilla 1 inner plate that bears two slender setae, the short dactylus of male gnathopod 2, and the presence of a group of long setae on anterior margins of pereopods 3 and 4 bases. Nucleotide sequences of mitochondrial cytochrome c oxidase subunit I obtained from the type specimens also differentiated this new species from the congeners. Ontogenetic morphological changes of the male gnathopod 2 are briefly discussed.

A new species of the caridean genus Alvinocaris, A. marimonte sp. nov., is described and illustrated on the basis of specimens collected from hydrothermal vents at three locations in Izu-Bonin and Mariana Arcs of the north-western Pacific Ocean, Myojin Knoll (1224 m depth), NW Eifuku (1574–1582 m depth) and NW Rota seamounts (525 m depth). It is the 17th species assigned to Alvinocaris, and differs from all other congeneric species in several morphological characteristics derived from the rostrum length and armature, the carapace armature, the shape of the posterior margin of the telson and the armature of the third and fourth pereopods. Results of molecular phylogenetic analyses based on partial sequences of the mitochondrial COI and nuclear ITS-I genes compared between specimens of A. marimonte sp. nov. and other known species of Alvinocaris supported our recognition of the new species, although available data are still limited. Alvinocaris marimonte sp. nov. is represented by two morphotypes, distinguished by the armature of the posterior margin of the telson (spines vs plumose setae), which were not detected as separate species by the molecular analysis. Our discovery of the intraspecific variation in the armature of the posterior margin of the telson led us to reassess diagnostic characters used to distinguish species within the genus Alvinocaris.