Bioluminescence is abundant in marine taxa, including copepods. Bioluminescent copepods flash or spout a cloud of bright blue light to stun, deter, or distract their predators in the dark depths of the midwater. Despite their sophisticated survival strategies, bioluminescent copepods are heavily preyed upon by a variety of predators, including bioluminescent fishes, squids, and decapods. Copepod bioluminescence is commonly attributed to the enzymatic oxidative reaction of coelenterazine, a substrate also used for luminescence reactions in wide range of marine bioluminescence across various taxa. It has been confirmed that some of those animals, such as hydrozoan jellies, a lophogastrid shrimp, and brittle stars are unable to biosynthesize coelenterazine. Instead, they acquire it from their diet to produce their own light, a process known as ‘semi-intrinsic bioluminescence’. Bioluminescent copepods, such as Metridia pacifica, can biosynthesize coelenterazine from three amino acids: one phenylalanine and two tyrosines. It is likely that other bioluminescent copepods also share this capability. As a consequence, bioluminescent copepods are considered a major source of bioluminescent substrate for a wide range and quantity of marine bioluminescent animals. This connection helps account for the richness of bioluminescence in marine midwater fauna.

Seasonal and interannual variations in phytoplankton primary production (PP) and in abundance, biomass, daily production rate (DPR) and food requirement of planktonic copepods (>200 µm) were investigated in the neritic area of Sagami Bay, Kanagawa, Japan, on 426 occasions from January 2006 to December 2023. Monthly/annual mean PP was 0.39–1.81 g C m⁻² d⁻¹. Copepod abundance, biomass and DPR revealed distinct seasonal variations, declining in May and remaining low in June–September, related to abiotic/biotic variables. These copepod parameters showed different interannual variations, related to anomalies of abiotic variables and climatic indices. Monthly/annual means of abundance, biomass and DPR of total copepods were 5.70×10²–3.17×10³ ind. m⁻³, 3.61–11.83 mg C m⁻³ and 0.38–1.28 mg C m⁻³ d⁻¹, while those of depth-integrated copepod secondary (SP) and tertiary production (TP) were 0.018–0.060 and 0.003–0.012 g C m⁻² d⁻¹, respectively. Monthly/annual mean transfer efficiency of PP–SP, PP–TP and SP–TP was 1.0–9.9%, 0.2–1.6% and 8.5–22.9%, respectively. Transfer efficiencies of PP–SP and PP–TP decreased exponentially with PP, and these correlations were statistically significant. Monthly/annual mean food requirement by copepod secondary and tertiary producers was estimated to be 0.064–0.216 and 0.009–0.041 g C m⁻² d⁻¹, corresponding to 3.5–34.0% and 0.6–5.3% of PP, respectively. Estimation methods for DPR calculated by several empirical growth models were discussed, based on gross growth efficiency (i.e. the ratio of DPR to food requirement).

We monitored the zooplankton community in the Salerno Gulf during a dredging operation, between 2020 and 2021, to study the spatial and temporal distribution. Salerno has upgraded its harbour, marking it as Europe’s most significant effort in marine sediment mobilization, and an environmental impact assessment protocol was developed to monitor the direct effects of dredging. During the present study the zooplankton community was dominated by copepods, with the main genera identified being Acartia, Clausocalanus, Corycaeus, Oncaea and Oithona. While the distributions of these genera remained consistent across the studied areas, variations in abundance were observed between the potentially impacted regions and the surrounding areas, likely linked to the dredging operations. One of the goals of this biomonitoring was to check for the presence of non-indigenous species. Two new records were discovered: the epipelagic harpacticoid Distioculus minor, which had not previously been found in this area, and the non-indigenous calanoid Pseudodiaptomus marinus. Further studies are needed to monitor the presence and potential dispersal of these new species, as well as the recovery of native species, to document a community that has not been previously recorded.

Copepods are one of the most diverse aquatic organisms and are critical to marine ecosystems. However, obtaining high-quality genomic data for copepods can be challenging due to their small size and low DNA content. Here, we relied on a technical strategy for sequencing the genome of three copepod species based on DNA enrichment from a single individual using the multiple displacement amplification method. This technique provides a reliable and efficient way to amplify small amounts of DNA, enabling the generation of sufficient DNA material for sequencing. We extracted DNA from a single individual and amplified it using the Repli-G kit (Qiagen). The amplified DNA was then sequenced using the Illumina NovaSeq platform. For one species, we also included long reads obtained with the Oxford Nanopore platform to perform a hybrid assembly. The resulting genome assemblies were evaluated in terms of contiguity and completeness. We also recovered nearly-circular mitochondrial genomes, revealing substantial rearrangements of gene features. Methods like this that facilitate the generation of high-quality genomic data will contribute to the field of copepodology, ranging from evolutionary studies, comparative genomics, phylogeny, and population genetics, among others.

We provide an overview of available knowledge on the Lake Baikal harpacticoid copepod fauna. The Harpacticoida is the third most species-rich group of crustaceans in Lake Baikal. They are abundant and are represented on all bottom biotopes. Although the species composition had been relatively stable and there were general assumptions about the origin of the Baikalian endemic fauna and the reasons for its immiscibility with the Siberian fauna, the application of molecular methods to their study has resulted in new insights. These include newly discovered species, species formerly classified as endemics being shown to be widespread in the Palearctic, and single species found to be probable species complexes. The research priorities identified at the beginning of the 21st century by Okuneva & Evstigneeva (2001) remain relevant: 1) re-describing most species and establishing type material in museum collections, 2) undertaking a complete revision of the Baikalian Canthocamptidae, 3) studying the behavior and biology of species, 4) using molecular sequence data to explore their evolutionary history.

In this study, the abundance and community composition of meiofauna on intertidal sandy beaches were investigated at four beaches in Rayong Province, Eastern Gulf of Thailand. A total of 12 meiofauna taxa were identified, of which nematodes was the most dominant (47.96%), followed by gastropods (13.34%) and copepods (8.04%). The highest values of Margalef’s taxon richness (d) and Shannon-Wiener diversity (H′) were found at Mae Ramphueng Beach with values of 1.44±0.25 (min=0.99, max=1.87) and 0.69±0.14 (min=0.37, max=0.94), respectively. In this study, seven families of meiobenthic copepods were identified, while no copepods were found at Saeng Chan Beach. Meiofaunal densities across the four sites ranged from 1 to 426 ind. 10 cm⁻², with an average of 104.16±10.60 ind. 10 cm⁻². The highest density was recorded at Mae Ramphueng Beach (196.80±28.75 ind. 10 cm⁻²), while the lowest was observed at Sang Chan Beach (15.56±3.81 ind. 10 cm⁻²). Spatial variation among the study sites was significant (F=28.01, p<0.01). Permutational MANOVA revealed significant differences in the meiofaunal composition among the four study sites (p<0.01), with Sang Chan Beach showing the most different community structure. Canonical analysis of principal coordinates (CAP) indicated that grain size, sea surface temperature (SST), and organic matter content were the key environmental factors influencing meiofaunal composition. This study provides essential baseline data on marine meiofauna in Thailand, supporting future ecological research.

Many specimens of the canuelloid copepod genus Longipedia (Copepoda: Canuelloida) were collected during a meiofauna survey in May 2023 at the intertidal zone at Mairood Sandy Beach of Trat Province, Eastern Gulf of Thailand. These specimens turned out to be a new species, Longipedia tratensis sp. nov. of the family Longipediidae. At present, 16 species of the genus Longipedia are known. Longipedia tratensis sp. nov. is the seventeenth species of the genus. The new species, L. tratensis sp. nov., is morphologically similar to L. thailandensis in the presence of an anterior unguiform projection on P2 enp-1, in the lack of a posterior armature element on P2 enp-2, and in the presence of a spiniform inner element on P4 enp-1. Longpedia tratensis sp. nov. differs from L. thailandensis in the following characters: number of setae on proximal endite of maxilla (four in L. tratensis sp. nov., but six in L. thailandensis), P1 coxa with small outer spinules proximally (present in L. tratensis sp. nov., absent in L. thailandensis), P1 basis without long setules on outer margin (with several long setules proximally in L. tratensis sp. nov.), relative length of the inner seta on P2 and P3 enp-1 (comparatively longer in L. tratensis sp. nov.), and structure of the male P5 (both baseoendopods confluent in L. thailandensis, but separated in L. tratensis sp. nov.).

Benthic copepods, the second most abundant meiofaunal taxa, are mainly composed of harpacticoids, which are generally sensitive to environmental stressors, such as oxygen depletion. However, harpacticoid copepods in the family Cletodidae are known to cope with hypoxic conditions. Tolerance of Cletodidae to oxygen depletion has been reported in a previous study in Omura Bay, a semi-enclosed, seasonally hypoxic bay in Nagasaki, western Kyushu, Japan. To gain insights into the genetic diversity of the copepods in the bay, we initiated DNA barcoding of selected specimens obtained from April through October 2022. Based on genetic comparison of a portion of the mitochondrial cytochrome c oxidase subunit I gene (COI), we found that all specimens of the family Cletodidae represented Geehydrosoma cf. intermedia, a member of a recently established genus. Morphological examination of their exoskeletons that had been secured after the DNA extraction provided good support for that tentative species assignment. Furthermore, it was suggested that G. cf. intermedia in Omura Bay is genetically differentiated from its counterparts in the eastern margin of Eurasia. Continued research, including a broader sampling of G. cf. intermedia specimens, is clearly required to clarify the genetic diversity of this harpacticoid copepod.

Grass puffer Takifugu alboplumbeus swims upstream in rivers to eliminate certain pathogens. In the Seto Inland Sea, Japan, and its neighboring brackish areas, the parasitism of ectoparasitic copepods infecting T. alboplumbeus was compared between the sea and brackish waters to clarify this hypothesis from April to November 2022. A marine station was set up off Takehara City, Hiroshima, and two stations for surveys in brackish waters were located at the mouths of two rivers flowing into the sea. The salinity of the bottom water at the two brackish stations was nearly 30, the same as the surface water at the marine station, whereas that of the surface water at the brackish stations was lower than 7. The host harbored four species of ectoparasitic copepods: Caligus fugu (adults and copepodids), Brachiochondria pinguis (adult females), Parabrachiella hugu (adult females), and Taeniacanthus yamaguchii (adults and copepodids). The adult and copepodid stages of C. fugu were found on the body surface and fins of the host, respectively, whereas those of the other species were found in the mouth cavities or gills. The mean abundance of all species and stages was significantly lower at the river mouths than that at the marine station. Parabrachiella hugu and B. pinguis firmly adhering to the mouth showed low prevalences at the brackish stations. In laboratory experiments, low salinity caused C. fugu to detach from their hosts and die. The present study demonstrated that freshwater bathing can efficiently eliminate freely moving ectoparasitic copepods.

During INDEX 2018 on a cruise as part of the Indian Ocean Exploration project in the area of the German exploration license for polymetallic sulfide deposits, a new species of Aphotopontius Humes, 1987 was collected. To date, there are 12 described species of the genus Aphotopontius found in deep sea hydrothermal vent fields around the world, with 3 species occurring in the Indian Ocean: A. kiost Lee J, Kim D & Kim IH, 2020, A. muricatus Lee J, Kim D & Kim IH, 2020 and A. limatulus Humes, 1987. The new species Aphotopontius ilianae sp. nov. shares most characters with its congeners but in particular with A. mammillatus Humes, 1987 and A. acanthinus Humes & Lutz, 1994, the presence of mammilliform processes on the basis of leg 1 and with A. kiost, the unsegmented leg 5 and endopod-1 of the maxilliped, armed with three setae. Confocal laser scanning microscope images and mtCOI barcoding complement the traditional taxonomic description. The present research not only describes a new species of Aphotopontius and extends the distribution of the genus to the Southeast Indian Ridge, but also provides 40 new sequences of dirivultids to the National Center for Biotechnology Information (NCBI) and explores the connectivity of multiple species of the genus.

Morphological and molecular analyses identifed Biacanthus debera n. sp. (Cyclopoida: Taeniacanthidae) as the second member of the previously monotypic genus Biacanthus which was originally proposed to accommodate Anchistrotos pleuronichthydis (type species by original designation). Biacanthus debera n. sp. is associated with the Longnosed-fivespot Flounder Pseudorhombus ocellifer (Paralichthyidae) and can be differentiated from B. pleuronichthydis by two key features: the tip of the terminal segment of the maxilliped showing a stronger curvature (approximately 90° vs 120°) and the presence of transverse rows of minute spinules on the ventral surface of the anal somite that are absent in the type species. Although recent phylogenetic studies have challenged the monophyly of the Bomolochidae and Taeniacanthidae, the present analysis based on 18S rDNA, 28S rDNA, and cytochrome c oxidase I (cox1) sequence data of a broad sampling of members from both families obtained in Japanese coastal waters revealed different phylogenetic relationships. Our analysis supported the early divergence of Pseudotaeniacanthus (here proposed as the type genus of Pseudotaeniacanthidae n. fam.) in apposition of two distinct monophyletic clades, representing the Taeniacanthidae and Bomolochidae, respectively. A comprehensive list of parasites known to infect P. ocellifer is provided.

The genus Acanthochondria has a global distribution and accommodates 54 valid species. Nineteen of those species have been reported from Japan. A new species of Acanthochondria parasitizing a deep-sea fish Chlorophthalmus albatrossis (Chlorophthalmidae), living at depths of 370 to 400 meters from the Bungo Channel, Japan, is described and illustrated. The new species, Acanthochondria toyoshiomaruae sp. nov., can be distinguished from its other 54 congeners by the following features: (1) short and stout body, 2.2 times as long as wide; (2) trunk stout, 0.9 times as long as wide (excluding the posterior process), 2.6 times wider than head; (3) cephalosome as long as wide; neck slightly narrower than head, as long as cephalosome; (4) funnel-shaped genito-abdomen and the abdomen gradually narrowing posteriorly, with dorsally curved apex; (5) egg strings multiseriate, cylindrical, highly coiled and longer than trunk. This is the first record of this genus from a chlorophthalmid host. A checklist of valid global species of Acanthochondria with hosts and distribution is also provided. Two species of Acanthochondria (A. bicornis and A. sagitta) are deviating from the generic diagnosis and Both of the species are regarded as Chondracanthidae incertae sedis.

The occurrence of adult females of two euryhaline ergasilid species of parasitic copepods—Ergasilus hypomesi and E. wilsoni on larval and juvenile Japanese smelt (Hypomesus nipponensis) was investigated in Lake Abashiri, a brackish lake in eastern Hokkaido, Japan, from May to August 2016. The first E. hypomesi infection in Japanese smelt was observed in late June, and the prevalence of parasitism (infection rate) increased to 13.9% in middle July and 35.7% in early August. The mean parasitism intensity of E. hypomesi was mostly stable (1.2 parasites/fish) in July and August. Meanwhile, the first infection by E. wilsoni occurred in mid-July, three weeks after the first E. hypomesi infection, and the prevalence of parasitism was 3.0% in July and 5.6% in August. The mean parasitism intensity by E. wilsoni was 1.1–1.3 parasites/fish. The prevalence of parasitism by E. hypomesi was higher than that of E. wilsoni, likely because of the low salinities in the lake. The timing of ergasilid infections may be affected by the morphological development of hosts. Most individuals of E. hypomesi attached to the fins and body surface of hosts, corresponding to the development of fins in these young fish. Meanwhile, because E. wilsoni attached to the gills mainly and the intensity of parasitism by E. wilsoni increased with the growth of host fish, their infection might be delayed until the complete development of gills.

The male of Dinemoleus indeprensus reported, to date, only on the megamouth shark, Megachasma pelagios, is described in detail. It has a 2-segmented abdomen, the outer margin of the distal endopodal segment of leg 3 is unmodified, and leg 4 has a 1-segmented exopod but the endopod is indistinctly 2-segmented. This combination of character states is unique in the family. Males are now known for 20 of the 22 genera currently accepted as valid within the Pandaridae, so a brief overview of sexual dimorphism within the family is presented and a key to genera, based on males, is provided to facilitate identification in the future.

Caligus grandiabdominalis was originally described from Indonesia based on adult females obtained from the redbelly yellowtail fusilier Caesio cuning, although it has not been subsequently reported. In this study, we describe the adult male of C. grandiabdominalis for the first time, along with a redescription of adult females, and present a molecular characterization based on materials collected from a fish market in Sabah, Malaysia, in 2022. Caligus grandiabdominalis is a member of the Caligus bonito group, with which it shares the following characteristics: (1) leg 4 three-segmented, (2) distal exopod segment of leg 4 with one outer and three terminal spines; (3) leg 1 with three well-developed setae along the inner margin of the distal exopod segment; and (4) a row of denticles present along the outer margin of the second endopod segment of leg 2. It can readily be distinguished from other species within the group by the absence of a sternal furca. The genital complex and abdomen of females were observed to differ in shape, which could be associated with the degree of maturation. In one of the females, the posterior corners of the genital complex were more conspicuously developed than in the original description. The genital complex and abdominal somites of the remaining three female specimens were not laterally expanded, consistent with the original description. COI sequence analysis revealed a perfect match for females, whereas the genetic distance between the sexes was 0.020. Consequently, molecular analysis revealed the examined individuals to be conspecific.

The flathead Platycephalus sp. 2 (“Magochi”) is a commercially and recreationally important marine fish in Japanese coastal waters. During the course of our surveys on the occurrence of metazoan parasites of this fish, we found that individuals caught from Tokyo Bay are commonly infected with the parasitic copepod Taeniacanthus platycephali (Yamaguti, 1939) and a monogenean parasite Platycephalotrema ogawai Kritsky & Nitta, 2019 on their gill filaments, and in a few cases can also be infected with another species of parasitic copepod Acanthochondria tchangi Yü, 1935 on the operculum and buccal cavity. Infection levels and microhabitat use of these parasites were therefore investigated, and the relationship with host condition was tentatively assessed for these two common gill parasites. As a result of examinations of the host obtained from seven coastal sites, the present study suggested that (1) P. ogawai is the numerically dominant-ectoparasite, (2) the abundance of the parasitic copepod T. platycephali increases significantly with host body size, (3) host body condition declines as P. ogawai abundance increases, and (4) P. ogawai represents a niche specificity in that it is abundant in the anterior gills, whereas T. platycephali does not follow such a bias.

The Spanish mackerel Scomberomorus commerson is an economically important fish species in Malaysian fisheries. This study investigates the diversity, prevalence, and intensity of parasitic copepods found in Spanish mackerel from Terengganu, Malaysia with a redescription of Caligus infestans. Nine fish specimens were collected from three wet markets and their gill filaments were examined for the presence of parasitic copepods. The collected copepod specimens were identified to the species level, revealing three species from the family Caligidae; Caligus asymmetricus, Caligus infestans and Caligus cybii as well as one species from the family Pseudocycnidae, Cybicola armatus. Species prevalence ranged from 22% to 100%, with C. armatus being the most dominant parasitic copepods. The Cytochrome c oxidase subunit I (COI) genes of all species were sequenced, providing the first COI sequences for Caligus infestans and C. asymmetricus, while confirming the species identification of C. cybii and C. armatus. These findings provide baseline data on the diversity of parasitic copepods of Scomberomorus commerson in Malaysia, along with new locality records and DNA information for all identified parasite species.

Based on the literature published between 1930 and 2024, this checklist assembles information on the parasitic copepods of fishes and other aquatic animals of Taiwan. During this 95-year period, a total of 233 valid species of parasitic copepods were reported, and they comprise 118 and 115 species in the orders Cyclopoida and Siphonostomatoida, respectively. The cyclopoid copepods span 14 families, consisting of the Anchimolgidae (13 spp.), Anthessiidae (1 sp.), Bomolochidae (26 spp.), Chondracanthidae (10 spp.), Clausiidae (1 sp.), Ergasilidae (3 spp.), Iveidae (1 sp.), Lernaeidae (2 spp.), Macrochironidae (1 sp.), Myicolidae (3 spp.), Rhynchomolgidae (1 sp.), Strepidae (1 sp.), Taeniacanthidae (23 spp.) and Xarifiidae (32 spp.). The siphonostomatoid copepods belong to 10 families, which are the Asterocheridae (2 spp.), Caligidae (78 spp.), Coralliomyzontidae (1 sp.), Dissonidae (2 spp.), Hatschekiidae (1 sp.), Lernaeopodidae (3 spp.), Lernanthropidae (18 spp.), Nicothoidae (1 sp.), Pandaridae (8 spp.) and Pennellidae (1 sp.). The reported copepods comprise both 175 and 58 species that are parasites of fishes and invertebrates, respectively. This checklist also includes records of the parasitic copepods not identified at the species level. Since the early 1960’s to date, Dr. Ju-shey Ho has greatly contributed to clarifying the fauna of the parasitic copepods of Taiwanese aquatic animals. Along with his Taiwanese coworkers, he reported 184 species (79% of the parasitic copepods known from Taiwan) and established one new family, three new genera, and 55 new species based on specimens from Taiwan. He also contributed to many excellent works of Taiwanese caligid copepods.

Copepods are known to have symbiotic relationships with protozoan organisms with significant parasitic or commensal associations with their adult or juvenile copepod hosts. However, taxonomic studies and distributional records of microcrustaceans and their symbionts remain incomplete in the Province of Sorsogon in the Bicol Region, Philippines. To address this knowledge gap, samples were collected from 19 sampling sites across Sorsogon Province across different freshwater bodies. Thermocyclops taihokuensis was the dominant cyclopoid copepod species in the epibiont-infested areas. Protozoan ciliates were prevalent in copepodites with an infestation rate of 0.27% (2 out of 747 copepodites examined). Epistylis niagarae were abundant on the urosome of a C5 copepodite from the Gogon reservoir site. While Zoothamnium sp. epibionts were dominant in the prosome of a C2 copepodite from an irrigation canal at the Prieto Diaz site. Both sampling sites are near potential habitats of freshwater fish for aquaculture. The findings of the preliminary survey provided the first accounts of copepod-epibiont interactions in Sorsogon’s inland freshwater bodies. The study also served as the first record of Zoothamnium and Epistylis ciliates as epibionts of copepods in the Philippines. The output of this analysis could contribute to more freshwater biodiversity studies in the Bicol region, promoting a holistic approach to exploring host-symbiont relationships in aquaculture habitats.

Members of the order Monstrilloida are enigmatic copepods, displaying a protelean life cycle that encompasses a modified nauplius acting as the infective stage, endoparasitic copepodids, and free-living, non-feeding, planktonic adults. Although adult monstrilloids can locally be very abundant in plankton samples, their larval endoparasitic stages are rarely encountered and, consequently, their host-parasite relationship remains poorly understood. During our study, two different morphotypes of larval monstrilloid copepods were found parasitizing two sympatric polychaete species (Haplosyllis spp.), which are themselves associated with a sponge host in the same location in Okinawa, Japan. Live observations of the infected worms enabled us to observe the emergence of monstrilloid copepodids from the host and their subsequent development into preadults and even adults. The release of the copepodid from the host was initiated and facilitated by the feeding tubes penetrating the host body wall. The two morphotypes, including their endoparasitic copepodid stages, differed significantly and could easily be distinguished by the number and shape of feeding tubes, color of ovaries and leg pigmentation. After molting outside the host, copepodids developed into morphologically distinct females and males which could be assigned to the genera Monstrilla Dana, 1849 and Cymbasoma Thompson, 1888, respectively. Molecular phylogenetic analyses based on both nuclear and mitochondrial markers confirmed that the morphotypes represent two different species. Sampling at several sites along the coast of Okinawa showed high infection rates in the Seragaki area throughout the year. The high prevalence of endoparasitic stages, in conjunction with the ready access to live material of the host, makes the Okinawan copepod-polychaete association a promising model for future research on the host-parasite interactions between monstrilloids and their invertebrate hosts.

The Chitonophilidae is a family of exclusively parasitic copepods that utilize various molluscs as hosts. Nucellicola is a monotypic genus of endoparasitic copepods found to parasitise the predatory dog whelk Nucella lapillus, showing a limited distribution along the east coast of the United Kingdom. Here, Nucellicola is reported for the first time from the Russian Arctic, in both the White Sea and Barents Sea, representing a significant range extension for the genus. The Arctic specimens were discovered during dissection of two members of the Buccinidae (true whelks), the common whelk, Buccinum undatum, and the common neptune, Neptunea despecta. Preliminary results based on 18S rDNA sequence data suggest the newly discovered chitonophilids likely represent two distinct species, different from Nucellicola holmanae but this requires confirmation with additional molecular markers.