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Marine copepod communities play crucial roles in ocean ecosystems. However, their accurate assessment remains challenging due to taxonomic complexities. This study combines morphological and DNA metabarcoding approaches to evaluate copepod diversity and community structure in the northern East China Sea. Zooplankton samples were collected from 10 stations along a coastal-offshore gradient in August 2019. Morphological analysis identified 34 species from 25 genera, while DNA metabarcoding detected 31 species from 20 genera. Both methods revealed distinct coastal and offshore assemblages, with Paracalanus parvus s.l. as the dominant species across all stations. A significant positive correlation was found between morphology-based individual counts and metabarcoding sequence reads (Spearman’s Rho  = 0.58, p  < 0.001), improving at the genus level ( Rho  = 0.70, p  < 0.001). Redundancy analysis revealed that salinity, temperature, and phytoplankton density significantly influenced copepod distribution. Although both approaches captured similar broad-scale patterns, they provided complementary insights into community structure. Morphological identification was more effective for detecting Cyclopoida diversity, whereas DNA metabarcoding had greater sensitivity for specific Calanoid species. This study underscores the value of integrating traditional and molecular methods for marine biodiversity assessment, especially in the context of global environmental changes.

The ecological role of planktonic harpacticoid copepods within marine ecosystems remains understudied. While primarily benthic, a subset of these organisms exhibits pelagic lifestyles. This research investigates the relationship between environmental factors and planktonic harpacticoid communities at Marina Beach, Chennai, a coastal area impacted by pollution from the Adyar and Cooum rivers. Zooplankton samples and environmental data were collected monthly from January to December 2021. Among the seven observed harpacticoid species, Longipedia weberi and Euterpina acutifrons displayed dominance, with mean abundances of 178.2 ± 190.54 ind./m 3 and 196.5 ± 155.8 ind./m 3 , respectively. Notably, the distributional pattern of L. weberi differed from that of E. acutiforns , with the former prevailing when micro-nutrient content in the water was high, particularly from June onwards. Regression analyses revealed a positive correlation between L. weberi abundance and nitrite and phosphate concentrations (R = 0.31 and 0.358, respectively). Furthermore, redundancy analysis indicated a close association between L. weberi and other nutrient parameters, suggesting its potential as an indicator species for coastal eutrophication along the Chennai coast. Throughout the study period, the diversity and distribution of harpacticoid copepods exhibited variations across different months.

The distribution of two species Eucyclops (Eucyclops) serrulatus (Fischer, 1851) s. lat. and Eucyclops (Eucyclops) agiloides (Sars G.O., 1909) s. lat. in the water bodies of the Old World is analysed in connection with the theory of continental drift. A hypothesis about the distribution of these morphologically related species and the role of the Tethys Sea in this distribution is presented. In the modern continental fauna of Eurasia, the northern records of E. agiloides s. lat. according to this hypothesis can be attributed to the remains of the fauna of the Paratethys. A similar distribution in South Asia is also shown by local populations of representatives of the genera Mesocyclops, Afrocyclops and Ochridocyclops, which supports the proposed hypothesis of a recent water connection across the continent in the form of Paratethys reservoirs in East Asia. Subspecies Eucyclops agiloides sarsi subsp.n. is described. The identification key for E. agiloides subspecies is provided.

In order to characterize the copepod communities associated with tubeworm and mussel aggregations around a hydrocarbon seep in the Green Canyon of the Gulf of Mexico, diversity, abundance, and community composition were analyzed. Also analyzed were species biogeography and the potential connectivity to other chemosynthesis-based habitats. Copepod abundance and biomass were very low among tubeworms and mussels, with 0.22 to 6.08 individuals per 10 cm 2 sampled area and 9.02 to 42.43 μg wet weight 10 cm 2 sampled area, respectively; but, abundance was significantly higher among the mussels. Fifty-five copepod species were identified, of which most were newly discovered and primarily belonging to the Harpacticoida order. Four copepod species were previously recorded from other food-rich and hard-substrata environments, such as hydrothermal vents or wood falls. Another four species showed close morphological proximity to species described from cold seeps, hydrothermal vents, and wood falls. Copepod diversity and community composition showed no significant differences between the foundation species. However, differences in the relative abundance and dominance of single species indicate a rather homogeneous community in mussel beds and a more heterogeneous community among tubeworms, indicating that foundation species may shape the abundance and community composition of associated copepods at cold seeps.

Calanoid, cyclopoid, harpacticoid and poecilostomatoid copepods were investigated over the year at five stations in the Chabahar Bay, Oman Sea. This area is under the influence of the Indian Ocean seasonal monsoons. The samples were collected using vertical plankton tows with 100 µm mesh nets. Copepods were identified into 20 genera and 59 species. Calanoid formed about 15% to 62% and cyclopoid 26% to 39% of total copepod abundance. Harpacticoid constituted about 6% in South West (SW)-monsoon and flourished well in pre (SW)-monsoon, formed 46% of copepod abundance. Poecilostomatoid accounted for approximately 5% to 13% of the total copepods. The most dominant species were Temora turbinata , Paracalanus elegans , Oithona nana and Euterpina acutifrons . The results showed that the species composition and distribution of copepods differed between the monsoon seasons, due to changes in hydrographic conditions. Furthermore, high abundance of small-sized copepods observed in offshore stations.

The metabolic theory of ecology (MTE) has explained the taxonomic richness of ectothermic species as an inverse function of habitat mean temperature. Extending this theory, we show that yearly temperature cycles reduce metabolic rates of taxa having short generation times. This reduction is due to Jensen’s inequality, which results from a nonlinear dependency of metabolic rate of organisms on temperature. It leads to a prediction that relatively lower species richness is found in habitats with larger amplitudes of yearly temperature cycles where mean temperatures and other conditions are similar. We show that metabolically driven generation time of a taxon also relates functionally to species richness, and similarly, its yearly cycles reduce richness. We test these hypotheses on marine calanoid copepods with 46,377 records of data collected by scientific cruise surveys in Mediterranean regions, across which the temperature amplitudes vary dramatically. We test both bio-energetic and phenomenological effects of temperature cycles on richness in 86 1° × 1° latitudinal and longitudinal spatial units. The models incorporated the effect of both periodic fluctuations and mean temperature explained 21.6% more variation in the data, with lower AIC, compared to models incorporated only the mean temperature. The study also gives insight into the basis of energetic-equivalence rule in MTE determining richness, which can be governed by generation time of taxon. The results of this study lead to the proposition that amplitude of yearly temperature cycles may contribute to both the longitudinal and the latitudinal differences in species richness and show how the metabolic theory can explain macro-ecological patterns arising from yearly temperature cycles.

Calanoids of the family Diaptomidae are the most widespread copepods in the lentic inland waters of the Palearctic region. In Italy, studies on the family date back to the end of 19th century. Since then, several papers contributed to increasing the knowledge on their presence, distribution, and ecological preferences. Nevertheless, new records for the area and the discovery of putative new species stress that the current knowledge on these inland water crustaceans is still far from being exhaustive. This paper presents an updated and annotated checklist and bibliography of the Diaptomidae of the Italian peninsula and surrounding islands, including Corsica and the Maltese islands, compiled through a critical review of the existing literature and carrying out further field research. The doubtful records reported in the literature are discussed and clarified. The updated checklist includes 30 diaptomid species and subspecies; among them, an alien species and three putative new species pending formal description are reported. About 20% of the observed species are endemic or subendemic to the study area. The faunal provinces ascribed to the Mediterranean limnofaunistic region host the highest species richness and contribute to the checklist with rare species and unique occurrences. The high species richness observed in the Mediterranean area supports the hypothesis of a long-lasting persistence of an ancient and peculiar copepod fauna.

This study explored how environmental variables influence the structure of epipelagic and pelagic copepod communities across tropical rivers, estuaries, and coastal ecosystems, considering the ongoing challenge of understanding the effects of these variables on copepod ecology in various tropical aquatic environments. Environmental variables were measured monthly over a complete 12-month cycle in the epipelagic (0.3 m below the surface) and pelagic (3.0 m below the surface) waters of the Kuantan River, which included two zones: downstream and upstream, as well as in its estuary and the nearest coast. This was done simultaneously with copepod data collection. Key findings indicated a total of 46 copepod species from 13 families were recorded. The highest species count was found at the coast (37 species), followed by the estuary (29), downstream (26), and upstream (18). Oithonoid copepods were dominant across all sampling zones (coast, 9.48 × 10 3 individuals m⁻ 3 , 28.3% of total copepods; estuary, 9.88 × 10 3 individuals m⁻ 3 , 30%; downstream, 7.81 × 10 3 individuals m⁻ 3 , 30.6%), except in the upstream zone, where freshwater cyclopoid copepods dominated (8.14 × 10 3 individuals m⁻ 3 , 41.9%) and harpacticoid copepods were absent. The average density of total copepods was similar in the coast (33.48 × 10 3 individuals m −3 ) and estuary (32.59 × 10 3 individuals m −3 ), both of which were greater than downstream (25.50 × 10 3 individuals m −3 ), followed by upstream (19.44 × 10 3 individuals m −3 ). The copepod species diversity was lower in the upstream zone (0.83) compared to the coast (1.20), estuary (1.17), and downstream (1.10) zones, with no difference among them. Water temperature (average range, 28.90–29.96 °C) negatively influenced the density of all observed copepod groups in all studied environments. Salinity did not limit the abundance of the copepod population at the coast and estuary but was a limiting factor for copepods in the river. The average salinity of as high as 0.97 PSU was harmful to observed freshwater copepod species, while its value ≤ 10.35 PSU performed as a controlling variable for observed marine species. The abundance of phytoplankton directly limited all observed copepod groups and indirectly amplified the negative effects of total suspended solids on cyclopoid and calanoid copepods. The remaining measured environmental factors such as pH, dissolved oxygen, turbidity, alkalinity, and phosphate did not affect the availability of any group of recorded copepods.

Aim Copepods are the key secondary producers in marine ecosystems, yet their in situ diet and symbionts remain underexplored due to technical challenges, limiting our understanding of their population dynamics and ecological functions. Using DNA metabarcoding, we jointly characterised the natural diet and symbionts of copepods across Chinese coastal waters, aiming to unveil their diversity, dynamics, and ecological drivers under various environmental conditions. Location Three coastal regions of Chinese seas: Qinhuangdao coastal waters (the Bohai Sea), the South Yellow Sea, the Yangtze River estuary and its adjacent waters. Methods Copepods were sorted and pooled by species for DNA extraction. The 18S rDNA of diet (gut content) and symbiont were amplified using modified copepod‐excluding eukaryote‐common primers and subjected to high‐throughput sequencing. Analyses covered OTU‐based diversity, feeding selectivity, host preference, and environmental drivers. Co‐occurrence network analysis characterised the potential association between prey and symbionts. Symbiotic ciliate genetic diversity was assessed via phylogenetic reconstruction. Results Our protocol effectively suppressed amplification from 13 copepod species while capturing all eukaryotic supergroups spanning over 30 phyla. Diverse diet and symbionts are associated with copepods, including taxa unreported in copepods or Chinese waters. Environmental filtering, rather than copepod identity, primarily shaped diet and symbiont communities, and prey‐symbiont associations also contributed. Copepods exhibited significant trophic plasticity, evidenced by wide feeding selectivity ratios across samples. Nine symbiotic ciliate ribogroups were revealed and unevenly distributed among hosts and regions, suggesting host specificity or environmental preference. Main Conclusions Our findings demonstrate the feasibility of DNA metabarcoding for profiling copepod natural diet and symbionts. We unveiled their higher diversity and dynamic variability than previously recognised, positioning copepods as amplifiers of hidden marine microbial diversity. Protecting the intricate trophic and symbiotic network associated with copepods is critical for future marine ecosystem conservation.

Many phytoplankton species produce toxic substances, but their functional role is unclear. Specifically, it remains uncertain whether these compounds have a toxic or deterrent effect on grazers; only, the latter is consistent with toxins as defensive tools. Here, we show that 10 of 12 species or strains of toxic dinoflagellates were consumed at lower rates than a similarly sized nontoxic dinoflagellate by a copepod. Through video observations of individual prey–grazer interactions, we further demonstrate that the dominating mechanism is through capture, examination, and subsequent rejection of vital cells, that is, a true deterrent effect that offers a straightforward explanation to its evolution. We argue that the diversity of grazer responses to toxic phytoplankton reported in the literature, including toxic effects, and the high diversity of toxin profiles between strains of the same phytoplankton species reflect different stages of an ever-ongoing evolutionary arms race, facilitated by rapid adaptation of grazers to toxic substances. We further argue that defensive toxicity requires a chemical signal exterior to the cell that informs the grazer about the toxicity of the cell. The signal can be the toxin itself or just an aposematic signal of toxicity. In the former case, allelochemical effects may emerge at high cell concentrations as a nonadaptive side effect of a predator defenses.