Urchin Density Research Articles

Marine Protected Areas That Preserve Trophic Cascades Promote Resilience of Kelp Forests to Marine Heatwaves.

Under accelerating threats from climate-change impacts, marine protected areas (MPAs) have been proposed as climate-adaptation tools to enhance the resilience of marine ecosystems. Yet, debate persists as to whether and how MPAs may promote resilience to climate shocks. Here, we use 38 years of satellite-derived kelp cover to empirically test whether a network of 58 temperate coastal MPAs in Central and Southern California enhances the resistance of kelp forest ecosystems to, and their recovery from, the unprecedented 2014-2016 marine heatwave regime that occurred in the region. We also leverage a 22-year time series of subtidal community surveys to mechanistically understand whether trophic cascades explain emergent patterns in kelp forest resilience within MPAs. We find that fully protected MPAs significantly enhance kelp forests' resistance to and recovery from marine heatwaves in Southern California, but not in Central California. Differences in regional responses to the heatwaves are partly explained by three-level trophic interactions comprising kelp, urchins, and predators of urchins. Urchin densities in Southern California MPAs are lower within fully protected MPAs during and after the heatwave, while the abundances of their main predators-lobster and sheephead-are higher. In Central California, a region without lobster or sheephead, there is no significant difference in urchin or kelp densities within MPAs as the current urchin predator, the sea otter, is protected statewide. Our analyses show that fully protected MPAs can be effective climate-adaptation tools, but their ability to enhance resilience to extreme climate events depends upon region-specific environmental and trophic interactions. As nations progress to protect 30% of the oceans by 2030, scientists and managers should consider whether protection will increase resilience to climate-change impacts given their local ecological contexts, and what additional measures may be needed.

Restricted recovery of Eisenia bicyclis kelp forest following sea urchin removal on the Pacific coast of Tohoku, Japan

Little information is available on the destruction of Eisenia bicyclis kelp forests (Phaeophyceae: Laminariales) due to overgrazing by sea urchins. In Shizugawa Bay, on the northeastern Pacific coast of Japan, overgrazing by the sea urchin Mesocentrotus nudus caused the destruction of E. bicyclis forests between 2014 and 2015. This study investigated the recovery of an adult kelp forest in relation to sea urchin densities through the continuous removal of sea urchins at 3 permanent experimental transects spanning 100 m2 for 3.8 yr from September 2015 to June 2019. The relationship between the densities of sea urchins and kelp juveniles was also analyzed. Estimates of the threshold density and biomass of sea urchins required to trigger the phase shift from an E. bicyclis forest to a barren were 15.5 ind. m-2 and 712.0 g m-2, respectively. The removal of sea urchins expanded the offshore kelp growth area. However, an increase in adult kelp following the return of juveniles was restricted to the nearshore half of the transects. The successful recruitment of kelp juveniles, followed by the recovery of adult kelp forest, requires reduction of the density of M. nudus of 40-50 mm diameter to 0 or 1 ind. m-2. Reducing sea urchin densities in autumn, when many move from barrens to invade kelp forests, is crucial to ensure the survival of kelp juveniles. This study highlights the presence of an intensified positive feedback mechanism of the barren state due to high grazing intensity and a weakened Oyashio Current.

Kelp forests versus urchin barrens: a comparison of ecosystem functions and services provided by two alternative stable marine habitats.

Kelp forests and urchin barrens are two stable states in rocky reef ecosystems, each providing unique ecosystem functions like habitat for marine species and primary production. While studies frequently show that kelp forests support higher levels of some ecosystem functions than urchin barren habitats, no research has yet compared average differences. To address this gap, we first conducted a meta-analysis of studies that directly compared the ecosystem functions, services and general attributes provided by each habitat. We also compiled individual studies on ecosystem properties from both habitats and qualitatively assessed the benefits provided. The meta-analysis included 388 observations from 55 studies across 14 countries. We found that kelp forests consistently delivered higher levels of ecosystem properties such as biodiversity, species richness, abalone abundance and sea urchin roe quality. Urchin barrens supported higher urchin density and crustose coralline algae cover. The qualitative review further supported these findings, showing that kelp forests ranked higher in 11 out of 15 ecosystem properties. These findings can help guide decisions on managing rocky reef habitats and demonstrate the benefits of preserving or expanding kelp forests.

Abalone recruitment patterns before and after sea urchin barrens formation in northern California: incorporating climate change

ABSTRACT Understanding the recruitment dynamics of invertebrates in kelp forests is critical to informing climate-ready restoration. Here we examine abalone and sea urchin recruitment (3–20 mm in size) patterns in northern California across a period of drastic change. Annual surveys were conducted before, during and after the MHW (2014–2016), the loss of a major predatory sea star (2012–2016) and the collapse of a bull kelp forest in 2014. Divers surveyed artificial reef recruitment modules (n = 12) over 20 years in an area that once supported dense bull kelp, Nereocystis leutkeana, forests and the world's largest recreational abalone fishery. From 2016 to 2022, we tracked the decline of red abalone, Haliotis rufescens, recruitment and the rise of purple sea urchin, Strongylocentrotus purpuratus, recruitment. Adult densities of purple sea urchins increased as did newly settled sea urchins (<3 mm), while adult and newly settled red abalone declined. Eight years after the kelp forest collapse, red abalone recruitment remained low and sea urchin recruitment continued to increase. Recruitment patterns can inform both abalone restoration targets and sea urchin dynamics as part of a more holistic kelp forest recovery plan that is responsive to climate change drivers.

Community Structure of Sea Urchins (Echinoidea) in the Intertidal Zone of Thomas Beach, Uluwatu

Thomas Beach, located in Uluwatu, Province of Bali, is a rocky shore inhabited by sea urchins. The research aims to discover the habitat characteristics and community structure of sea urchins (Echinoidea) in the intertidal zone of Thomas Beach. Data collection was conducted from April to May 2023 during the lowest tide. A quantitative method with modified line transects was used for data collection. Data collected included the number and species of sea urchins, water quality parameters (temperature, salinity, pH, dissolved oxygen, water clarity, and depth), and substrate types. Data were analyzed using Microsoft Excel, including individual density, diversity index, uniformity index, and dominance index. The research findings revealed five species of sea urchins: Echinotrix calamaris, Echinometra oblonga, Echinometra mathaei, Tripneustes gratilla, and Stomopneustes variolaris. The highest density of sea urchins was found in E. oblonga (16.91 ind/m2). The diversity index values ranged from 0.4936 to 0.8721, indicating a low level of diversity. The uniformity index values ranged from 0.4493 to 0.784, suggesting the community is relatively unstable. The dominance index values ranged from 0.5036 to 0.7166, indicating a moderate level of dominance. The aquatic habitat characteristics, based on the research findings, it was determined that a temperature of 30℃, salinity ranging from 31.77 to 32.76, pH of 8, dissolved oxygen (DO) of 5 mg/l, water clarity of 100%, and a depth ranging from 8.4 to 33.93 cm. The substrate type was rocky at station 1, sandy with seagrass at station 2, and rocky sandy at station 3.

Ocean warming undermines the recovery resilience of New England kelp forests following a fishery-induced trophic cascade.

Ecological theory predicts that kelp forests structured by trophic cascades should experience recovery and persistence of their foundation species when herbivores become rare. Yet, climate change may be altering the outcomes of top-down forcing in kelp forests, especially those located in regions that have rapidly warmed in recent decades, such as the Gulf of Maine. Here, using data collected annually from 30+ sites spanning >350 km of coastline, we explored the dynamics of Maine's kelp forests in the ~20 years after a fishery-induced elimination of sea urchin herbivores. Although forests (Saccharina latissima and Laminaria digitata) had broadly returned to Maine in the late 20th century, we found that forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest. Forests collapsed in the southwest apparently because ocean warming has-directly and indirectly-made this area inhospitable to kelp. Indeed, when modeling drivers of change using causal techniques from econometrics, we discovered that unusually high summer seawater temperatures the year prior, unusually high spring seawater temperatures, and high sea urchin densities each negatively impacted kelp abundance. Furthermore, the relative power and absolute impact of these drivers varied geographically. Our findings reveal that ocean warming is redefining the outcomes of top-down forcing in this system, whereby herbivore removal no longer predictably leads to a sustained dominance of foundational kelps but instead has led to a waning dominance (northeast) or the rise of a novel phase state defined by "turf" algae (southwest). Such findings indicate that limiting climate change and managing for low herbivore abundances will be essential for preventing further loss of the vast forests that still exist in northeast Maine. They also more broadly highlight that climate change is "rewriting the rules" of nature, and thus that ecological theory and practice must be revised to account for shifting species and processes.

Sea urchin roe quality within urchin barrens and improvement through kelp restoration

AbstractSea urchin overgrazing is a major cause of kelp loss and consequently sea urchin removal is frequently promoted as a tool for restoring kelp in urchin barrens. While many sea urchin species support high‐end roe fisheries, roe condition is generally lower in barrens than in kelp forests due to reduced food availability. Understanding how roe condition varies between kelp and barren habitats, and in response to kelp restoration, is key to developing effective, culturally and socially acceptable approaches to sea urchin removal and for assessing the potential benefits of kelp restoration. We assessed whether sea urchin roe quality within urchin barrens in northeastern Aotearoa New Zealand can be sufficient for harvest, and whether sea urchin removal can ultimately enhance urchin roe condition (by increasing food quality and quantity). We monitored roe condition (roe weight and gonadosomatic index [GSI]) in kelp forest and urchin barrens monthly for one year at two sites, and quarterly over two years at four sites following large‐scale (1.6–2 ha) sea urchin removal from urchin barrens. While roe condition was generally lower in barrens, the magnitude of this difference varied seasonally and among sites. Sea urchin roe quality within barrens was sufficient for commercial harvest (GSI ≥ 6%) during spring–summer at three sites. Following large‐scale sea urchin removal, kelp and fucoid density increased over two years (~9–58 plants m−2), but remained low in barren control areas. Although variable, sea urchins from removal areas (restored kelp) typically showed increased GSI, roe weight, and improved roe color within nine months following removal. These results suggest that, at some locations, harvest may provide a viable method for removing urchins from urchin barrens for kelp restoration purposes. However, to ensure sea urchin densities are reduced and maintained below the critical levels needed to achieve restoration outcomes, strategic approaches with critical evaluation of methods are required. Furthermore, we demonstrate that effectively removing urchins and promoting kelp recovery can ultimately improve urchin roe quality.

Dynamic of coral recruits in the Karimunjawa National Park, Central Java, Indonesia

Abstract. Tarigan SAR, Munasik, Wijayanti DP, Muhidin, Rohman EA, Pardede S. 2024. Dynamic of coral recruits in the Karimunjawa National Park, Central Java, Indonesia. Biodiversitas 25: 869-880. After a disturbance event, coral reefs can recover naturally by recruiting new corals. These can be affected by environmental factors like the substrate's physical and biological structure, predation, and accidental mortality of recruits by grazers, as well as the number and size of parent corals supplying larvae. This study examined the connection between newly recruited corals and biological factors such as sea urchin density, herbivorous fish abundance, and hard coral coverage. The study was monitored changes in coral cover, juvenile coral density, herbivore abundance, and hard coral coverage at 43 locations and two depths (shallow; 2-3 m) and deep (8-10 m) from 2013 to 2022. The locations were distributed across six different zone systems: the core zone, protection zone, tourism zone, traditional fisheries zone, aquaculture zone, and rehabilitation zone. Multiple Linae Regression, ANOVA test, and Principal Component Analysis were employed to assess the relationship between coral recruitment and other variables. Results indicated that the coral recruitment density was not significantly different when comparing different zoning systems (two-way ANOVA test, P-value&gt;0.05). Based on the PCA analysis, we found that in 2013 and 2019, excavator, sea urchin, browser, and hard coral have a positive relationship with coral recruitment, which implies that coral recruitment would increase as sea urchin, browser, and hard coral increase. Meanwhile, in 2019 and 2022, coral recruitment has a negative relationship with scraper, and also with hard coral growth (although only in 2022), implying that coral recruitment would decrease if scraper and hard coral increase. The study recommends restoring the role of herbivorous species in Karimunjawa National Park (KNP) by prioritizing them in conservation efforts and managing their populations.

Predators of the sea urchin Diadema mexicanum (Diadematoida: Diadematidae) at the Eastern Tropical Pacific coral reefs

Introduction: The coral reefs of Isla del Coco National Park are some of the most pristine ecosystems on Earth. The sea urchin Diadema mexicanum (Diadematoida: Diadematidae) is a common inhabitant with a pivotal role in the ecology of this unique ecosystem.&#x0D; Objective: In this study, we identified the predominant predators of D. mexicanum and estimated the predatory consumption rates. We also determined predation rates at different sea urchin sizes and at sites with contrasting refuge availability.&#x0D; Methods: We use field video recording observations and tethering experiments.&#x0D; Results: The balistid Sufflamen verres and the labrid Bodianus diplotaenia were identified as predators of small and medium size D. mexicanum; the labrids Thalassoma grammaticum and B. diplotaenia (juvenile) and the tetraodontid Arothron meleagris were attempted predators; and Canthigaster punctatissima and Holacanthus passer were scavengers. Larger sea urchins (&gt; 30 mm) were also preyed upon during the tethering experiments. Furthermore, a clear effect of the site on survival of the different sea urchins’ sizes was noted. No difference in the sea urchin predator biomass was found among sites, which highlights the importance of site complexity on survival. At high and medium complexity sites, large individuals had better survival, while at the low complexity site, there was almost no differences in survival rates among the three size classes. Our results also show that a high abundance of these predatory fishes, above 0.04 ind m-2, guarantees a low sea urchin density. Below this threshold, a higher variability in sea urchin density is observed Despite not being registered with the video recordings, lobsters were observed once preying upon a large sea urchin individual.&#x0D; Conclusions: This study identifies a keystone fish guild with high predation rate for Diadema in the National Park, which suggests that protective actions have positively benefited predatory fish and lobster populations.

Optimizing the biomass balance of macroalgae and sea urchins in kelp beds by removing the urchins

ABSTRACT In this study, we conducted sea urchin removal (SUR) experiments at two sites along Hokkaido’s southwestern and northwestern coasts in the Sea of Japan and one site along the northeast coast in the Sea of Okhotsk, to elucidate the biomass balance between macroalgae and sea urchins at different urchin barren sites. The experiments resulted in the restoration of macroalgal biomass at all sites and an increase in macroalgal diversity, indicating that it was the impact of high grazing pressure reducing the macroalgae, regardless of the particular experimental site. Notably, the recovery of the macroalgal biomass was characterized by increased kelp biomass, including Saccharina japonica var. religiosa and S. japonica var. ochotensis. In addition, negative correlations were observed between macroalgal biomass and sea urchin density. The lowest sea urchin density inducing an increase of macroalgal biomass was approximately the same in the reproductive growth phase (0.1 kg m−2) and in the vegetative growth phase (0.15 kg m−2) of Saccharina spp., the dominant kelps, regardless of the experimental site. These values can be helpful indicators for managing sea urchin density in order to restore macroalgae. In contrast, the maximum sea urchin density inducing the lowest macroalgal biomass varied markedly depending on the study site, indicating that different levels of kelp productivity are influenced by oceanographic conditions. Moreover, a strong positive correlation was observed between macroalgal biomass and sea urchins’ gonadosomatic index (GSI). Based on this correlation, macroalgal biomass supporting sea urchins with commercial value (GSI ≥ 15%) was estimated to be 1.6 kg m−2. This sea urchin density value is also a helpful indicator for managing macroalgal biomass and commercially valuable sea urchins. Moreover, this value will depend on the influence of the oceanographic environment of the site, particularly the nutrient conditions, on macroalgal productivity. In addition, we have also discussed how macroalgae and sea urchins can best coexist at the experimental sites.

To the assessment of green sea urchin stocks in the coastal zone of the Barents Sea

The purpose of the work is to assessment the state of stocks of the green sea urchin (Strongylocentrotus droebachiensis) in the coastal zone of the Barents Sea.Methods used — to achieve the set goal, standard methods for collecting hydrobiological material and calculating the abundance and biomass of hydrobionts were used.Novelty — the data obtained as a result of the research made it possible to update information on the state of the green sea urchin stocks on the coasts of Western and Eastern Murman.Results — the total and commercial reserves of sea urchin were determined on an area of 2812 thousand m2. The total stock of sea urchins was 3.58 thousand tons, the commercial one — 2.17 thousand tons. The total density of urchins in the studied water area was 32 specimens/m², the commercial one — 10.3 specimens/m². The most promising areas for fishing have been identified.Practical significance — thanks to the use of the research results, adding them to materials from other sources, it will be possible to more accurately predict the volumes and timing of fishing. Mining companies will receive more complete information about promising fishing areas and possible catch volumes.

Modeling grazer-mediated effects of demographic and material connectivity on giant kelp metapopulation dynamics

From dispersal-based metapopulations to meta-ecosystems that arise from flows of non-living materials, spatial connectivity is a major driver of population dynamics. One potentially important process is material transport between populations also linked by individual dispersal. Here, I explored material and demographic connectivity in metapopulations of giant kelp Macrocystis pyrifera, a foundation species that produces both detritus and reproductive spores. Kelp detritus (drift) subsidizes grazers, helping maintain the kelp forest ecosystem state. Drift could potentially be exchanged among kelp patches, but this is less studied than spore dispersal. Therefore, I built an ordinary differential equation (ODE) model to investigate conditions under which drift and/or spore connectivity promotes the kelp forest state. I fit statistical models (generalized linear mixed models, GLMMs) to observational data and used the GLMM’s predictions to validate the ODE model. My results suggest kelp patch dynamics are best explained by connectivity of both drift and spores, and that the impacts of these forms of connectivity depend on local grazer (urchin) abundance. Both models predicted greater kelp persistence in well-connected patches across a range of urchin densities. These effects were largely driven by drift, which reduced grazing in recipient patches and thereby enhanced spore recruitment. While testing these predictions will require greater empirical quantification of interpatch drift transport, my findings indicate drift connectivity may be an important spatial process in kelp forest systems. More broadly, this work highlights the role of meta-ecosystem dynamics within a single ecosystem type, reinforcing the need to expand traditional metapopulation perspectives to consider multiple forms of spatial connectivity.

The effect of a harmful algal bloom (Karenia selliformis) on the benthic invertebrate community and the sea otter (Enhydra lutris) diet in eastern Hokkaido.

In recent decades, the locally extinct sea otter (Enhydra lutris lutris) has been recolonizing the coast of eastern Hokkaido. Their diet includes benthic invertebrates such as bivalves, sea urchins, snails, and chitons. In the fall of 2021, a harmful algal bloom (HAB) of Karenia selliformis occurred across Hokkaido's northern and eastern coasts, leading to a massive mortality of sea urchins. This dinoflagellate produces a neurotoxin (gymnodimine) implicated in shellfish poisoning. To determine the effect of the HAB on the marine community, we conducted benthic surveys using SCUBA and visually monitored the prey items of the sea otters in the affected area from 2020 to 2023. Following the HAB, we observed an 82% decrease in benthic sea urchin density (number m2), leading to their complete absence from the diet of sea otters. Conversely, bivalve density increased six-fold, accompanied by a nearly two-fold rise in their percentage in the sea otters' diet. Minimal changes were observed in the density of chitons and snails, with no significant alteration in the sea otters' diet. Despite these changes, the impact of the HAB on otters' dietary preferences was temporary, as the percentage of dietary sea urchins began recovering one year later. Sea otters augmented their diet with bivalves to compensate for the reduced availability of sea urchins during the HAB with no apparent effects on the number of sea otters or their health. Our results highlight the adaptability of sea otters to adjust their diet according to prey availability.

Lobster predation on barren-forming sea urchins is more prevalent in habitats where small urchins are common: a multi-method diet analysis

Context In Tasmania, Australia, the government’s response to range-extending, barren-forming longspined sea urchin (Centrostephanus rodgersii) includes rebuilding of southern rock lobster (Jasus edwardsii) stocks to increase predation. But lobster preference for native species and continued barren expansion challenges the control efficacy. Aims To determine the impact of lobster predation on C. rodgersii in different habitats. Methods Multi-method dietary analysis consisting of stomach contents, faecal DNA and stable isotopes was performed on 64 lobsters from four habitats varying in barren extent and density of urchins and lobsters. Key results C. rodgersii contributed to lobster diet in all barren habitats and was found in lobsters of every size class. Stable isotope and DNA analyses showed that C. rodgersii was more common in lobster diet than were targeted native species at incipient barren sites. Surprisingly, in extensive barrens C. rodgersii is less prominent in lobster diet. Conclusions Combined with site-specific urchin population data, our findings indicated that lobster predation may be less effective at sites where most C. rodgersii individuals have reached a size refuge than at sites dominated by small urchins. Implications Lobster predation may provide a useful control for smaller C. rodgersii, but top–down predatory control may be constrained at sites dominated by urchins that exceed the size suitable for lobster predation.

Large‐scale one‐off sea urchin removal promotes rapid kelp recovery in urchin barrens

Sea urchin overgrazing is a leading cause of kelp forest loss and in such cases their removal is increasingly advocated for kelp forest restoration. However, refining removal approaches is needed to improve the efficiency and success of restoration, as most previous removal studies have been small scale and require ongoing removals to maintain low densities and allow kelp recovery. We investigated the effectiveness of one‐off urchin removal from large, semi‐discrete areas of urchin barrens as a tool to promote kelp recovery. We removed sea urchins (Evechinus chloroticus) from four areas (1.6–2 ha) of urchin barrens in northeastern New Zealand. Exposed urchins were reduced to approximately 7% of initial densities and remained low, yet cryptic urchin densities increased after 2 years. Kelp (Ecklonia radiata) and fucoid (Sargassum sinclairii, Carpophyllum spp.) densities increased rapidly in removal areas, but remained constant or declined in adjacent control urchin barren areas. Macroalgal canopy recovery varied among and within removal areas, but increased on average from approximately 5 to 43% in 2 years. Densities of large sea urchin predators did not increase with kelp recovery, likely due to ongoing fishing within removal areas. Our results demonstrate that a single urchin removal from large, semi‐discrete areas of urchin barrens can effectively and efficiently promote rapid multi‐species macroalgal recovery without additional actions (e.g. repeated urchin removals or macroalgae enhancement) for at least 2 years. However, this approach does not restore whole ecosystems and consequently restoration benefits through kelp recovery will be temporary without longer‐term urchin management and/or rebuilding of predator populations.

Grazing pressure of sea urchin Mesocentrotus nudus estimated from seawater temperature and wave height, and its effects on the early growth of annual kelp Saccharina japonica in northeastern Japan

Grazing by the sea urchin Mesocentrotus nudus plays a vital role in the development of marine forests of annual kelp, Saccharina japonica, along the Pacific coast of northeastern Japan. The grazing rate of sea urchins, which are rigorously temperature dependent, affects the chance of germination/survival of the kelp during winter/spring when seawater temperatures in this region are highly variable among years. It has previously been found that if the mean seawater temperature between February and March remained below 5.5 °C, kelp forests in this region would establish later in the year. However, the mechanism leading to this success or failure of kelp forests to develop has yet to be investigated in detail.Laboratory experiments confirmed that the grazing rate and movement of the sea urchin decreased as seawater temperature declined from 12 °C to 3 °C. Field observations, using interval cameras from autumn to spring, revealed that the density of sea urchins on artificial reefs decreased at lower seawater temperatures and/or higher significant wave heights (SWH). Using these relationships, the grazing pressure by sea urchins (= grazing rate × density) in the site was estimated based on seawater temperature and SWH. Comparing the grazing pressures among past years with different states of kelp forests, conditions needed to establish kelp forests were revealed. Extremely low grazing pressure (< 0.5 g m−2 day−1) lasted for a total of 31 days in 2015 when kelp grew densely, while this condition occurred on 11 and 14 days respectively in 2019 and 2022 when kelp was sparse. Kelp did not grow in 2016 when the grazing pressure continuously remained above 0.5 g m−2 day−1. At this study site, such combinations of seawater temperature and SWH as 4 °C and 1.0 m, 6 °C and 2.3 m, and 8 °C and 3.7 m would equate to a grazing pressure of 0.5 g m−2 day−1. To develop a dense kelp forest, grazing pressure was required to be lower than 0.5 g m−2 day−1 for about a month as in 2015. Though these figures should be adjusted, a model and scheme of this study will be applied to the other areas where kelp forest restoration/management plans are required.

Status of Coral Reef Communities on the Caribbean Coast of Costa Rica: Are We Talking about Corals or Macroalgae Reefs?

In the past decades, one of the most widely reported phenomena on Caribbean reefs is the general fall in coral cover and rise in macroalgae. Reefs with low coral cover and high macroalgal abundances are often presumed to provide poorer ecosystem functions and services. In this study, we assessed the condition of coral reefs on the Caribbean Coast of Costa Rica and determined how eight key ecosystem metrics varied in response to different coral and fleshy macroalgae covers. Most reefs surveyed had high fleshy macroalgae and low live coral covers, with an average (±SD) of 31 ± 28% and 14 ± 13% per site. The value of many of the ecosystem metrics estimated for coral reefs of the region appears to be lower than what has been reported for other areas in the Caribbean. We found that the rugosity, urchin density, fish richness, total fish biomass, large fish density, and the potential fishery value of the reef were higher in sites with low fleshy macroalgae covers (&lt;10%). Our results concur with the prevailing paradigm that an increase in macroalgae abundance could reduce the ecosystem services provided by coral reefs.

Effects of artificial light at night on the mobility of the sea urchin Paracentrotus lividus

Light pollution poses a significant global threat to biodiversity, driven by the increasing coastal urbanization and the resulting growth of artificial light at night (ALAN). However, to date, the scientific community has focused mainly on studying its ecological effects within the terrestrial environment. It is only recently that attention has turned to coastal marine systems which are crucial due to their essential contribution at the ecosystem level. These environments, characterized by their high productivity, also play a crucial role in protecting coasts against erosion. The aim of this case study was to investigate the possible effects of ALAN on the sea urchin species Paracentrotus lividus in four areas of an Italian rocky coast, selected according to a gradient of light intensity (0, 0.4, 3 and 25 lux), from April 2022 to February 2023. Effects of ALAN were examined by measuring the density and size of sea urchins and also their reactivity to a stress condition through an innovative technique of overturning sea urchins to study their physiological response in the presence or absence of artificial light. In addition, the permanence of sea urchins in the four areas was evaluated through an efficient tagging test. Results show how these organisms, typically nocturnal, suffer negative effects of ALAN in terms of minor density and mobility, expressed as the speed of response to an adverse event, compared to a dark area.

The behavior of sympatric sea urchin species across an ecosystem state gradient.

In temperate macroalgal forests, sea urchins are considered as a keystone species due to their grazing ability. Given their potential to shape benthic communities, we monitored the habitat use by three sympatric sea urchin species and compared their behaviors in a vegetated habitat (VH) and an adjacent isoyake habitat (IH). We monitored the environmental conditions and sea urchin density along deep and shallow transects of the VH and IH for over a year. The benthic rugosity at both sites were also surveyed. A mark-recapture experiment was conducted on the two most abundant sea urchins, Diadema setosum and Heliocidaris crassispina, to elucidate sea urchin movement patterns and group dynamics. We found that exposure to waves was highest at the VH while the IH was sheltered. The deep IH experienced the least amount of light due to high turbidity. Water temperature patterns were similar across sites. The VH benthic topography was more rugose compared to the smoother and silt-covered IH substate. Peak macroalgal bloom occurred three months earlier in IH, but macroalgae persisted longer at the shallow VH. Among the sympatric sea urchins, H. crassispina was most abundant at the shallow VH and was observed in pits and crevices. The most abundant across IH and in the deep VH was D. setosum, preferring either crevices or free-living, depending on hydrodynamic conditions. The least abundant species was D. savignyi, and most often observed in crevices. Small and medium sea urchins were most often observed at the IH site, whereas larger sea urchins were more likely observed at the VH. The mark-recapture study showed that D. setosum was found to displace further at the IH, and H. crassispina was more sedentary. Additionally, D. setosum was always observed in groups, whereas H. crassispina was always solitary. The behaviors of sympatric urchins, Diadema savignyi, D. setosum and H. crassispina, differed in response to changes in the benthic environment and physical conditions. Sea urchin displacement increased when rugosity and wave action were low. Habitat preference shifted to crevices in seasons with high wave action. In general, the mark-recapture experiment showed that sea urchins displaced further at night.

Invasive Fish and Sea Urchins Drive the Status of Canopy Forming Macroalgae in the Eastern Mediterranean.

Canopy-forming macroalgae, such as Cystoseira sensu lato, increase the three-dimensional complexity and spatial heterogeneity of rocky reefs, enhancing biodiversity and productivity in coastal areas. Extensive loss of canopy algae has been recorded in recent decades throughout the Mediterranean Sea due to various anthropogenic pressures. In this study, we assessed the biomass of fish assemblages, sea urchin density, and the vertical distribution of macroalgal communities in the Aegean and Levantine Seas. The herbivore fish biomass was significantly higher in the South Aegean and Levantine compared to the North Aegean. Very low sea urchin densities suggest local collapses in the South Aegean and the Levantine. In most sites in the South Aegean and the Levantine, the ecological status of macroalgal communities was low or very low at depths deeper than 2 m, with limited or no canopy algae. In many sites, canopy algae were restricted to a very narrow, shallow zone, where grazing pressure may be limited due to harsh hydrodynamic conditions. Using Generalized Linear Mixed Models, we demonstrated that the presence of canopy algae is negatively correlated with the biomass of the invasive Siganus spp. and sea urchins. The loss of Cystoseira s.l. forests is alarming, and urgent conservation actions are needed.