Shifts in survival and reproduction after chronic warming enhance the potential of a marine copepod to persist under extreme heat events.

The study of a species' thermal tolerance and vital rates responses provides useful metrics to characterize its vulnerability to ocean warming. Under prolonged thermal stress, plastic and adaptive processes can adjust the physiology of organisms. Yet it is uncertain whether the species can expand their upper thermal limits to cope with rapid and extreme changes in environmental temperature. In this study, we reared the marine copepod Paracartia grani at control (19°C) and warmer conditions (25°C) for >18 generations and assessed their survival and fecundity under short-term exposure to a range of temperatures (11-34°C). After multigenerational warming, the upper tolerance to acute exposure (24 h) increased by 1-1.3°C, although this enhancement decreased to 0.3-0.8°C after longer thermal stress (7 days). Warm-reared copepods were smaller and produced significantly fewer offspring at the optimum temperature. No shift in the thermal breadth of the reproductive response was observed. Yet the fecundity rates of the warm-reared copepods in the upper thermal range were up to 21-fold higher than the control. Our results show that chronic warming improved tolerance to stress temperatures and fecundity of P. grani, therefore, enhancing its chances to persist under extreme heat events.

Ecophysiological response of marine copepods to dietary elemental imbalances.

We assessed the effects of nutrient imbalanced diets on the feeding, reproduction and gross-growth efficiency of egg production of the copepod Paracartia grani. The cryptophyte Rhodomonas salina, cultivated under balanced (f/2) and imbalanced growth conditions (N and P limitation), served as prey. Copepod C:N and C:P ratios increased in the imbalanced treatments, particularly under P limitation. Feeding and egg production rates did not differ between the balanced and N-limited treatments but decreased under P limitation. We found no evidence of compensatory feeding in P. grani. C gross-growth efficiency averaged 0.34 in the balanced treatment and declined to values of 0.23 and 0.14 for the N- and P-limited treatments, respectively. Under N limitation, N gross-growth efficiency increased significantly to a mean value of 0.69, likely as a result of increasing the nutrient absorption efficiency. P gross-growth efficiency reached values > 1 under P limitation, involving the depletion of body P. Hatching success was >80%, with no differences among diets. Hatched nauplii, however, had lower size and slower development when the progenitor was fed a P-limited diet. This study highlights the effects of P limitation in copepods, which are more constraining than N, and the presence of maternal effects driven by prey nutritional composition that ultimately may affect population fitness.

The effect of short-term temperature exposure on vital physiological processes of mixoplankton and protozooplankton.

Sudden environmental changes like marine heatwaves will become more intense and frequent in the future. Understanding the physiological responses of mixoplankton and protozooplankton, key members of marine food webs, to temperature is crucial. Here, we studied two dinoflagellates (one protozoo- and one mixoplanktonic), two ciliates (one protozoo- and one mixoplanktonic), and two cryptophytes. We report the acute (24 h) responses on growth and grazing to a range of temperatures (5-34 °C). We also determined respiration and photosynthetic rates for the four grazers within 6 °C of warming. The thermal performance curves showed that, in general, ciliates have higher optimal temperatures than dinoflagellates and that protozooplankton is better adapted to warming than mixoplankton. Our results confirmed that warmer temperatures decrease the cellular volumes of all species. Q10 coefficients suggest that grazing is the rate that increases the most in response to temperature in protozooplankton. Yet, in mixoplankton, grazing decreased in warmer temperatures, whereas photosynthesis increased. Therefore, we suggest that the Metabolic Theory of Ecology should reassess mixoplankton's position for the correct parameterisation of future climate change models. Future studies should also address the multigenerational response to temperature changes, to confirm whether mixoplankton become more phototrophic than phagotrophic in a warming scenario after adaptation.

Reduction in thermal stress of marine copepods after physiological acclimation.

We studied the phenotypic response to temperature of the marine copepod Paracartia grani at the organismal and cellular levels. First, the acute (2 days) survival, feeding and reproductive performances at 6-35°C were determined. Survival was very high up to ca. 30°C and then dropped, whereas feeding and fecundity peaked at 23-27°C. An acclimation response developed after longer exposures (7 days), resulting in a decline of the biological rate processes. As a consequence, Q10 coefficients dropped from 2.6 to 1.6, and from 2.7 to 1.7 for ingestion and egg production, respectively. Due to the similarity in feeding and egg production thermal responses, gross-growth efficiencies did not vary with temperature. Respiration rates were less sensitive (lower Q10) and showed an opposite pattern, probably influenced by starvation during the incubations. The acclimation response observed in the organismal rate processes was accompanied by changes in body stoichiometry and in the antioxidant defense and cell-repair mechanisms. Predictions of direct effects of temperature on copepod performance should consider the reduction of Q10 coefficients due to the acclimation response. Copepod population dynamic models often use high Q10 values and may overestimate thermal effects.

Thermal Acclimation and Adaptation in Marine Protozooplankton and Mixoplankton.

Proper thermal adaptation is key to understanding how species respond to long-term changes in temperature. However, this is seldom considered in protozooplankton and mixoplankton experiments. In this work, we studied how two heterotrophic dinoflagellates (Gyrodinium dominans and Oxyrrhis marina), one heterotrophic ciliate (Strombidium arenicola), and one mixotrophic dinoflagellate (Karlodinium armiger) responded to warming. To do so, we compared strains adapted at 16, 19, and 22°C and those adapted at 16°C and exposed for 3 days to temperature increases of 3 and 6°C (acclimated treatments). Neither their carbon, nitrogen or phosphorus (CNP) contents nor their corresponding elemental ratios showed straightforward changes with temperature, except for a modest increase in P contents with temperature in some grazers. In general, the performance of both acclimated and adapted grazers increased from 16 to 19°C and then dropped at 22°C, with a few exceptions. Therefore, our organisms followed the "hotter is better" hypothesis for a temperature rise of 3°C; an increase of >6°C, however, resulted in variable outcomes. Despite the disparity in responses among species and physiological rates, 19°C-adapted organisms, in general, performed better than acclimated-only (16°C-adapted organisms incubated at +3°C). However, at 22°C, most species were at the limit of their metabolic equilibrium and were unable to fully adapt. Nevertheless, adaptation to higher temperatures allowed strains to maintain physiological activities when exposed to sudden increases in temperature (up to 25°C). In summary, adaptation to temperature seems to confer a selective advantage to protistan grazers within a narrow range (i.e., ca. 3°C). Adaptation to much higher increases of temperatures (i.e., +6°C) does not confer any clear physiological advantage (with few exceptions; e.g., the mixotroph K. armiger), at least within the time frame of our experiments.

Predator Chemical Cue Effects on the Diel Feeding Behaviour of Marine Protists.

We have assessed the effect of copepod chemical cues on the diel feeding rhythms of heterotrophic and mixotrophic marine protists. All phagotrophic protists studied exhibited relatively high diurnal feeding rates. The magnitude of the diel feeding rhythm, expressed as the quotient of day and night ingestion rates, was inversely related to the time that phagotrophic protists were maintained in the laboratory in an environment without predators. In the case of the recently isolated ciliate Strombidium arenicola, the rhythm was lost after a few months. When challenged with chemical alarm signals (copepodamides) from the copepod Calanus finmarchicus at realistic concentrations (0.6-6 pM), S. arenicola partially re-established diurnal feeding. Conversely, the amplitude of the diel feeding rhythm for the ciliate Mesodinium rubrum was not affected by copepodamides, although the 24-h integrated food intake increased by approximately 23%. For the dinoflagellates Gyrodinium dominans and Karlodinium armiger, copepodamides significantly reduced the amplitude of their diel feeding rhythms; significant positive effects on total daily ingestion were only observed in G. dominans. Finally, the dinoflagellate Oxyrrhis marina, isolated >20 years ago, showed inconsistent responses to copepodamides, except for an average 6% increase in its total ingestion over 24 h. Our results demonstrate that the predation risk by copepods affects the diel feeding rhythm of marine protists and suggests a species-specific response to predation threats.

Non-lethal effects of the predator Meganyctiphanes norvegica and influence of seasonal photoperiod and food availability on the diel feeding behaviour of the copepod Centropages typicus.

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.

Effects of prey trophic mode on the gross-growth efficiency of marine copepods: the case of mixoplankton.

Copepod reproductive success largely depends on food quality, which also reflects the prey trophic mode. As such, modelling simulations postulate a trophic enhancement to higher trophic levels when mixotrophy is accounted in planktonic trophodynamics. Here, we tested whether photo-phagotrophic protists (mixoplankton) could enhance copepod gross-growth efficiency by nutrient upgrading mechanisms compared to obligate autotrophs and heterotrophs. To validate the hypothesis, we compared physiological rates of the copepod Paracartia grani under the three functional nutrition types. Ingestion and egg production rates varied depending on prey size and species, regardless of the diet. The gross-growth efficiency was variable and not significantly different across nutritional treatments, ranging from 3 to 25% in the mixoplanktonic diet compared to autotrophic (11-36%) and heterotrophic (8-38%) nutrition. Egg hatching and egestion rates were generally unaffected by diet. Overall, P. grani physiological rates did not differ under the tested nutrition types due to the large species-specific variation within trophic mode. However, when we focused on a single species, Karlodinium veneficum, tested as prey under contrasting trophic modes, the actively feeding dinoflagellate boosted the egestion rate and decreased the copepod gross-growth efficiency compared to the autotrophic ones, suggesting possible involvement of toxins in modulating trophodynamics other than stoichiometric constraints.

Towards an Understanding of Diel Feeding Rhythms in Marine Protists: Consequences of Light Manipulation.

Temporal programs synchronised with the daily cycle are of adaptive importance for organisms exposed to periodic fluctuations. This study deepens into several aspects of the exogenous and endogenous nature of microbial grazers. We investigated the diel rhythms of cell division and feeding activity of four marine protists under different light regimes. In particular, we tested if the feeding cycle of protistan grazers could be mediated by a light-aided enhancement of prey digestion, and also explored the consequences of cell division on diel feeding rhythms. Cell division occurred at night for the heterotrophic dinoflagellates Gyrodinium dominans and Oxyrrhis marina. In contrast, the mixotrophic dinoflagellate Karlodinium armiger and the ciliate Strombidium sp. mostly divided during the day. Additionally, a significant diurnal feeding rhythm was observed in all species. When exposed to continuous darkness, nearly all species maintained the cell division rhythm, but lost the feeding cycle within several hours/days (with the exception of O. marina that kept the rhythm for 9.5 days). Additional feeding experiments under continuous light also showed the same pattern. We conclude that the feeding rhythms of protistan grazers are generally regulated not by cell division nor by the enhancement of digestion by light. Our study, moreover, indicates that the cell division cycle is under endogenous control, whereas an external trigger is required to maintain the feeding rhythm, at least for most of the species studied here.

Tetrabromobisphenol A inhibits carboxylesterase activity of marine organisms from different trophic levels.

Tetrabromobisphenol A (TBBPA), a brominated flame retardant used in synthetic polymers and electronics, is present in the aquatic environment and recent evidence suggests it can be potentially biomagnified in the marine ecosystem. However, the toxicity of TBBPA in the marine biota has not been investigated in detail. In this study we aimed to understand the role of carboxylesterases (CEs) in xenobiotic metabolism under the exposure of marine organisms to a chemical of environmental concern, TBBPA. Specifically, we tested for in vitro inhibition of CE activity in a range of marine organisms covering different ecological niches, from species from low (mussels and copepods), medium (sardines and anchovies) and high trophic levels (tuna). The results revealed that the highest inhibition of CE activity to 100 µM TBBPA was recorded in mussels (66.5% inhibition) and tunids (36.3-76.4%), whereas copepods and small pelagic fish showed comparatively lower effects (respectively, 30% and 36.5-55.6%). Our results suggest that CE-mediated detoxification and physiological processes could be compromised in TBBPA-exposed organisms and could ultimately affect humans as many of them are market species.

Publisher Correction: Sex-Dependent Effects of Caloric Restriction on the Ageing of an Ambush Feeding Copepod.

A correction to this article has been published and is linked from the HTML version of this paper. The error has been fixed in the paper.

Sex-Dependent Effects of Caloric Restriction on the Ageing of an Ambush Feeding Copepod.

Planktonic copepods are a very successful group in marine pelagic environments, with a key role in biogeochemical cycles. Among them, the genus Oithona is one of the more abundant and ubiquitous. We report here on the effects of caloric (food) restriction on the ageing patterns of the copepod Oithona davisae. The response of O. davisae to caloric restriction was sex dependent: under food limitation, females have lower age-specific mortality rates and longer lifespans and reproductive periods; male mortality rates and life expectancy were not affected. Males are more active swimmers than females, and given their higher energetic demands presumably generate reactive oxygen species at higher rates. That was confirmed by starvation experiments, which showed that O. davisae males burn through body reserves much faster, resulting in shorter life expectancy. Compared with common, coastal calanoid copepods, the effects of caloric restriction on O. davisae appeared less prominent. We think this difference in the magnitude of the responses is a consequence of the distinct life-history traits associated with the genus Oithona (ambush feeder, egg-carrier), with much lower overall levels of metabolism and reproductive effort.

Feeding behaviour of the nauplii of the marine calanoid copepod Paracartia grani Sars: Functional response, prey size spectrum, and effects of the presence of alternative prey.

Laboratory feeding experiments were conducted to study the functional response and prey size spectrum of the young naupliar stages of the calanoid copepod Paracartia grani Sars. Experiments were conducted on a range of microalgal prey of varying sizes and motility patterns. Significant feeding was found in all prey of a size range of 4.5-19.8 µm, with Holling type III functional responses observed for most prey types. The highest clearance rates occurred when nauplii fed on the dinoflagellate Heterocapsa sp. and the diatom Thalassiosira weissflogii (respectively, 0.61 and 0.70 mL ind-1 d-1), suggesting an optimal prey:predator ratio of 0.09. Additional experiments were conducted to examine the effects of the presence of alternative prey (either Heterocapsa sp. or Gymnodinium litoralis) on the functional response to the haptophyte Isochrysis galbana. In the bialgal mixtures, clearance and ingestion rates of I. galbana along the range of the functional response were significantly reduced as a result of selectivity towards the larger, alternative prey. Paradoxically, relatively large prey trigger a perception response in the nauplii, but most likely such prey cannot be completely ingested and a certain degree of sloppy feeding may occur. Our results are further evidence of the complex prey-specific feeding interactions that are likely to occur in natural assemblages with several available prey types.

Ageing and Caloric Restriction in a Marine Planktonic Copepod.

Planktonic copepods are a key group in the marine pelagic ecosystem, linking primary production with upper trophic levels. Their abundance and population dynamics are constrained by the life history tradeoffs associated with resource availability, reproduction and predation pressure. The tradeoffs associated with the ageing process and its underlying biological mechanisms are, however, poorly known. Our study shows that ageing in copepods involves a deterioration of their vital rates and a rise in mortality associated with an increase in oxidative damage (lipid peroxidation); the activity of the cell-repair enzymatic machinery also increases with age. This increase in oxidative damage is associated with an increase in the relative content of the fatty acid 22:6(n-3), an essential component of cell membranes that increases their susceptibility to peroxidation. Moreover, we show that caloric (food) restriction in marine copepods reduces their age-specific mortality rates, and extends the lifespan of females and their reproductive period. Given the overall low production of the oceans, this can be a strategy, at least in certain copepod species, to enhance their chances to reproduce in a nutritionally dilute, temporally and spatially patchy environment.

Copepod foraging on the basis of food nutritional quality: can copepods really choose?

Copepods have been considered capable of selective feeding based on several factors (i.e., prey size, toxicity, and motility). However, their selective feeding behaviour as a function of food quality remains poorly understood, despite the potential impact of such a process on copepod fitness and trophodynamics. In this study, we aimed to evaluate the ability of copepods to feed selectively according to the nutritional value of the prey. We investigated the feeding performance of the calanoid copepod Acartia grani under nutritionally distinct diets of the dinoflagellate Heterocapsa sp. (nutrient-replete, N-depleted and P-depleted) using unialgal suspensions and mixtures of prey (nutrient-replete vs. nutrient-depleted). Despite the distinct cell elemental composition among algal treatments (e.g., C:N:P molar ratios) and the clear dietary impact on egg production rates (generally higher number of eggs under a nutrient-replete diet), no impact on copepod feeding rates was observed. All unialgal suspensions were cleared at similar rates, and this pattern was independent of food concentration. When the prey were offered as mixtures, we did not detect selective behaviour in either the N-limitation (nutrient-replete vs. N-depleted Heterocapsa cells) or P-limitation (nutrient-replete vs. P-depleted Heterocapsa cells) experiments. The lack of selectivity observed in the current study contrasts with previous observations, in which stronger nutritional differences were tested. Under normal natural circumstances, nutritional differences in natural prey assemblages might not be sufficiently strong to trigger a selective response in copepods based on that factor alone. In addition, our results suggest that nutritional quality might depend not only on the growing conditions but also on the inherent taxonomical properties of the prey.

Accumulation and cycling of polycyclic aromatic hydrocarbons in zooplankton.

Planktonic food webs play an important role driving the environmental fate of persistent organic pollutants, and POP accumulation in phytoplankton has been previously studied for its importance as a first step in the aquatic food webs. However, little is known about the accumulation and cycling of organic pollutants between zooplankton and water. The present study shows the results of laboratory experiments on the bioconcentration (by passive uptake) of polycyclic aromatic hydrocarbons in phytoplankton (Rhodomonas salina) and accumulation in copepods (Paracartia(acartia) granm), by ingestion and diffusion. Both bioconcentration (BCF) and bioaccumulation (BAF) factors show significant correlation with the octanol-water partition coefficient (Kow) for phytoplankton and zooplankton. The BCF values for phytoplankton were 2 orders of magnitude higher than those for copepods. The analysis of fecal pellets shows that elimination by defecation is mainly significant for PAHs taken up from ingested phytoplankton but not due to passive uptake. However, the dominant elimination mechanisms are by far metabolism and diffusive depuration. Indeed, the mass balance suggests that metabolism of PAHs by copepods is a significant process that could play a role in the fate of PAHs in the water column. Uptake, depuration, eggestion, and ingestion rates increased with hydrophobicity of the chemical, while the metabolism rate was slightly higher for the less hydrophobic compounds. Passive partitioning dominated the accumulation of POPs in zooplankton. The derivation of all the uptake and loss rate constants for PAHs opens the doorto future modeling studies of the role of zooplankton in PAH cycling in the marine environment.

Lethal and sublethal effects of naphthalene and 1,2-dimethylnaphthalene on naupliar and adult stages of the marine cyclopoid copepod Oithona davisae.

Short-term (24h) exposure experiments have been conducted to determine the effects of two environmental relevant polycyclic aromatic hydrocarbons (PAHs), naphthalene (NAPH) and dimethylnaphthalene (C2-NAPH), on the naupliar and adult stages of the marine cyclopoid copepod Oithona davisae. To resemble more realistic conditions, those exposure experiments were conducted under the presence of food. The naupliar stages evidenced lower tolerance to PAH exposure regarding narcotic and lethal effects than adults. Copepod feeding activity showed to be very sensitive to the presence of the studied PAHs, detrimental effects occurring at toxic concentrations ca. 2-3 fold lower than for narcotic effects. In addition we report PAH-mediated changes in cell size and growth rate of the prey item, the heterotrophic dinoflagellate Oxyrrhis marina, that could indirectly affect copepod feeding and help explain hormesis-like responses in our feeding experiments.

Phylogenetic position of the copepod-infesting parasite Syndinium turbo (Dinoflagellata, Syndinea).

Sequences were determined for the nuclear-encoded small subunit (SSU) rRNA and 5.8S rRNA genes as well as the internal transcribed spacers ITS1 and ITS2 of the parasitic dinoflagellate genus Syndinium from two different marine copepod hosts. Syndinium developed a multicellular plasmodium inside its host and at maturity free-swimming zoospores were released. Syndinium plasmodia in the copepod Paracalanus parvus produced zoospores of three different morphological types. However, full SSU rDNA sequences for the three morphotypes were 100% identical and also their ITS1-ITS2 sequences were identical except for four base pairs. It was concluded that the three morphotypes belong to a single species that was identified as Syndinium turbo, the type species of the dinoflagellate subdivision Syndinea. The SSU rDNA sequence of another Syndinium species infecting Corycaeus sp. was similar to Syndinium turbo except for three base pairs and the ITS1-ITS2 sequences of the two species differed at 34-35 positions. Phylogenetic analyses placed Syndinium as a sister taxon to the blue crab parasite Hematodinium sp. and both parasites were affiliated with the so-called marine alveolate Group II. This corroborates the hypothesis that marine alveolate Group II is Syndinea.

Predicting single and mixture toxicity of petrogenic polycyclic aromatic hydrocarbons to the copepod Oithona davisae.

In the present study, the acute toxicity of 10 polycyclic aromatic hydrocarbons (PAHs) associated with the Prestige fuel oil spill (Spain, 2002) were evaluated, either as single substances or in mixtures, in adults of the copepod Oithona davisae. All but dimethylphenanthrene had negative effects on O. davisae survival at concentrations below their water solubility, with 48-h median lethal concentrations for naphthalene and pyrene of 56.1 and 0.8 micromol/L, respectively, making these the least and most toxic compounds. Polycyclic aromatic hydrocarbons had narcotic effects on copepods, as evidenced by the lack of motility at lower concentrations than those causing death. Naphthalene showed the greatest narcotic effects, and phenanthrene showed minor effects. Acute toxicity of the tested PAHs was inversely related (r2 = 0.9) with their octanol-water partition coefficient, thereby confirming the validity of the baseline quantitative structure-activity regression models for predicting the toxicity of PAH compounds in copepod species. When supplied in mixtures, the toxic effect of PAHs was additive. These results indicate that the many PAHs in an oil spill can be considered unambiguous baseline toxicants (class 1) acting additively as nonpolar narcotics in copepods; hence, their individual and combined toxicity can be predicted using their octanol-water partition coefficient.