Microplastic (MP) particles have been reported from surface waters to the deepest trenches in every ocean (Auta et al., 2017; Richon et al., 2022). The rise in oceanic contamination by these small (<5 mm) and sometimes down to the micro- to nanometer scale (Barnes et al., 2009; Ter Halle et al., 2017) anthropogenic debris may lead to widespread contamination of oceanic food webs. MP particles or fibers have become emergent pllutants in estuarine and marine ecosystems. Food-web contamination by MP has been observed in fish and other higher trophic levels (Neves et al., 2015; Ugwu et al., 2021; Nair and Perumal, 2022; Avignon et al., 2023).

Recent studies have found MPs in wild-caught and commercial fish stomachs and guts (Jabeen et al., 2017; Baalkhuyur et al., 2018; Halstead et al., 2018; Kumari et al., 2023b), indicating large-scale MP contamination of the oceanic food web caused by direct MP ingestion (Roch et al., 2020), indirect MP ingestion, such as consumption of contaminated prey, or MP entanglements (Setälä et al., 2014; Nelms et al., 2018; Zhang et al., 2019). The ubiquitous nature of MP pollution in air, water, and soil has become a major concern among environmental activists, scientists, and citizens alike (Cunningham et al., 2020). In estuarine and brackish water ecosystems, copepods and rotifers play an important role in linking nano-and microplankton to higher trophic levels (Beaugrand et al., 2003; Tseng et al., 2011). Food-web contamination may begin with MP ingestion by zooplankton, which is the primary consumer and constitutes the lowest heterotrophic trophic level of aquatic ecosystems (Kumar, 2005; Amélineau et al., 2016; Richon et al., 2022). Zooplankton contamination with MPs may also have several biological implications, such as decreased growth, reproduction, and lower survival rates (Cole et al., 2019; Isinibilir et al., 2023). Their position in the food web is at the interface between the primary producers and higher trophic levels (Kumar, 2005; Kerfoot and DeMott, 2019). Thus, MP ingestion by zooplankton may act as an entry point for MPs into the marine food web (Gunaalan et al., 2023).

The ingestion of inert particles such as plastic beads by calanoid copepods has been frequently documented (Bundy and Vanderploeg, 2002; Ferrao Filho et al., 2005). With the help of mechanoreception ability, calanoid copepods can detect boundary layer disturbances and ingest non-motile inert particles in proximity to the first antennae of copepods (Bundy et al., 1998; Ayukai, 2020). These copepods redirect and ingest large inert particles entrained in their feeding currents (Vanderploeg et al., 1990). MPs are chemically inert and morphologically similar to the natural food of copepods, and may affect them through entanglement, ingestion, and interference with other natural food organisms (Bhuyan, 2022). The smaller size and higher availability of MPs increases the probability of ingestion by marine organisms (Woods et al., 2020). The patchy heterogeneous distribution of MPs in estuaries (Akdogan et al., 2023; Rajan et al., 2023) and oceans (Pan et al., 2021) may also lead to zooplankton in some parts of the ocean being much more vulnerable to MPs than the rest. The extent of spread is also on the rise at an alarming rate, with more recent studies such as Kanhai et al. (2020) showing the presence of MPs even in the sea ice cores of the Arctic central basin. A range of zooplankton taxa have been shown to be able to ingest MPs of similar size to algal prey (7 μm–30 μm; 650 MP mL⁻¹–3,000 MP mL⁻¹), while smaller microplastics (3.8 μm; 40 MP mL⁻¹ × 10³ MP mL⁻¹) can externally adhere to a copepod’s functional appendages (Cole et al., 2015). Ingestion of plastic or latex beads, used as mimics or tracers of prey, by copepods has been reported by Frost (1977); Paffenhöfer and Van Sant (1985), and Cole et al. (2015). The feeding capacity and metabolic rates of calanoids, Centropages typicus and Calanus helgolandicus were significantly reduced in the presence of 6 μm–12 μm polystyrene beads (>4,000 MPs mL⁻¹) (Cole et al., 2013; Isinibilir et al., 2020). However, another recent study by Svetlichny et al. (2021) showed an increase in the consumption rate of pure microplastics in a mixture of microplastic beads (6μm diameter) and algae Rhodomonas salina (5 μm–10 μm size range) with equal concentrations of approximately 5,000 cells/beads mL⁻¹. The presence of micro- and nano-sized polystyrene in the environment has been observed to alter the feeding behavior of diving beetle (Cybister japonicas) and crucian carp (Carassius carassius) (Kim et al., 2018). The negative effects of MPs on physiology, organ damage, and even mortality have been shown in the laboratory (Franzellitti et al., 2019; Isinibilir et al., 2023), but only limited information is available on the behavioral impacts, feeding response, and functional and numerical responses of brackish water copepods commonly occurring in regions with higher MP pollution (Franzellitti et al., 2019; Cunningham et al., 2021; Rajan et al., 2023).

The ingestion of MP by zooplankton not only affects the aquatic food web, but also the entire ecosystem (Alava et al., 2023). Isinibilir et al. (2020) highlighted the negative effects of microplastic intake on the energy metabolism and swimming frequency of the calanoid copepod C. helgolandicus. As feeding is fundamental to the energetic requirements of an organism, MPs are likely to negatively affect zooplankton health by inducing a false sense of satiety, bioaccumulation in organisms, and blocking gut passage. Since copepods are an important component of the fish diet (Kerfoot and DeMott, 2019), they can also lead to trophic transfer of MPs to higher trophic levels, thereby posing a threat to the ecological functioning of the entire aquatic ecosystem.

Most feeding studies related to food selection have shown that copepods can distinguish between algal-flavored and unflavored artificial particles (DeMott, 1986), algae and polystyrene microspheres of similar size (DeMott, 1988), live and dead prey of the same species (Paffenhöfer and Van Sant, 1985; DeMott, 1988), toxic and non-toxic algae (Fulton and Paerl, 1987; DeMott and Moxter, 1991; Kumar, 2003), nitrogen-sufficient and nitrogen-limited algae of the same species (Cowles et al., 1998), algae and ciliates (Hartmann et al., 1993), ciliates and rotifers (Rao and Kumar, 2002), and larger and smaller rotifers (Dhanker et al., 2012). In contrast, some studies have reported that calanoid copepods are unable to exercise selection in regions with higher levels of suspended solids (Arendt et al., 2011) and tidally influenced estuaries with high sediment concentrations.

Previous studies have shown the effect of algae on animal prey consumption rate and alterations in the functional response of copepods (Kumar and Rao, 1999; Enríquez-García et al., 2013). Being numerically dominant and a preferred live food for fish larvae, Psudodiaptomus annadalei exerts a significant impact on the estuarine food web in the Indo-Pacific. Recent studies have demonstrated the predation efficiency of P. annandalei in natural setups, and ciliates and rotifers are important food items in their diet (Dhanker et al., 2012). P. annandalei is a generalist species with a large dietary niche breadth ranging from algal size, i.e., 7 μm–20 μm (Lee et al., 2010; Beyrend-Dur et al., 2011; Dhanker et al., 2015), to the size (150 μm–220 μm) of a rotifer (Dhanker et al., 2012). P. annandalei is able to select similarly sized rotifer over ciliate diet and show prey switching behavior on the basis of relative abundance of prey species. Dhanker et al. (2012) determined that the predatory success of P. annandalei on rotifers and ciliates followed an inverse size function, as observed for other invertebrate predators (Rao and Kumar, 2002; Hwang et al., 2009). However, there is limited knowledge regarding its ecology and functional feeding response. These organisms are currently facing potential threats from rapidly increasing MP concentrations in their immediate surroundings. MPs have been recorded in various domains of marine and estuarine environments (Li et al., 2019), which are supposed to be the habitat of the calanoid copepod P. annandalei, in the tropical and subtropical Indo-Pacific region. Another calanoid copepod, C. helgolandicus, showed a gradual decrease in food consumption rate during the 6-day experiment, feeding on a mixture of the microalgae R. salina and MP (Isinibilir et al., 2020). In a short-term experiment to evaluate the effect of initial MP concentration on consumption rate, Isinibilir et al. (2020) recorded a linear increase in MP consumption rates in C. helgolandicus with increasing concentration of MP in the medium, showing a Type-I functional response. Recent studies have shown the ability of copepods to distinguish between the chemical properties of MPs with the help of their mouthparts (Xu et al., 2022); however, despite the study showing the preference of zooplankton for its natural prey when both MP and natural prey are present, they do not shed light on the effect of MPs on the overall functional response of zooplankton. However, it is worth noting that the functional response can be best assessed in the presence of alternative prey (Iyer and Rao, 1996; Chan et al., 2017) as the functional response elucidates predator feeding rates with varying food densities (Solomon, 1949; Smith and Smith, 2020), thus, providing an estimation of energy transfer across trophic levels, understanding the stability and mechanism of coexistence of prey and predators (Oaten and Murdoch., 1975; Kumar and Rao, 2001). Xu et al. (2022) compares the ingestion of natural prey to MPs, it does not attempt to address and compare the number of natural prey consumed in presence and absence of MP.

Current knowledge of MP-imposed impacts on zooplankton largely focuses on the potential of zooplankton to engulf plastics. Most studies have used ingestion rates of MP as a parameter to describe the effects of MP on copepods (Jeong et al., 2017; Li et al., 2019).

Scientific knowledge of the functional response to a gradient of food density in MP-contaminated environment is essential to elucidate the adaptability of P. annandalei in an MP-contaminated environment and its response to the differential density of natural food. To date, no information is available on the estuarine calanoid, P. annandalei, whose natural habitats are contaminated by MP. A thorough understanding of the effects of MPs on the feeding ecology and foraging strategy of micro-meso-sized organisms is relevant to understand the effects of MPs on ecosystem functioning. To elucidate the impact of MPs on ecosystem functioning and feeding ecology, it is important to address the effects on the functional and numerical responses of an organism at the base of the food chain. Herein, we demonstrated the effects of MPs falling within the dietary niche size of the copepod P. annandalei. We also compared the effects of microalgae as an alternative food type in the medium to determine whether MP-imposed effects are similar to, or different from, alternative natural diets.

Thus, in the present study, we investigated the effects of MPs on the functional responses of P. annandalei (adult and copepodid stages, C2–C3) feeding on the rotifer B. rotundiformis and ciliate F. microstoma. We hypothesized that the presence of MPs along with the rotifer or ciliate would negatively influence prey ingestion rates in the copepod, P. annandalei, by inducing a false sense of satiety upon uptake of MPs, which would be reflected as an alteration in its functional response.

The presence of MPs significantly affected the functional response curves of adult P. annandalei feeding on either exclusive ciliate or exclusive rotifer diets. In general, P. annandalei adults exhibited a classical Type-II functional response to ciliates or rotifers when offered as monospecific or in combination with algae ( Figure 1 ). Where no clear asymptote was observed, the proportion of prey consumed in relation to initial prey density declined significantly in the MP-contaminated medium ( Figure 2 ). The prey ingestion rate initially increased with increasing prey density but reached a plateau, after which the prey ingestion rates slightly declined ( Figures 1A, B ). The maximum number of rotifers consumed at the highest prey density (160 ind. 100 mL⁻¹) in medium without MP was ~89.2 ± 5.51, while the maximum number of rotifers consumed by three adults P. annandalei at the highest prey density was (~40.4 ± 4.26) in the MP contaminated medium. The same prey species and density when offered in combination with MP showed a deviation from the Type-II functional response for copepodites ( Figure 2 ), as the gradual decrease in the proportion of prey consumed was not significant (Pearson’s correlation; R² = 0.018; P = 0.086) with increasing prey concentration ( Figure 3 ) for copepodites (C3–C4).

The copepodites (C3–C4) of P. annandalei exhibited different responses to the prey type and density. The functional response curve for copepodites with ciliate diet showed a Type-II curve, where the rate of prey consumption increased with an increase in prey density but eventually levelled off and declined thereafter. Type-II functional responses of copepodites were recorded with monospecific rotifer and ciliate prey types ( Figure 3 ). In the presence of microalgae, copepodites showed a stronger response to ciliate density (R² = 0.77; P <0.001), exhibiting a Type-II functional response ( Figure 3 ). The presence of MPs along with natural prey affected prey consumption in both adults and copepodites. The functional response curve of copepodites showed a Type-II functional response with rotifers both with a monospecific diet and with algae ( Figure 2A ), but the functional response curve in the presence of MPs showed linear regression with a negative slope ( Figure 3A ). The gradual decline in the proportion of prey consumed showed a highly significant correlation with the increasing initial prey density ( Figure 4 ). Copepodites did not exhibit any response with rotifers or ciliates to the changed initial prey availability in the medium treated with MP ( Figure 3 ). In the MP medium, the proportion of initial prey consumed by the copepodites did not correlate significantly (R² = 0.02, MP-rotifer; R² = 0.2, MP-ciliate) with increasing initial prey density ( Figure 4 ).

In the present study, adults in all cases ingested natural rotifer and ciliate prey, but the presence of MPs differentially affected ingestion depending on food type. Regardless of prey density, the prey ingestion rates were significantly lower ( Figure 5 ; p <0.01; one-way ANOVA) for rotifer prey than for ciliate prey. The ingestion of natural prey by P. annandalei adults and copepodites varied significantly with prey density, MPs concentration, and their interaction ( Figure 5 ; p <0.001; two-factor ANOVA). MP contamination did not affect prey ingestion in adults or copepodites at lower prey densities. The number of preys consumed in case of rotifer diet was maximum at the maximum prey density (160 ind. 100 mL⁻¹) ~80.4 ± 2.7 which reduced by ~60% in presence of MPs. A similar decrease in prey consumption at maximum density was observed in the case of ciliate F. microstoma (maximum prey consumed with only ciliate prey was ~117 ± 4.55, while in the presence of MPs, ciliate consumption was reduced to ~39.8 ± 2.305 ( Figure 5 ). At medium and high prey densities, we recorded a 40%–66% reduction in ciliate ingestion and 20%–60% reduction in rotifer ingestion by P. annandalei adults ( Figure 5 ; Table 2 ). In the case of copepodites, we recorded a 40% to 98% reduction in ciliate consumption and a 50% to 91% reduction in rotifer consumption due to the presence of MP in the medium ( Figure 5 ; Table 2 ).

The main aim of this study was to obtain definitive information on the impact of MP contamination on feeding rates in P. annandalei, a common brackish water copepod species, with natural prey. In the natural setup, the distribution of prey species and MP (Rajan et al., 2023) are patchy, where copepods encounter an array of prey concentrations (Hwang et al., 2010). Various types of MP, similar to the size range of microzooplankton, are abundant in coastal and estuarine ecosystems (Cole et al., 2019; Rajan et al., 2023).

Evaluation of MP imposed alterations in consumer behavior is complex because of (i) patchy distribution and natural concentrations of MP, (ii) limited information on MP concentration-specific ingestion of natural diet, and (iii) lack of quantitative data on behavioral responses of zooplankton at varying MP concentrations. Unless colonized by biota or affected by external pollutants, MP are inert particles, and the main mode of effects on consumers is food dilution and behavioral responses, such as functional and numerical responses of consumers to MP, as the polymer structure and chemical properties of particles are less important (Mehinto et al., 2022). While food dilution outweighs other effects, the major response of organisms with a dietary niche size overlapping with MP would be the adjustment of feeding rates, i.e., functional response.

Our results on prey ingestion by P. annandalei adults and copepodites were within the ranges reported for related taxa (Hwang et al., 2009; Dhanker et al., 2012; He et al., 2020). Recent studies have also confirmed ingestion of MP by copepods (Desforges et al., 2015; Cheng et al., 2020; Bai et al., 2021); however, this is the first study to understand the effect of MP on natural prey ingestion in P. annandalei. Contributing to the size range, sorption potential, hydrophobicity, wide occurrence in the global ocean (Foekema et al., 2013; Mathalon and Hill, 2014; Ugwu et al., 2021), and bioavailability to zooplankton (Cole et al., 2013; Setälä et al., 2014), ichthyoplankton (Rochman et al., 2013; Batel et al., 2016), and other meso-macro-sized invertebrates (Browne et al., 2008; Graham and Thompson, 2009; Gray and Weinstein, 2017; Kumari et al., 2023a), MPs are likely to have large-scale effects on marine ecosystem functioning. Various experimental results have revealed lethal and sublethal effects on different organisms (von Moos et al., 2012; Rochman et al., 2013; Au et al., 2015; Gray and Weinstein, 2017).

P. annandalei showed a direct functional response to varying concentration of prey in the medium. Thus, the presence of MPs (size range 15 μm–30 μm diameter) in the environment of P. annandalei directly affects the functional response and feeding behavior of both adults and the developmental stages of copepods. The prey ingestion rate showed a decline in the presence of MPs in all cases, suggesting interference of MPs in the feeding behavior and feeding efficiency of P. annandalei. Copepods can ingest plastics by either selective grazing or by indiscriminately ingesting particles with natural prey (Traboni et al., 2023). Copepods can also actively refuse MPs or passively reduce MP intake by physical impacts/interference or chemical toxicity associated with plastics (leachates or sorbed pollutants) (Cássio et al., 2022). Similarly, the presence of plastics in the environment, along with natural prey, may obstruct predator–prey interactions, leading to a reduced prey ingestion rate (Turner and Tester, 1989). The linear increase in prey ingestion by copepods with increasing prey density suggests a Type-I functional response, which assumes a linear increase in food ingestion rate with food concentration, either for all initial food concentrations or only for the particular maximum concentration, beyond which the feeding rate is constant. This could be attributed to the adaptive feeding response of P. annandalei, as has also been reported for other copepods (Abrams, 1990; Kumar and Rao, 1999; Kerfoot and DeMott, 2019). The MP incurred reduction in ciliate and rotifer prey ingestion, recorded in this study, indicating the adaptive feeding strategy, as adults of P. annandalei did not exhibit any discernible reduction in prey consumption at lower concentrations; however, at higher concentrations, it showed a 20% to 60% reduction in rotifer ingestion and a 40% to 66% reduction in ciliate ingestion. Thus, the presence of a more satiating prey has the potential to reduce MP uptake by copepods. P. annandalie is mainly a suspension feeder copepod, though occasionally capture a food item through a surprise jerky movement of appendages. A previous study recorded different feeding modes for P. annandalei when dealing with different prey sizes and mobilities (Dhanker et al., 2013). MP-driven alterations in the behavioral and physiological responses of the calanoid copepods Acartia clausi and C. typicus have been attributed to differences in the feeding strategies of the two species (Svetlichny et al., 2021). Further comparative studies are required to understand the role of different feeding strategies in the uptake of MP by P. annandalei.

In the present study, copepods showed neither linear nor curvilinear relationships with the initial prey concentrations in the medium with MP. Conclusively P. annandalei was much less efficient in feeding B. rotundiformis or F. microstoma when MP was present, indicating a negative impact of MPs contamination. The ability of copepods, such as Temora longicornis, to selectively refuse MPs and their preference for natural prey has been previously demonstrated (Xu et al., 2022). Overall, copepods have been shown to be equipped with diverse sensors on their antennae, feeding appendages or body surfaces for detecting either hydromechanical or chemical signals emitted by their prey (Turner and Tester, 1989; Vanderploeg et al., 1990; DeMott, 1995). The reduced frequency of prey predator encounters in the presence of MPs may be a factor in changes in the functional response. In addition, accidental or voluntary intake of MPs by P. annandalei could induce false feelings of satiety that may reduce the hunger level of copepods (Gommer et al., 2018; Botterell et al., 2019). MPs acceptance and rejection by copepods create a challenging environment for copepods to extract natural food particles from the environment because of the morphological similarity of MPs with the natural diet (Nielsen et al., 2020) and microbial coating on MP surfaces as biofilms (Zimmermann et al., 2019). The purpose of this study was to evaluate the effect of MP on the functional response (prey consumption rate) of P. annandalei. The complete life cycle of this species has been studied on an exclusive algal diet (Beyrend-Dur et al., 2011; Nielsen et al., 2020), and lower availability of microalgae showed an increase in MP ingestion and retention in P. annandalei (Cheng et al., 2020); therefore, it is also relevant to study the effect of MP on the clearance rate.

Earlier reports suggest ingestion of inorganic particles of low nutritional value (Arendt et al., 2011) resulting in pseudo-satiation (Turner and Tester, 1989). However, a lack of nutrients adversely affects an organism’s reproductive potential and inclusive fitness (White and Dagg, 1989; Kang, 2012).

The brackish water, egg-carrying euryhaline copepod, P. annandalei, is distributed widely in tropical and subtropical Indo-Pacific with an ability to survive in altered marine and brackish waters (Beyrend-Dur et al., 2011; Beyrend-Dur et al., 2013; Blanda et al., 2015). This species is widely used as a live feed in the aquaculture industry; however, the harvest of P. annandalei in aquaculture ponds has declined in recent years (Blanda et al., 2015). The present results emphasize the need to assess MP contamination in aquaculture ponds and further large-scale studies of the numerical response of P. annandalei in the presence and absence of MP. The impact of MPs on the functional responses of P. annandalei is of particular concern because they have the potential for trophic transfer to fish and humans (Lee et al., 2010; Nelms et al., 2018; Ugwu et al., 2021). Furthermore, P. annandalei which is a filter-feeder, is more vulnerable to MPs ingestion. It is important to consider that a cocktail of various additives and chemical contaminants (Rochman et al., 2019; Zimmermann et al., 2019) are adsorbed by plastic surfaces and ingested along with microplastics, which further adds to toxicity and oxidative stress (Jeong et al., 2017; Qiao et al., 2019). Although the present study does not suggest any toxic effects of MPs, the effects on functional responses are suggestive of chronic effects on ecosystem functioning of planktonic organisms, particularly at the base of the pelagic food webs, hints at the knock-on alterations in ecosystem functioning, resulting in trophic cascades (Galloway et al., 2017; Botterell et al., 2019; Nair and Perumal, 2022). MPs are expected to become more prevalent in aquatic environments because of their extensive use and dumping in aquatic ecosystems (Zeynep et al., 2023). Their potential to transport and adsorb toxic materials, as well as their absorbance by living cells, tissues, and organs of aquatic organisms, and interfere with physiological functions, all cause significant health risks. Could the observed MP imposed alterations in functional responses have implications for the numerical responses of copepods? The predator’s functional response is hypothesized to be an important determinant of the population structure and numerical responses (Dhanker et al., 2012; Dhanker et al., 2015).

To support the generalizability of the present results, we recommend studying a range of brackish water zooplankton species used as live feed in the aquaculture industry, by applying different types of MP particles.

This paper reports significant changes in the functional response curve of the adult and copepodid stages of P. annandalei when MPs are present along with natural prey. These results clearly suggest that MP contamination adversely affects the feeding rates of P. annandalei adults and juveniles fed on their natural diets. The impact of MPs on the functional responses of P. annandalei and prey ingestion rates is of particular concern because they have the potential for trophic transfer to fish and humans. Ingestion of inert particles, such as MP, gives a sense of false satiety, which in turn results in an altered prey selection pattern (Gommer et al., 2018) and reduced saturation level prey intake. Further research is needed to fully understand the feeding modes in zooplankton species, prey behavior, prey selection patterns, and trophic transfer to fish in MP-contaminated environments.

The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

The manuscript presents research on animals that do not require ethical approval for their study.

JS: Conceptualization, Writing - review & editing. MR: Writing – original draft, Writing – review & editing. RG-o: Writing – original draft, Writing – review & editing.