Sea urchins mediate the availability of kelp detritus to benthic consumers

When species don't move together: Non-concurrent range shifts in Eastern Pacific kelp forest communities

Species range shifts due to changing ocean conditions are occurring around the world. As species move, they build new interaction networks as they shift from or into new ecological communities. Typically, species ranges are modeled individually, but biotic interactions have been shown to be important to creating more realistic modeling outputs for species. To understand the importance of consumer interactions in Eastern Pacific kelp forest species distributions, we used a Maxent framework to model a key foundation species, giant kelp (Macrocystis pyrifera), and a dominant herbivore, purple sea urchins (Strongylocentrotus purpuratus). With neither species having previously been modeled in the Eastern Pacific, we found evidence for M. pyrifera expansion in the northern section of its range, with no projected contraction at the southern range edge. Despite its known co-occurrence with M. pyrifera, models of S. purpuratus showed a non-concurrent southern range contraction and a co-occurring northern range expansion. While the co-occurring shifts may lead to increased spatial competition for suitable substrate, this non-concurrent contraction could result in community wide impacts such as herbivore release, tropicalization, or ecosystem restructuring.

Marine heatwave temperatures enhance larval performance but are meditated by paternal thermal history and inter-individual differences in the purple sea urchin, Strongylocentrotus purpuratus

Marine heatwave (MHW) events, characterized by periods of anomalous temperatures, are an increasingly prevalent threat to coastal marine ecosystems. Given the seasonal phenology of MHWs, the full extent of their biological consequences may depend on how these thermal stress events align with an organism's reproductive cycle. In organisms with more complex life cycles (e.g., many marine invertebrate species) the alignment of adult and larval environments may be an important factor determining offspring success, setting the stage for MHW events to influence reproduction and development in situ. Here, the influence of MHW-like temperatures on the early development of the California purple sea urchin, Strongylocentrotus purpuratus, were explored within the context of paternal thermal history. Based on temperature data collected during MHW events seen in Southern California from 2014-2020, adult urchins were acclimated to either MHW or non-MHW temperatures for 28 days before their sperm was used to produce embryos that were subsequently raised under varying thermal conditions. Once offspring reached an early larval stage, the impact of paternal and offspring environments were assessed on two aspects of offspring performance: larval size and thermal tolerance. Exposure to elevated temperatures during early development resulted in larger, more thermally tolerant larvae, with further influences of paternal identity and thermal history, respectively. The alignment of paternal and offspring exposure to MHW temperatures had additional positive benefits on larval thermal tolerance, but this tolerance significantly decreased when their thermal experience mismatched. As the highest recorded temperatures within past MHW events have occurred during the gametogenesis of many kelp forest benthic marine invertebrate species, such as the purple sea urchin, such parental mediated impacts may represent important drivers of future recruitment and population composition for these species.

The Blob marine heatwave transforms California kelp forest ecosystems

Ocean warming has both direct physiological and indirect ecological consequences for marine organisms. Sessile animals may be particularly vulnerable to anomalous warming given constraints in food acquisition and reproduction imposed by sessility. In temperate reef ecosystems, sessile suspension feeding invertebrates provide food for an array of mobile species and act as a critical trophic link between the plankton and the benthos. Using 14 years of seasonal benthic community data across five coastal reefs, we evaluated how communities of sessile invertebrates in southern California kelp forests responded to the "Blob", a period of anomalously high temperatures and low phytoplankton production. We show that this event had prolonged consequences for kelp forest ecosystems. Changes to community structure, including species invasions, have persisted six years post-Blob, suggesting that a climate-driven shift in California kelp forests is underway.

Detrital supply suppresses deforestation to maintain healthy kelp forest ecosystems

Herbivores can reach extraordinary abundances in many ecosystems. When herbivore abundance is high, heavy grazing can severely defoliate primary producers and, in some cases, even drive ecosystem to undergo regime shifts from a high productivity state to a denuded, low productivity state. While the phenomenon of herbivore-driven regime shifts is well documented, we only partially understand the mechanisms underlying these events. Here, we combine herbivory experiments with 21 years of long-term monitoring data of kelp forest ecosystems to test the hypothesis that herbivores drive regime shifts when herbivory exceeds primary production. To test this hypothesis, we quantified how the foraging habits of an important group of marine herbivores-sea urchins-change with increases in sea urchin biomass and trigger regime shifts to a foundation species, giant kelp (Macrocystis pyrifera). Using experiments, we quantified how the grazing capacity of urchins increases as urchin biomass increases, then we combined these estimates of urchin grazing capacity with estimates of kelp production to predict when and where urchin grazing capacity exceeded kelp production. When grazing capacity exceeded kelp production, sea urchins caused a 50-fold reduction in giant kelp biomass. Our findings support the hypothesis that the balance between herbivory and production underlies herbivore-driven regime shifts in Southern California kelp forests and provides insight into when and where urchins are likely to force regime shifts in kelp forest ecosystems.

Digestive Enzyme Activities and Gut Emptying Are Correlated with the Reciprocal Regulation of TRPA1 Ion Channel and Serotonin in the Gut of the Sea Urchin Strongylocentrotus intermedius

The energetic link in the benthic community is based on physiological characteristics of the low food absorption efficiency of sea urchins. Low food absorption efficiency of sea urchins is correlated with the activity of digestive enzymes and the duration of food in their gut. Thus, the digestive enzymes activities (pepsin and amylase enzyme activities) and gut emptying are important indicators in assessing nutrient digestion and absorption in sea urchins. In the present study, the relationship between these indicators and molecules related to digestive physiology were quantified in sea urchins. We found (1) an inter-regulatory relationship existed between Transient receptor potential cation channel, subfamily A, member 1 (TRPA1), and serotonin (5-hydroxytryptamine; 5-HT) in the gut of Strongylocentrotus intermedius; (2) digestive enzyme activities were negatively correlated with the TRPA1 and concentration of 5-HT in the gut of S. intermedius; (3) gut emptying rate was positively correlated with TRPA1 and concentration of 5-HT in the gut of S. intermedius. The present study revealed that the digestion and absorption of food are correlated with the TRPA1 and 5-HT in the gut of S. intermedius, which provides valuable information about the digestive physiology of sea urchins. This novel finding is relevant to understanding the low food digestibility of sea urchins. It also provides valuable information to the digestive physiology of sea urchins, which are key to maintaining the stability of food webs in the marine ecosystem.

Macroalgae and interspecific alarm cues regulate behavioral interactions between sea urchins and sea cucumbers

Sea urchins and sea cucumbers are mutually beneficial organisms in kelp ecosystem. As herbivores, sea urchins process kelp through feeding and egestion, providing inaccessible food for benthic consumers such as sea cucumbers. Sea urchins in turn profit from the sediment cleaned by sea cucumbers. However, behavioral interactions between them remain poorly understood, which greatly hampers our understanding on the relationship between ecologically important benthic species in marine ecosystems and the regulating mechanism. The present study investigated behavioral interactions between sea urchins Strongylocentrotus intermedius and sea cucumbers Apostichopus japonicus in laboratory conditions. We revealed that the presence of sea urchins caused significant higher speed movement of A. japonicus. Interestingly, the negative effects of S. intermedius on A. japonicus were significantly reduced in the shared macroalgal area. For the first time, we found the interspecific responses to alarm cues between sea cucumbers and sea urchins. Conspecific responses were significantly larger than the interspecific responses in both sea urchins and sea cucumbers. This indicates that interspecific response to alarm cues is an efficient approach to anti-predation and coexistence in mutually beneficial organisms. The present study shed light on the interspecific relationships and coexistence between sea urchins and sea cucumbers in kelp ecosystem.

Physical and chemical threats posed by micro(nano)plastic to sea urchins

The awareness of the plastic issue is rising in recent years. Our seas and coastal seawaters are investigated with the aim to evaluate the possible fate, behavior and the impact of these novel contaminants upon marine biota. In particular, benthic organisms are exposed to micro(nano)plastics that sink and accumulated on the seabed. Sea urchins can be prone to the plastic impact for all their lifespan with effect that can be extended upon the trophic cascade since their key role as grazer organisms. Moreover, they are largely used in the assessment of contaminant impact both as adult individuals and as early larval stages. This review analyzes the recent literature about the chemical and physical hazards posed by diverse polymers to sea urchins, in relation to their peculiar characteristics and to their size. The search was based on a query of the keyword terms: microplastic _ OR nanoplastic_AND Sea urchins in Web of Science and Google Scholar. The effects provoked by exposure of different sea urchin biological form are highlighted, considering both laboratory exposure and collection in real world. Additional focus has also been given upon the exposure methods utilized in laboratory test and in the existing limitations in the testing procedures. In conclusion, the micro(nano)plastics major impact seemed to be attributable to leaching compounds, however variability and lacking of realisms in the procedures do not allow a full understanding of the hazard posed by micro(nano)plastics for sea urchins. Finally, the work provides insights into the future research strategies to better characterize the actual risk for sea urchins.

Stable Isotope Analysis of Food Web Structure and the Contribution of Carbon Sources in the Sea Adjacent to the Miaodao Archipelago (China)

The littoral zones around archipelagos are highly productive coastal habitats that serve as biodiversity hotspots and provide valuable ecosystem services that are different from those of the pelagic and profundal zones. The littoral zone has complex basal carbon sources from different primary producers and is an important ocean–land transition area. Macroalgae are the main primary producers of the littoral zone, but their carbon contribution to consumers is rarely studied. Basal carbon sources determine the structure of the food web. In order to determine the contribution of basal carbon sources and the food web structure of the littoral zone, we used carbon and nitrogen stable isotope techniques and a Bayesian mixing model to study the autumn benthic food web in the sea adjacent to the Miaodao Archipelago. The potential carbon sources of the benthic food web biota in the sea adjacent to the Miaodao Archipelago in autumn are mainly algae (including phytoplankton and macroalgae) and SOM, but the contribution of POM is low. Macroalgae may play a more important role in the littoral zone benthic food webs. Although there are certain uncertainties in the model results, invertebrates and fish have obvious differences in their use of carbon sources. The trophic importance of different primary producers varies with functional feeding groups, and the resource utilization of different functional feeding groups may have certain spatial characteristics.

Intraspecific variation and energy channel coupling within a Chilean kelp forest

The widespread importance of variable types of primary production, or energy channels, to consumer communities has become increasingly apparent. However, the mechanisms underlying this "multichannel" feeding remain poorly understood, especially for aquatic ecosystems that pose unique logistical constraints given the diversity of potential energy channels. Here, we use bulk tissue isotopic analysis along with carbon isotope (δ13 C) analysis of individual amino acids to characterize the relative contribution of pelagic and benthic energy sources to a kelp forest consumer community in northern Chile. We measured bulk tissue δ13 C and δ15 N for >120 samples; of these we analyzed δ13 C values of six essential amino acids (EAA) from nine primary producer groups (n = 41) and 11 representative nearshore consumer taxa (n = 56). Using EAA δ13 C data, we employed linear discriminant analysis (LDA) to assess how distinct EAA δ13 C values were between local pelagic (phytoplankton/particulate organic matter), and benthic (kelps, red algae, and green algae) endmembers. With this model, we were able to correctly classify nearly 90% of producer samples to their original groupings, a significant improvement on traditional bulk isotopic analysis. With this EAA isotopic library, we then generated probability distributions for the most important sources of production for each individual consumer and species using a bootstrap-resampling LDA approach. We found evidence for multichannel feeding within the community at the species level. Invertebrates tended to focus on either pelagic or benthic energy, deriving 13-67% of their EAA from pelagic sources. In contrast, mobile (fish) taxa at higher trophic levels used more equal proportions of each channel, ranging from 19% to 47% pelagically derived energy. Within a taxon, multichannel feeding was a result of specialization among individuals in energy channel usage, with 37 of 56 individual consumers estimated to derive >80% of their EAA from a single channel. Our study reveals how a cutting-edge isotopic technique can characterize the dynamics of energy flow in coastal food webs, a topic that has historically been difficult to address. More broadly, our work provides a mechanism as to how multichannel feeding may occur in nearshore communities, and we suggest this pattern be investigated in additional ecosystems.

How sea urchins face microplastics: Uptake, tissue distribution and immune system response

Plastic pollution represents one of the major threats to the marine environment. A wide range of marine organisms has been shown to ingest microplastics due to their small dimensions (less than 1 mm). This negatively affects some biological processes, such as feeding, energy reserves and reproduction. Very few studies have been performed on the effect of microplastics on sea urchin development and virtually none on adults. The aim of this work was to evaluate the uptake and distribution of fluorescent labelled polystyrene microbeads (micro-PS) in the Mediterranean sea urchin Paracentrotus lividus and the potential impact on circulating immune cells. Differential uptake was observed in the digestive and water vascular systems as well as in the gonads based on microbeads size (10 and 45 μm in diameter). Treatment of sea urchins with particles of both sizes induced an increase of the total number of immune cells already after 24 h. No significant differences were observed among immune cell types. However, the ratio between red and white amoebocytes, indicative of sea urchin healthy status, increased with both particles. This effect was detectable already at 24 h upon exposure to smaller micro-PS (10 μm). An increase of intracellular levels of reactive oxygen and nitrogen species was observed at 24 h upon both micro-PS exposure, whereas at later time these levels became comparable to those of controls. A significant increase of total antioxidant capacity was observed after treatment with 10 μm micro-PS. Overall data provide the first evidence on polystyrene microbeads uptake and tissue distribution in sea urchins, indicating a stress-related impact on circulating immune cells.

Carbon export is facilitated by sea urchins transforming kelp detritus

With the increasing imperative for societies to act to curb climate change by increasing carbon stores and sinks, it has become critical to understand how organic carbon is produced, released, transformed, transported, and sequestered within and across ecosystems. In freshwater and open-ocean systems, shredders play a significant and well-known role in transforming and mobilizing carbon, but their role in the carbon cycle of coastal ecosystems is largely unknown. Marine plants such as kelps produce vast amounts of detritus, which can be captured and consumed by shedders as it traverses the seafloor. We measured capture and consumption rates of kelp detritus by sea urchins across four sampling periods and over a range of kelp detritus production rates and sea urchin densities, in northern Norway. When sea urchin densities exceeded 4 m-2, the sea urchins captured and consumed a high percentage (ca. 80%) of kelp detritus on shallow reefs. We calculated that between 1.3 and 10.8 kg of kelp m-2 are shredded annually from these reefs. We used a hydrodynamic dispersal model to show that transformation of kelp blades to sea urchin feces increased its export distance fourfold. Our findings show that sea urchins can accelerate and extend the export of carbon to neighboring areas. This collector-shredder pathway could represent a significant flow of small particulate carbon from kelp forests to deep-sea areas, where it can subsidize benthic communities or contribute to the global carbon sink.

Overwintering tropical herbivores accelerate detritus production on temperate reefs

The tropicalization of temperate marine ecosystems can lead to increased herbivory rates, reducing the standing stock of seaweeds and potentially causing increases in detritus production. However, long-term studies analysing these processes associated with the persistence of tropical herbivores in temperate reefs are lacking. We assessed the seasonal variation in abundances, macrophyte consumption, feeding modes and defecation rates of the range-extending tropical rabbitfish Siganus fuscescens and the temperate silver drummer Kyphosus sydneyanus and herring cale Olisthops cyanomelas on tropicalized reefs of Western Australia. Rabbitfish overwintered in temperate reefs, consumed more kelp and other macrophytes in all feeding modes, and defecated more during both summer and winter than the temperate herbivores. Herbivory and defecation increased with rabbitfish abundance, but this was dependent on temperature, with higher rates attained by big schools during summer and lower rates in winter. Still, rabbitfish surpassed temperate herbivores, leading to a fivefold acceleration in the transformation of macrophyte standing stock to detritus, a function usually attributed to sea urchins in kelp forests. Our results suggest that further warming and tropicalization will not only increase primary consumption and affect the habitat structure of temperate reefs but also increase detritus production, with the potential to modify energy pathways.

Sea urchins mediate the availability of kelp detritus to benthic consumers

Detritus can fundamentally shape and sustain food webs, and shredders can facilitate its availability. Most of the biomass of the highly productive giant kelp, Macrocystis pyrifera, becomes detritus that is exported or falls to the seafloor as litter. We hypothesized that sea urchins process kelp litter through shredding, sloppy feeding and egestion, making kelp litter more available to benthic consumers. To test this, we conducted a mesocosm experiment in which an array of kelp forest benthic consumers were exposed to 13C- and 15N-labelled Macrocystis with or without the presence of sea urchins, Strongylocentrotus purpuratus. Our results showed that several detritivore species consumed significant amounts of kelp, but only when urchins were present. Although they are typically portrayed as antagonistic grazers in kelp forests, sea urchins can have a positive trophic role, capturing kelp litter before it is exported and making it available to a suite of benthic detritivores.