Over the past decade, microplastic research on ingestion and impacts in marine biota has received significant attention. Zooplankton has become a subject of interest due to their crucial role in marine food webs. This review focuses on trends in nano-and microplastics (NMPs) ingestion studies in marine zooplankton. Four groups of organisms were considered: protozoans, holoplankton, meroplankton and ichthyoplankton. Of 120 reviewed articles, holoplankton was the most studied group, with laboratory experiments dominating over field studies. Although NMPs sizes and polymer types are diversifying in laboratory experiments, their characteristics are still far from representing the complexity of NMPs found in nature. Polystyrene (as polymer type) and beads (as shape) are overrepresented in laboratory experiments (54% and 79%, respectively). Furthermore, most NMPs concentrations used in the laboratory exceed those found in the field. The units used to report ingestion of NMPs in zooplankton vary greatly, with "microplastics per individual" being the most frequently used. In addition, certain planktonic groups (e.g., protozoans and ichthyoplankton) and behavioral traits, such as ambush feeding, have been poorly investigated. This variability hampers comparisons between studies and thus mechanistic insights into NMPs ingestion in marine zooplankton.<p>This review identifies research gaps and it highlights the ongoing disparity between environmental and laboratory conditions in zooplankton ingestion studies. We encourage the scientific community to harmonize the reporting units for NMPs ingestion and focus on more environmentally realistic studies with a trait-based approach. Transitioning towards more hypothesis-driven experiments is crucial to clarify the mechanistic importance of environmentally relevant microplastic features.</p>

The entry of microplastics (MPs) into marine food webs is a major environmental concern. We investigated how the behavior of planktonic copepods influences the risk of MPs to enter marine food webs by applying a trait-based approach and by combining experiments (bottle incubations and video observations) with biogeographical analyses. We aimed to evaluate which type of feeding behavior is most risky in terms of MP ingestion and which marine geographical areas are more susceptible to MP ingestion by planktonic copepods. We used different species as models of the main foraging behaviors in planktonic copepods: feeding-current, cruising, ambush, and mixed behavior feeding. All behaviors showed a similarly low risk of MP ingestion, up to 1 order of magnitude lower than for similar-sized microalgae. We did not observe any influence of the prey type or MP size (8 and 20 μm) on MP ingestion for any of the behaviors. By mapping the global distribution of feeding behaviors, we showed that feeding-current feeding is the most common behavior, but the risk of MP ingestion remains equally low across the global ocean, independently of the predominant behavior. Overall, our results suggest a low risk of MP ingestion by planktonic copepods and therefore a minimal risk of trophic transfer of MPs via marine pelagic copepods in marine ecosystems.

Planktonic copepods are the most abundant animals in the ocean and key players in global biochemical processes. Recent modeling suggests that zooplankton ingestion of microplastics (MPs) can disrupt the biological carbon pump and accelerate a global loss of oceanic oxygen. Here we investigate the behavioral responses and ingestion rates of a model feeding-current generating copepod when exposed to microplastics of different characteristics by small-scale video observations and bottle incubations. We found that copepods rejected 80% of the microplastics after touching them with their mouth parts, in essence exhibiting a kind of taste discrimination. High rejection rates of microplastics were independent of polymer type, shape, presence of biofilms, or sorbed pollutant (pyrene), indicating that microplastics are unpalatable for feeding-current feeding copepods and that post-capture taste discrimination is a main sensorial mechanism in the rejection of microplastics. In an ecological context, taking into account the behaviors of planktonic copepods and the concentrations of microplastics found in marine waters, our results suggest a low risk of microplastic ingestion by zooplankton and a low impact of microplastics on the vertical exportation of fecal pellets.

Planktonic copepods have sexual dimorphism that can lead to differences in starvation tolerance between genders. Additionally, mating may be energetically costly and thus reduce starvation tolerance. We investigated the influence of sexual dimorphism and mating on starvation tolerance of copepods with different feeding behaviours: Oithona nana (ambusher), Temora longicornis (feeding-current feeder) and Centropages typicus (cruiser). Males of C. typicus and O. nana had a starvation tolerance lower than females, whereas T. longicornis had a similar starvation tolerance between genders. Only O. nana males and females had reduced starvation tolerance when both genders were incubated together, which suggests that mating activities in ambushers have an energetic cost higher than in active feeding copepods. C:N ratios showed a non-significant difference between genders, which indicates that gender differences in starvation tolerance are not due to dissimilarities in lipid reserves. Gender differences in starvation tolerance can be partially explained by body size differences between sexes. This indicates a minor influence of mate-seeking behaviour on male starvation tolerance, likely due to reduced mate-searching behaviour under prolonged starvation. Our results demonstrate that sexual dimorphism can result in different starvation tolerance between copepod genders and that a negative effect of mating on starvation tolerance depends on the foraging strategy.