Zooplankton-microplastic exposure in Delaware coastal waters: Atlantic blue crab (Callinectes sapidus) larvae case study.

High microplastic concentrations in the Delaware Bay have prompted concern regarding harm to local species. We consider the extent to which the zooplankton is exposed to bay-derived microplastics, focusing on Atlantic blue crabs (Callinectes sapidus) during offshore larval migration. We simulate regional flow fields for a spawning season in the Delaware coastal system to advect passive Lagrangian microplastic and zooplankton tracers. Microplastic exposure levels are estimated from tracer distributions. Field sampling of zooplankton and microplastic concentrations for the Delaware Bay mouth and the adjacent shelf in August 2020 is utilized to appraise model performance. Three mechanisms elevating microplastics exposure are identified: zooplankton transport into microplastic-laden tidelines, displacement of microplastics into the buoyant outflow current, and aggregation in offshore plume fronts. Organization via the above mechanisms substantially enhance microplastic exposures over zooplankton migrations (by an average factor of at least 3.8).

Spectral and RGB analysis of the light climate and its ecological impacts using an all-sky camera system in the Arctic.

The ArcLight observatory provides an hourly continuous time series of all-sky images providing light climate data (intensity, spectral composition, and photoperiod) from the Arctic (Svalbard at 79°N). Until recently, no complete annual time series of light climate relevant for biological processes has been provided from the high Arctic because of insufficient sensitivity of commercial light sensors during the Polar Night. The ArcLight set up is unique, as it provides both all-sky images and the corresponding integrated spectral irradiance in the visible part of the solar electromagnetic spectrum (E P A R ). Here we present a further development providing hourly diel-annual dynamics from 2020 of the irradiance partitioned into the red, green, and blue parts of the solar spectrum and illustrate their relation to weather conditions, and sun and moon trajectories. We show that there is variation between the RGB proportions of irradiance throughout the year, with the blue part of the spectrum showing the greatest variation, which is dependent on weather conditions (i.e., cloud cover). We further provide an example of the biological impact of these spectral variations in the light climate using in vivo Chl a-specific absorption coefficients of diatoms (mean of six low light acclimated northern-Arctic bloom-forming species) to model total algal light absorption (AQ t o t a l ) and the corresponding fraction of quanta used by Photosystem II (AQPSII) (O 2 production) in RGB bands and the potential impacts on the photoreceptor response, suggesting periods where repair and maintenance functions dominate activity in the absence of appreciable levels of red or green light. The method used here can be applied to light climate data and spectral response data worldwide to give localized ecological models of AQ.

Physiological and behavioral evidence for multiple spectral channels in the larval stomatopod visual system.

Larval stomatopods have generally been described as having a typical larval crustacean compound eye, which lacks the visual pigment diversity and morphological specializations of the well-studied stomatopod adult eye. However, recent work has suggested that larval stomatopod eyes are more complex than previously described. In this study, we provide physiological and behavioral evidence of at least three distinct photoreceptor classes in three species of larval stomatopods: Gonodactylellus n. sp., Gonodactylaceus falcatus and Pullosquilla n. sp. First, electroretinogram recordings were used to measure the spectral sensitivity of each species. Evidence for at least three spectral classes were identified in each: an ultraviolet, peaking at 340-376 nm; a short-wavelength blue, peaking at 455-464 nm; and a long-wavelength orange, peaking at 576-602 nm. Next, the behavioral response to light was investigated. We found that each species demonstrated positive phototactic responses to monochromatic stimuli across the UV-visible spectrum. In wavelength preference trials, distinct preferences among species were identified when different colored light stimuli were presented simultaneously. All species displayed a strong response to the UV stimulus, as well as responses to blue and orange stimuli, although at different response strengths, but no response to green. The results of this study demonstrate that larval stomatopods not only have multiple physiologically active spectral classes but they also display clear and distinct responses to wavelengths across the spectrum. We propose that the spectral classes demonstrated in each are related to visually guided ecological tasks of the larvae, which may differ between species.

Methods for controlled preparation and dosing of microplastic fragments in bioassays.

Microplastic fragments (microfragments) are among the most abundant microplastic shapes found in marine ecosystems throughout the world. Due to their limited commercial availability, microfragments are rarely used in laboratory experiments. Here a novel method of microfragment production has been developed and validated. Polyethylene and polypropylene plastic stock (2 and 3 mm thick respectively) was ground using a cryomill, washed, and rinsed through a stack of sieves. Microfragments were prepared at three distinct size classes (53-150, 150-300, 300-1000 µm) and were confirmed to be accurate and consistent in size. Employing a novel ice cap dosing technique, microfragments were accurately dosed into experimental vials while excluding headspace, facilitating particle suspension without the aid of chemical surfactants. A proof of principle ingestion experiment confirmed the bioavailability of 53-150 µm polyethylene microfragments to brine shrimp Artemia sp. Together, these methods provide a controlled way to produce and dose microplastic fragments for experimental and analytical research.

Animal behavior is central in shaping the realized diel light niche.

Animal behavior in space and time is structured by the perceived day/night cycle. However, this is modified by the animals' own movement within its habitat, creating a realized diel light niche (RDLN). To understand the RDLN, we investigated the light as experienced by zooplankton undergoing synchronized diel vertical migration (DVM) in an Arctic fjord around the spring equinox. We reveal a highly dampened light cycle with diel changes being about two orders of magnitude smaller compared to the surface or a static depth. The RDLN is further characterized by unique wavelength-specific irradiance cycles. We discuss the relevance of RDLNs for animal adaptations and interactions, as well as implications for circadian clock entrainment in the wild and laboratory.

Ultraviolet vision in larval Neogonodactylus oerstedii.

Stomatopod crustaceans have among the most complex eyes in the animal kingdom, with up to 12 different color detection channels. The capabilities of these unique eyes include photoreception of ultraviolet (UV) wavelengths (<400 nm). UV vision has been well characterized in adult stomatopods but has not been previously demonstrated in the comparatively simpler larval eye. Larval stomatopod eyes are developmentally distinct from their adult counterpart and have been described as lacking the visual pigment diversity and morphological specializations found in adult eyes. However, recent studies have provided evidence that larval stomatopod eyes are more complex than previously thought and warrant closer investigation. Using electroretinogram recordings in live animals we found physiological evidence of blue- and UV-sensitive photoreceptors in larvae of the Caribbean stomatopod species Neogonodactylus oerstedii. Transcriptomes of individual larvae were used to identify the expression of three distinct UV opsin mRNA transcripts, which may indicate the presence of multiple UV spectral channels. This is the first paper to document UV vision in any larval stomatopod, expanding our understanding of the importance of UV sensitivity in plankton. Larval stomatopod eyes are more complex and more similar to adult eyes than expected, showing previously uncharacterized molecular diversity and physiological functions.

Photophysiological cycles in Arctic krill are entrained by weak midday twilight during the Polar Night.

Light plays a fundamental role in the ecology of organisms in nearly all habitats on Earth and is central for processes such as vision and the entrainment of the circadian clock. The poles represent extreme light regimes with an annual light cycle including periods of Midnight Sun and Polar Night. The Arctic Ocean extends to the North Pole, and marine light extremes reach their maximum extent in this habitat. During the Polar Night, traditional definitions of day and night and seasonal photoperiod become irrelevant since there are only "twilight" periods defined by the sun's elevation below the horizon at midday; we term this "midday twilight." Here, we characterize light across a latitudinal gradient (76.5° N to 81° N) during Polar Night in January. Our light measurements demonstrate that the classical solar diel light cycle dominant at lower latitudes is modulated during Arctic Polar Night by lunar and auroral components. We therefore question whether this particular ambient light environment is relevant to behavioral and visual processes. We reveal from acoustic field observations that the zooplankton community is undergoing diel vertical migration (DVM) behavior. Furthermore, using electroretinogram (ERG) recording under constant darkness, we show that the main migratory species, Arctic krill (Thysanoessa inermis) show endogenous increases in visual sensitivity during the subjective night. This change in sensitivity is comparable to that under exogenous dim light acclimations, although differences in speed of vision suggest separate mechanisms. We conclude that the extremely weak midday twilight experienced by krill at high latitudes during the darkest parts of the year has physiological and ecological relevance.

All-sky camera system providing high temporal resolution annual time series of irradiance in the Arctic.

The ArcLight observatory provides hourly continuous time series of light regime data (intensity, spectral composition, and photoperiod) from the Arctic, Svalbard at 79° N. Until now, no complete annual time series of biologically relevant light has been provided from the high Arctic due to insufficient sensitivity of commercial light sensors during the Polar Night. We describe a camera system providing all-sky images and the corresponding integrated spectral irradiance (EPAR) in energy or quanta units, throughout a complete annual cycle. We present hourly-diel-annual dynamics from 2017 to 2020 of irradiance and its relation to weather conditions, sun and moon trajectories.

A marine zooplankton community vertically structured by light across diel to interannual timescales.

The predation risk of many aquatic taxa is dominated by visually searching predators, commonly a function of ambient light. Several studies propose that changes in visual predation will become a major climate-change impact on polar marine ecosystems. The High Arctic experiences extreme seasonality in the light environment, from 24 h light to 24 h darkness, and therefore provides a natural laboratory for studying light and predation risk over diel to seasonal timescales. Here, we show that zooplankton (observed using acoustics) in an Arctic fjord position themselves vertically in relation to light. A single isolume (depth-varying line of constant light intensity, the value of which is set at the lower limit of photobehaviour reponses of Calanus spp. and krill) forms a ceiling on zooplankton distribution. The vertical distribution is structured by light across timescales, from the deepening of zooplankton populations at midday as the sun rises in spring, to the depth to which zooplankton ascend to feed during diel vertical migration. These results suggest that zooplankton might already follow a foraging strategy that will keep visual predation risk roughly constant under changing light conditions, such as those caused by the reduction of sea ice, but likely with energetic costs such as lost feeding opportunities as a result of altered habitat use.

Effects of a bacteria-produced algicide on non-target marine invertebrate species.

Harmful algal blooms (HABs) affect both freshwater and marine systems. Laboratory experiments suggest an exudate produced by the bacterium Shewanella sp. IRI-160 could be used to prevent or mitigate dinoflagellate blooms; however, effects on non-target organisms are unknown. The algicide (IRI-160AA) was tested on various ontogenetic stages of the copepod Acartia tonsa (nauplii and adult copepodites), the blue crab Callinectes sapidus (zoea larvae and megalopa postlarvae), and the eastern oyster Crassostrea virginica (pediveliger larvae and adults). Mortality experiments with A. tonsa revealed that the 24-h LC50 was 13.4% v/v algicide for adult females and 5.96% for early-stage nauplii. For C. sapidus, the 24-h LC50 for first-stage zoeae was 16.8%; results were not significant for megalopae or oysters. Respiration rates for copepod nauplii increased in the 11% concentration, and in the 11% and 17% concentrations for crab zoeae; rates of later stages and oysters were unaffected. Activity level was affected for crab zoeae in the 1%, 11%, and 17% treatments, and for oyster pediveliger larvae at the 17% level. Activity of later stages and of adult copepods was unaffected. Smaller, non-target biota with higher surface to volume could be negatively impacted from IRI-160AA dosing, but overall the taxa and stages assayed were tolerant to the algicide at concentrations required for dinoflagellate mortality (EC50 = ~ 1%).

Functional trait thermal acclimation differs across three species of mid-Atlantic harmful algae.

Characterizing the thermal niche of harmful algae is crucial for understanding and projecting the effects of future climate change on harmful algal blooms. The effects of 6 different temperatures (18-32 °C) on the growth, photophysiology, and toxicity were examined in the dinoflagellate Karlodinium veneficum, and the raphidophytes, Heterosigma akashiwo and Chattonella subsalsa from the Delaware Inland Bays (DIB). K. veneficum and H. akashiwo had skewed unimodal growth patterns, with temperature optima (Topt) at 28.6 and 27.3 °C respectively and an upper thermal niche limit of 32 °C. In contrast, C. subsalsa growth increased linearly with temperature, suggesting Topt and upper thermal boundaries >32 °C. K. veneficum photosystem II (PSII) photochemical efficiency remained stable across all temperatures, while H. akashiwo PSII efficiency declined at higher temperature and C. subsalsa was susceptible to low temperature (~18 °C) photoinactivation. Cell toxicity thermal response was species-specific such that K. veneficum toxicity increased with temperature above Topt. Raphidophyte toxicity peaked at 25-28 °C and was in close agreement with Topt for growth in H. akashiwo but below C. subsalsa maximal growth. The mode of toxicity was markedly different between the dinoflagellate and the raphidophytes such that K. veneficum had greater hemolytic activity while the raphidophytes had pronounced fish gill cell toxicity. These results and patterns of natural abundance for these algae in the DIB suggest that continued ocean warming may contribute to C. subsalsa bloom formation while possibly promoting highly toxic blooms of K. veneficum.

Artificial light during the polar night disrupts Arctic fish and zooplankton behaviour down to 200 m depth.

For organisms that remain active in one of the last undisturbed and pristine dark environments on the planet-the Arctic Polar Night-the moon, stars and aurora borealis may provide important cues to guide distribution and behaviours, including predator-prey interactions. With a changing climate and increased human activities in the Arctic, such natural light sources will in many places be masked by the much stronger illumination from artificial light. Here we show that normal working-light from a ship may disrupt fish and zooplankton behaviour down to at least 200 m depth across an area of >0.125 km2 around the ship. Both the quantitative and qualitative nature of the disturbance differed between the examined regions. We conclude that biological surveys in the dark from illuminated ships may introduce biases on biological sampling, bioacoustic surveys, and possibly stock assessments of commercial and non-commercial species.

Observations and Simulations of Microplastic Debris in a Tide, Wind, and Freshwater-Driven Estuarine Environment: the Delaware Bay.

Microplastic (MP) in estuarine and coastal environments remains poorly characterized, despite the importance of these physically dynamic regions as a buffer between land, freshwater environments, and the open ocean where plastic debris accumulates. We sampled MP particles to determine concentration, size, and type in Delaware Bay and numerically simulated transport and distribution at a high spatiotemporal resolution of positively buoyant particles, representing common MP types. Baywide MP concentrations averaged between 0.19 and 1.24 pieces m-3 depending on size fraction (300-1000 and 1000-5000 µm) and sampling month (April and June 2017). Upper bay stations, which are located in or near the estuarine turbidity maximum, had higher MP concentrations than lower bay and New Jersey shore stations. Fragments were predominately polyethylene, and filaments predominately polypropylene. Model results suggest that buoyant particles quickly (i.e., within hours) organize in patchy, highly inhomogeneous distributions, creating "hot spots" of MP. In the presence of variable currents driven by buoyancy, wind, and tides, we predict high spatial and temporal variability of MP distributions in Delaware Bay; MP concentrations could vary by a factor of 1000 within a tidal cycle at our sample locations. Collectively, these observations and simulations provide a baseline of MP concentrations in Delaware Bay along with broader, contextual understanding for how measurements reflect MP concentrations in a dynamic estuarine system.

Loss of buoyancy control in the copepod Calanus finmarchicus.

A mechanism is demonstrated that could explain large-scale aggregations of lipid-rich copepods in the surface waters of marine environments. Laboratory experiments establish that changes in salinity and temperature induce lipid-mediated buoyancy instability that entrains copepods in surface waters. Reduced hydrostatic pressure associated with forced ascent of copepods at fjordic sills, shelf breaks and seamounts would also reduce the density of the lipid reserves, forcing copepods and particularly those in diapause to the surface. We propose that salinity, temperature and hydrodynamics of the physical environment, in conjunction with the biophysical properties of lipids, explain periodic high abundances of lipid-rich copepods in surface waters.

Cerebral photoreception in mantis shrimp.

The currently unsurpassed diversity of photoreceptors found in the eyes of stomatopods, or mantis shrimps, is achieved through a variety of opsin-based visual pigments and optical filters. However, the presence of extraocular photoreceptors in these crustaceans is undescribed. Opsins have been found in extraocular tissues across animal taxa, but their functions are often unknown. Here, we show that the mantis shrimp Neogonodactylus oerstedii has functional cerebral photoreceptors, which expands the suite of mechanisms by which mantis shrimp sense light. Illumination of extraocular photoreceptors elicits behaviors akin to common arthropod escape responses, which persist in blinded individuals. The anterior central nervous system, which is illuminated when a mantis shrimp's cephalothorax protrudes from its burrow to search for predators, prey, or mates, appears to be photosensitive and to feature two types of opsin-based, potentially histaminergic photoreceptors. A pigmented ventral eye that may be capable of color discrimination extends from the cerebral ganglion, or brain, against the transparent outer carapace, and exhibits a rapid electrical response when illuminated. Additionally, opsins and histamine are expressed in several locations of the eyestalks and cerebral ganglion, where any photoresponses could contribute to shelter-seeking behaviors and other functions.

Multiple spectral channels in branchiopods. I. Vision in dim light and neural correlates.

Animals that have true color vision possess several spectral classes of photoreceptors. Pancrustaceans (Hexapoda+Crustacea) that integrate spectral information about their reconstructed visual world do so from photoreceptor terminals supplying their second optic neuropils, with subsequent participation of the third (lobula) and deeper centers (optic foci). Here, we describe experiments and correlative neural arrangements underlying convergent visual pathways in two species of branchiopod crustaceans that have to cope with a broad range of spectral ambience and illuminance in ephemeral pools, yet possess just two optic neuropils, the lamina and the optic tectum. Electroretinographic recordings and multimodel inference based on modeled spectral absorptance were used to identify the most likely number of spectral photoreceptor classes in their compound eyes. Recordings from the retina provide support for four color channels. Neuroanatomical observations resolve arrangements in their laminas that suggest signal summation at low light intensities, incorporating chromatic channels. Neuroanatomical observations demonstrate that spatial summation in the lamina of the two species are mediated by quite different mechanisms, both of which allow signals from several ommatidia to be pooled at single lamina monopolar cells. We propose that such summation provides sufficient signal for vision at intensities equivalent to those experienced by insects in terrestrial habitats under dim starlight. Our findings suggest that despite the absence of optic lobe neuropils necessary for spectral discrimination utilized by true color vision, four spectral photoreceptor classes have been maintained in Branchiopoda for vision at very low light intensities at variable ambient wavelengths that typify conditions in ephemeral freshwater habitats.

Multiple spectral channels in branchiopods. II. Role in light-dependent behavior and natural light environments.

Light is a primary environmental factor used by aquatic invertebrates for depth selection behavior. Many branchiopod crustaceans live in ephemeral aquatic habitats. All branchiopod crustaceans studied to date express four or more visual opsins in their compound eyes. We asked whether two branchiopods, Triops longicaudatus and Streptocephalus mackini, use multiple spectral channels to regulate their position in the water column. At the lowest intensities that elicited photonegative behavior, both species had broad spectral bandwidths, suggesting they use multiple spectral photoreceptor classes. Male S. mackini were more likely to maintain a vertical position 8.0-12.0 cm below the surface than females, independently of whether females were present. Male photopositive behavior at low intensity was restricted to a narrow bandwidth centered at 532 nm, suggesting a single photoreceptor class is used to maintain position above females. We compared ephemeral pools from two regions in Arizona and found that diffuse light attenuation coefficients were two orders of magnitude greater than the most heavily attenuating coastal waters. At less than 1 m of depth, pools were often dimmer than terrestrial habitats under starlight. Soil particle size distribution in each region affected spectral light environments, and behavioral responses of field-caught shrimp were adapted to the spectral properties of their region. The results suggest that branchiopods predominantly use luminance vision summed from multiple spectral photoreceptor classes for depth selection in dim, spectrally variable environments. The neuroanatomical basis for summation is described in a companion paper.

Use of an Autonomous Surface Vehicle reveals small-scale diel vertical migrations of zooplankton and susceptibility to light pollution under low solar irradiance.

Light is a major cue for nearly all life on Earth. However, most of our knowledge concerning the importance of light is based on organisms' response to light during daytime, including the dusk and dawn phase. When it is dark, light is most often considered as pollution, with increasing appreciation of its negative ecological effects. Using an Autonomous Surface Vehicle fitted with a hyperspectral irradiance sensor and an acoustic profiler, we detected and quantified the behavior of zooplankton in an unpolluted light environment in the high Arctic polar night and compared the results with that from a light-polluted environment close to our research vessels. First, in environments free of light pollution, the zooplankton community is intimately connected to the ambient light regime and performs synchronized diel vertical migrations in the upper 30 m despite the sun never rising above the horizon. Second, the vast majority of the pelagic community exhibits a strong light-escape response in the presence of artificial light, observed down to 100 m. We conclude that artificial light from traditional sampling platforms affects the zooplankton community to a degree where it is impossible to examine its abundance and natural rhythms within the upper 100 m. This study underscores the need to adjust sampling platforms, particularly in dim-light conditions, to capture relevant physical and biological data for ecological studies. It also highlights a previously unchartered susceptibility to light pollution in a region destined to see significant changes in light climate due to a reduced ice cover and an increased anthropogenic activity.

Kairomones from an estuarine fish increase visual sensitivity in brine shrimp (Artemia franciscana) from Great Salt Lake, Utah, USA.

Chemical cues from fish, or kairomones, often impact the behavior of zooplankton. These behavioral changes are thought to improve predator avoidance. For example, marine and estuarine crustacean zooplankton become more sensitive to light after kairomone exposure, which likely deepens their vertical distribution into darker waters during the day and thereby reduces their visibility to fish predators. Here, we show that kairomones from an estuarine fish induce similar behavioral responses in adult brine shrimp (Artemia franciscana) from an endorheic, hypersaline lake, Great Salt Lake, Utah, USA. Given downwelling light stimuli, kairomone-exposed A. franciscana induce a descent response upon dimmer light flashes than they do in the absence of kairomones. Using extracellular electroretinogram (ERG) recordings, we also find that kairomones induce physiological changes in the retina that may lead to increased visual sensitivity, suggesting that kairomone-induced changes to photobehavior are mediated at the photoreceptor level. However, kairomones did not induce structural changes within the eye. Although A. franciscana inhabit endorheic environments that are too saline for most fish, kairomones from an estuarine fish amplify photobehavior in these branchiopod crustaceans. The mechanism for this behavioral change has both similarities to and differences from that described in marine malacostracan crustaceans.

Fish kairomones induce spine elongation and reduce predation in marine crab larvae.

Specialized defense strategies are induced in zooplankton upon detection of predator chemical cues or kairomones. These defenses are well-described for freshwater zooplankters, with morphological defenses being particularly striking, but few studies have reported kairomone-induced morphological defenses in marine zooplankton. Here, we compare morphological responses to kairomones in the larvae of two marine crab species, estuarine mud crabs (Rhithropanopeus harrisii) and Asian shore crabs (Hemigrapsus sanguineus). When reared in the presence of fish kairomones, spine length increased by 2-3% in larval R. harrisii, while no morphological changes were identified in H. sanguineus. In subsequent feeding assays with a co-occurring fish predator (Atlantic silversides, Menidia menidia), consumption of R. harrisii was lower on larvae that had been reared with kairomones. In addition, we found that broods with smaller larvae are more likely to exhibit increases in spine length after kairomone exposure. Hence, the observed morphological response is likely influenced by larval size.