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HABs pose a threat to coastal ecosystems, the economic sector and human health, and are expanding globally. However, their influence on copepods, a major connector between primary producers and upper trophic levels, remains essentially unknown. Microalgal toxins can eventually control copepod survival and reproduction by deterring grazing and hence reducing food availability. We present several 24-h experiments in which the globally distributed marine copepod, Acartia tonsa, was exposed to different concentrations of the toxic dinoflagellate, Alexandrium minutum, grown under three N:P ratios (4:1, 16:1 and 80:1), with the simultaneous presence of non-toxic food (the dinoflagellate Prorocentrum micans). The different N:P ratios did not affect the toxicity of A. minutum, probably due to the low toxicity of the tested strain. Production of eggs and pellets as well as ingested carbon appeared to be affected by food toxicity. Toxicity levels in A. minutum also had an effect on hatching success and on the toxin excreted in pellets. Overall, A. minutum toxicity affected the reproduction, toxin excretion and, to an extent, the feeding behavior of A. tonsa. This work indicates that even short-term exposure to toxic A. minutum can impact the vital functions of A. tonsa and might ultimately pose serious threats to copepod recruitment and survival. Still, further investigation is required for identifying and understanding, in particular, the long-term effects of harmful microalgae on marine copepods.

Marine micro-organisms must cope with complex flow patterns and even turbulence as they navigate the ocean. To survive they must avoid predation and find efficient energy sources. A major difficulty in analysing possible survival strategies is that the time series of environmental cues in nonlinear flow is complex and that it depends on the decisions taken by the organism. One way of determining and evaluating optimal strategies is reinforcement learning. In a proof-of-principle study, Colabrese et al. (Phys. Rev. Lett., vol. 118, 2017, 158004) used this method to find out how a micro-swimmer in a vortex flow can navigate towards the surface as quickly as possible, given a fixed swimming speed. The swimmer measured its instantaneous swimming direction and the local flow vorticity in the laboratory frame, and reacted to these cues by swimming either left, right, up or down. However, usually a motile micro-organism measures the local flow rather than global information, and it can only react in relation to the local flow because, in general, it cannot access global information (such as up or down in the laboratory frame). Here we analyse optimal strategies with local signals and actions that do not refer to the laboratory frame. We demonstrate that symmetry breaking is required to find such strategies. Using reinforcement learning, we analyse the emerging strategies for different sets of environmental cues that micro-organisms are known to measure.

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.

Blooms of the toxic dinoflagellate Karenia mikimotoi cause devastation to marine life, including declines of fitness and population recruitment. However, little is known about the effects of them on benthic copepods. Here, we assessed the acute and chronic effects of K. mikimotoi on the marine benthic copepod Tigriopus japonicus. Results showed that adult females maintained high survival (>85%) throughout 14-d incubation, but time-dependent reduction of survival was detected in the highest K. mikimotoi concentration, and nauplii and copepodites were more vulnerable compared to adults. Ingestion of K. mikimotoi depressed the grazing of copepods but significantly induced the generation of reactive oxygen species (ROS), total antioxidant capacity, activities of antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase), and acetylcholinesterase. Under sublethal concentrations for two generations, K. mikimotoi reduced the fitness of copepods by prolonging development time and decreasing successful development rate, egg production, and the number of clutches. Our findings suggest that the bloom of K. mikimotoi may threaten copepod population recruitment, and its adverse effects are associated with oxidative stress.

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.

Two new species of copepods of the genus Elaphoidella Chappuis, 1929 were discovered in a cave and a spring in northeastern Thailand. The first species, E. phuphamanensis sp. nov., belongs to species-group VII sensu Lang. It is most similar to E. turgisetosa Petkovski, 1980 in the armament of the male third exopod of the fourth swimming leg and the shape and armament of the fifth swimming leg in both sexes. However, it is easily distinguished from other congeners by the segmentation of the first swimming leg, the endopod of the fourth swimming leg, and the armature of the third exopod of swimming legs 2–4 in both sexes. The second species, E. propecabezasi sp. nov., is located in species-group I sensu Lang, where the male does not have a transformed seta on the third exopod of the fourth swimming leg and the female fifth swimming leg has four baseoendopodal robust setae, unequal in length. It is most similar to E. cabezasi Petkovski, 1982 and E. paraaffinis Watiroyram, Sanoamuang and Brancelj, 2017 in having the same armature formula as endopods 1–2 of female swimming legs 1–4. However, the ornamentation of the anal operculum, the shape of the caudal ramus, and the armature of the fifth swimming leg in both sexes distinguish them from each other. A rare gynandromorphic specimen of E. propecabezasi sp. nov. was recorded, and a revised key to Elaphoidella species in Southeast Asia is provided.

Calanoida is a representative oligo-mesotrophic indicator species, frequently used as an index for assessing lake ecosystems’ health. Additionally, they function as trophic intermediates in the food web, connecting primary producers and higher consumers within lake ecosystems’ food chains. However, research cases that present the implications of habitat environment assessments, such as ecosystem structure and water quality, represented by these taxa remain insufficient. In this study, we conducted research across 49 lakes in South Korea, analyzing the occurrence characteristics of Calanoida and examining the correlations between Calanoida abundance and water quality parameters and the morphological-based functional groups of phytoplankton. Calanoida were more frequently observed in lakes characterized by greater size, increased depth, and reduced levels of anthropogenic land use. Furthermore, Calanoida occurrence was more probable in environments characterized by lower electrical conductivity and suspended solids concentrations. Their presence was also associated with conditions where Large mucilaginous phytoplankton (MBFG7), which includes cyanobacteria, and Large filamentous phytoplankton (MBFG3) were prevalent. An analysis of the environmental factors influencing the increase in Calanoida abundance revealed an inverse relationship between their abundance and water quality factors, including nutrient levels. This trend was observed consistently across all genera. Additionally, Calanoida were observed to maintain a high abundance in environments where the presence of the Large mucilaginous phytoplankton group (MBFG7) was relatively high. In contrast, Cyclopoida exhibited varying occurrence characteristics by genus in response to different water quality factors. Based on these results, we suggest that Calanoida, commonly used as an indicator of mesotrophic conditions, can also serve as a valuable indicator for evaluating the functionality of the food web. While Calanoida struggle to inhabit environments characterized by degraded water quality, they demonstrate the ability to adapt and persist in environments containing large, mucilaginous, or filamentous phytoplankton species that are typically challenging for other zooplankton to graze.

Copepods serve as a major link in marine food webs, bridging the energy transfer from primary producers to higher trophic levels. Oceanic warming is linked to reduced concentrations of essential fatty acids (FA) in phytoplankton, namely eicosapentaenoic acid (EPA, 20:5ω3) and docosahexaenoic acid (DHA, 22:6ω3), and it remains largely unknown if copepods have the capacity to endure. The calanoid Temora longicornis and the harpacticoid Platychelipus littoralis were chosen to analyse their FA and biosynthesis activity in response to a long-chain polyunsaturated FA (LC-PUFA) deficient diet (Dunaliella tertiolecta) along a temperature gradient. Copepods were fed D. tertiolecta labelled with the stable isotope carbon-13 (13C) to quantify carbon assimilation into their total FA and de novo EPA and DHA biosynthesis after 6 days incubated at 11, 14, 17, 20 and 23 °C. The calanoid had increased mortality with warming, whereas the harpacticoid exhibited high survival across the thermal gradient. After the incubation, P. littoralis assimilated minimal amounts of dietary carbon into its total FA in comparison to T. longicornis. T. longicornis depleted their field EPA and DHA stores more rapidly, whereas P. littoralis maintained its relative storage of EPA and DHA and absolute concentrations of DHA. T. longicornis displayed higher fractions of de novo EPA and DHA biosynthesis than P. littoralis at all temperatures, with the exception of DHA at 23 °C. Within our experimental incubation period both species were unable to meaningfully upgrade the LC-PUFA deficient algae to biosynthesize de novo EPA and DHA as a relevant source for higher trophic levels.

A new species belonging to the genus Parathalestris Brady & Robertson, 1873 was identified and described here based on samples recently collected from a survey on fouling organisms at the Zhonggang wharf in Qingdao, China. The new species differs from its congeners in the following characteristics: caudal rami 2.0 times longer than wide, with bulbous setae II and setae III of caudal ramus issuing terminally and laterodistally, respectively; exopod of antenna two-segmented, with two setae on first segment and four setae on second one;maxilla with two bipinnate setae on proximal endite, and three spinulose setae on middle and distal endites; basis of maxilliped with concave inner edge; P1 exopod and endopod approximately same length, enp-1 about 6.7 times as long as greatest width, with inner seta reaching proximal 2/3 of segment; baseoendopod and exopod of female P5 with transversal rows of spinules on its surface, exopod bearing six marginal setae; male P1 carrying an inner curved spine on basis; exopod of male P5 bearing six elements. The DNA barcode (COI) sequence of the new species was obtained and submitted to GenBank. This is the first report of Parathalestris from the China Seas.