The feeding activity of copepods is crucial for the structure and functioning of marine ecosystems. Quantification of feeding rates of different copepod life stages across a range of prey densities (functional response) is essential knowledge for improvement of plankton dynamic models. In this study, we conducted experiments to compare the feeding functional responses of nauplii, copepodites, and adults of the marine copepod Paracartia grani (formerly Acartia grani) when grazing on the flagellate Rhodomonas salina. We found that all copepod stages followed a sigmoid curve in their functional responses (Holling Type III model), indicating a metabolic threshold constraining foraging effort at low prey densities. Maximum clearance rates of nauplii and copepodites increased with body mass with a power scaling factor of 1, but maximum clearance rates of adults did not follow the pattern observed for juvenile stages, likely because of the relatively small prey size used in the experiments. Copepod maximum ingestion rates, however, showed allometric scaling along ontogeny that was closer to the typical 0.75 power law and seemed to not be so dependent on prey size. The insights obtained from our study highlight stage-specific differences in copepod feeding activity, and can help improve our capability to estimate the energy flow through copepods in marine food webs, This study was framed within the project FERMI (CGL2014-59227-R, MINECO/AEI/FEDER, UE). Manuel Olivares was supported financially by an FPU grant (FPU15/01747) from the Spanish Ministry of Education, Culture and Sport, Number of experiment, date of experiment, temperature, bottle volume, prey species, prey species AphiaID, copepod species, copepod species AphiaID, copepod stage, prey concentration, prey size, number of copepods per bottle, copepod size, incubation time, clearance rate, ingestion rate, Peer reviewed

Marine planktonic copepods frequently exhibit diel feeding cycles coupled with vertical migrations. However, copepod feeding rhythms can be influenced by factors others than different food availability between layers. In this study, we determined the changes in the diel feeding behaviour of two marine copepod species (Centropages typicus and Paracartia grani) through multigenerational laboratory rearing, ontogeny, and upon the exposure to predator chemical cues. The wild females of both C. typicus and P. grani showed marked feeding rhythms with higher ingestion rates at night. The diel rhythms of C. typicus were maintained in the first laboratory-reared generation (F1), suggesting maternal effects, but disappeared in the following generations. The P. grani females of a long-term culture (> 10 years) also showed no differences in their day–night feeding activity. Ontogenetic variations were detected in the F1 generation of C. typicus: feeding rhythms were absent in naupliar stages, but adults fed more intensely at night. In the case of the cultured P. grani, in general none of the stages showed feeding rhythms. Laboratory-reared C. typicus (8–11 generations) did not recover back the natural cyclic feeding when exposed to jellyfish and fish exudates, indicating that either predation risk does not significantly affect their diel feeding activity or predator-induced responses in marine copepods might not involve chemical signalling. Our study confirms that feeding-related functional traits of marine copepods can experience quick multigenerational changes in the laboratory; consequently, cultured copepods might not be good models for studies involving their diel feeding behaviour, This study was framed within the Project FERMI (CGL2014-59227-R, MINECO/AEI/FEDER, UE). Manuel Olivares is supported financially by a FPU Grant (FPU15/01747) from the Spanish Ministry of Education, Culture and Sport, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), Number of experiment, date of experiment, temperature, bottle volume, prey species, prey species AphiaID, copepod species, copepod species AphiaID, copepod generation, copepod stage, day/night, presence/absence of predator exudates, predator species, predator species AphiaID, prey concentration, prey size, number of copepods per bottle, copepod size, incubation time, clearance rate, ingestion rate, Peer reviewed

Predators can induce changes in the diel activity patterns of marine copepods. Besides vertical migration, diel feeding rhythms have been suggested as an antipredator phenotypic response. We conducted experiments to assess the non-lethal direct effects of the predator Meganyctiphanes norvegica (northern krill) on the diel feeding patterns of the calanoid copepod Centropages typicus. We also analysed the influence of seasonal photoperiod and prey availability on the intensity of copepod feeding rhythms. We did not detect any large effect of krill presence on the diel feeding behaviour of copepods, either in day-night differences or total daily ingestions. Seasonal photoperiod and prey availability, however, significantly affected the magnitude of copepod feeding cycles, with larger diel differences in shorter days and at lower prey concentrations. Therefore, the role of non-lethal direct effects of predators on the diel feeding activity of marine copepods remain debatable and might not be as relevant as in freshwater zooplankton, The research leading to these results received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 730984, ASSEMBLE Plus project. The open access publication fee was covered by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI). This study was framed within the project FERMI (CGL2014–59227-R, MINECO/AEI/FEDER, UE), and M.O. was supported financially by an FPU grant (FPU15/01747) from the Spanish Ministry of Education, Culture and Sport, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), Number of experiment, date of experiment, temperature, bottle volume, prey species, prey species AphiaID, copepod species, copepod species AphiaID, predator species, predator species AphiaID, day/night, predator/no predator, prey concentration, prey size, number of copepods per bottle, copepod size, number of predators per bottle, predator size, incubation time, number of copepod faecal pellets, faecal pellet volume, Peer reviewed