Mass Mortalities Of Sea Urchins Research Articles

Lethal effects of the harmful dinoflagellate Karenia selliformis (Gymnodiniales, Dinophyceae) on two juvenile kelp sporophytes Saccharina japonica and S. sculpera (Laminariales, Phaeophyceae)

A huge red tide caused mainly by Karenia selliformis in the eastern Hokkaido Pacific Ocean during autumn of 2021 was associated with mass mortalities of sea urchins, salmon, and other marine organisms. Bleaching of seaweeds, including kelps, was reported in the areas with red tide. Thus, K. selliformis is suspected to be harmful to kelp; however, the harmful effects are not yet known. This study aimed to verify the harmfulness of K. selliformis to cultured juvenile sporophytes of two commercially valuable kelp species, Saccharina japonica and Saccharina sculpera. Juvenile kelp sporophytes were exposed to seawater with four different densities of red tide-causing K. selliformis (0, 200, 1000, 2000 cells mL−1), and changes in thallus color over time were examined. Some thalli of both kelps were visibly bleached within 12 h of K. selliformis exposure; no thallus of the negative control was bleached. The ratio of bleached thalli differed significantly depending on the density of K. selliformis and kelp species; S. japonica was more sensitive than S. sculpera. During the follow-up period, most bleached thalli, especially those exposed to high-density K. selliformis seawater, finally withered. The median lethal density (LD50) of K. selliformis within 144 h after 48 h exposure was estimated as 218 cells mL−1 and 2285 cells mL−1 for juvenile thalli of S. japonica and S. sculpera, respectively. These results suggested the lethal effects of K. selliformis to kelp leaves, suggesting the potential risk posed to cultured and natural kelp communities by K. selliformis red tides.

Recurrent large‐scale sea urchin mass mortality and the establishment of a long‐lasting alternative macroalgae‐dominated community state

AbstractRelationships between sea urchins and noncrustose macroalgae are typically nonlinear, and an abrupt phase shift from a barren to a macroalgae‐dominated state and vice versa happens when the sea urchin abundance exceeds a critical threshold. Here, we have used two mass mortality events as a natural experiment to study progressive changes and dynamics between these stable states. In 2009–2010 and 2018, two mass mortalities decimated the populations of the sea urchin Diadema africanum in the Canary Islands (Eastern Subtropical Atlantic). In these islands, this sea urchin is a key‐herbivore controlling the growth of macroalgae, and sea urchin barrens have persisted for more 60 yr. Mass mortality events differed in intensity both in space and time, and two phase shifts from sea urchin barrens to macroalgal beds were detected in both when sea urchin density declined below a critical threshold of 4 indv. · m−2. The first mass mortality was greater in La Palma Island, consequently in this island a long‐lasting alternative state dominated by noncrustose macroalgae has been established. Subsequently, a recurrent mass mortality also favored the transition from the sea urchin barrens to noncrustose macroalgal beds in Tenerife Island. After mass mortalities, D. africanum populations were reduced by 94–98% in both islands. No signs of recovery of these populations have been identified after phase shifts; ecosystem feedbacks may be the cause of the low population density of D. africanum.

Sea urchin mass mortality rapidly restores kelp forest communities

Giant kelp Macrocystis pyrifera is a foundational species that forms a 3-dimensional habitat and supports numerous high-value fisheries species. Constant grazing of kelp holdfasts by overabundant sea urchins causes catastrophic ecological and economic impacts on rocky reefs worldwide. Overgrazing creates urchin barrens that persist for decades in the absence of ecological forcing that would shift the ecosystem back to a kelp-dominated state. Annual surveys of kelp forest and urchin barren sites in the Southern California Bight were performed from 2011 to 2020 to assess changes in kelp forest communities as a result of restoration efforts through sea urchin culling. However, that time period also encompassed a sea urchin mass mortality event. Following drastic reductions of sea urchin densities, rocky reefs returned to a kelp-dominated state within approximately 6 mo and remained stable through the remainder of the study. Benthic cover, fish, and kelp and macroinvertebrate communities inside former urchin barrens became more similar to that of kelp forest reference sites and continued to do so for the next 5 yr. Giant kelp density increased significantly compared to existing kelp forests, while benthic indicators of urchin dominance (i.e. crustose coralline algae and bare rock cover) decreased. Kelp restoration through sea urchin culling essentially mimics sea urchin mass mortality events. If culling can produce similar declines in urchin density, it may be a viable management tool to rapidly restore persistent urchin barrens at moderate spatial scales, while a mass mortality event can drive recovery of kelp forest communities at more extensive spatial scales.

Uncommon southwest swells trigger sea urchin disease outbreaks in Eastern Atlantic archipelagos.

Recurrent sea urchin mass mortality has recently affected eastern Atlantic populations of the barren‐forming sea urchin Diadema africanum. This new episode of die‐off affords the opportunity to determine common meteorological and oceanographic conditions that may promote disease outbreaks. The population dynamics of this sea urchin species are well known—urchin barrens have persisted for many decades along most of the coastlines off the archipelagos of Madeira, Selvages, and the Canary Islands, where they limit macroalgae biomass growth. However, this new and explosive mortality event decimated the sea urchin population by 93% on Tenerife and La Palma Islands. Two severe episodes of southwestern rough sea that led to winter storms, in February 2010 (Xynthia) and February 2018 (Emma), preceded both mass mortality events. The autumn and winter months of those years were anomalous and characterized by swells with an average wave height above 2 m that hit the south and southwest sides of the islands. The amoeba Paramoeba brachiphila was the only pathogen isolated this time from the moribund and dead sea urchins, suggesting that the amoeba was the primary cause of the mortality. This new sea urchin die‐off event supports the “killer‐storm” hypothesis that has been already described for western Atlantic coasts. These anomalous southwest storms during winters generate pronounced underwater sediment movement and large‐scale vertical mixing, detected in local tide gauge, which may promote paramoebiasis. This study presents valuable insights about climate‐mediated changes in disease frequency and its impacts on the future of coastal marine ecosystems in the Atlantic.

Detection and quantification of a keystone pathogen in a coastal marine ecosystem

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 606:79-90 (2018) - DOI: https://doi.org/10.3354/meps12776 Detection and quantification of a keystone pathogen in a coastal marine ecosystem Robyn Buchwald, Robert E. Scheibling*, Alastair G. B. Simpson Department of Biology, Dalhousie University, Halifax, NS, B3H 4R2, Canada *Corresponding author: rescheib@dal.ca ABSTRACT: The pathogenic amoeba Paramoeba invadens causes recurrent mass mortalities of green sea urchins Strongylocentrotus droebachiensis in coastal Nova Scotia, Canada, driving regime shifts from urchin barrens to kelp beds. Outbreaks of the disease (paramoebiasis) are sporadic, and the source population(s) and epizootiology of the amoeba are poorly understood. We developed PCR-based detection of P. invadens in urchin tissue, sediment, and seawater. Primers specific to the P. invadens nuclear SSU rRNA gene were designed and used in PCR and qPCR analyses to better detect and quantify P. invadens during, following, and in the absence of a natural disease outbreak. A comparison of pathogen load in asymptomatic and symptomatic sea urchins indicated a lower threshold of ~1 cell mg-1 tissue for observing overt signs of paramoebiasis in urchins. P. invadens was detected for the first time in sediment during and following an outbreak of disease in 2014. It also was detected in low abundance (<10 cells l-1) in seawater in fall 2015 in the absence of sea urchin mass mortality or a strong storm event, but not under similar conditions in summer/fall 2016 and 2017. The ability to detect and quantify this pathogen in sea urchins and environmental samples sheds new light on mechanisms of introduction, spread, and persistence of P. invadens along the Nova Scotian coast and the role of large-scale meteorological events and ocean warming in these processes. KEY WORDS: Epizootic · Ecosystem dynamics · Paramoeba invadens · Strongylocentrotus droebachiensis · Quantitative real-time PCR · Pathogenic amoebae · Sea urchins Full text in pdf format Supplementary material PreviousNextCite this article as: Buchwald R, Scheibling RE, Simpson AGB (2018) Detection and quantification of a keystone pathogen in a coastal marine ecosystem. Mar Ecol Prog Ser 606:79-90. https://doi.org/10.3354/meps12776 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 606. Online publication date: November 15, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

POPULATION PHARMACOKINETICS OF ENROFLOXACIN AND ITS METABOLITE CIPROFLOXACIN IN THE GREEN SEA URCHIN (STRONGYLOCENTROTUS DROEBACHIENSIS) FOLLOWING INTRACOELOMIC AND IMMERSION ADMINISTRATION.

Sea urchin mass mortality events have been attributed to both infectious and noninfectious etiologies. Bacteria, including Vibrio spp. and Pseudoalteromonas spp., have been isolated during specific mortality events. Aquarium collection sea urchins are also subject to bacterial infections and could benefit from antimicrobial treatment, but pharmacokinetic studies have been lacking for this invertebrate group until recently. This study evaluated the pharmacokinetics of enrofloxacin and its active metabolite ciprofloxacin in the green sea urchin (Strongylocentrotus droebachiensis) after intracoelomic injection and medicated bath immersion administration. The utility of a population pharmacokinetic method using nonlinear mixed effects modeling (NLME) was also evaluated. Thirty sea urchins were assigned to either the injection or immersion group. Twelve study animals and three untreated controls were utilized for each administration method: enrofloxacin 10 mg/kg intracoelomic injection or a 6-hr enrofloxacin 10 mg/L immersion. Each animal was sampled four times from 0 to 120 hr. Water samples were collected during immersion treatment and posttreatment time points in both groups. Hemolymph and water sample drug concentrations were analyzed using high-performance liquid chromatography, and pharmacokinetic parameters were determined using an NLME population pharmacokinetic method. Enrofloxacin concentrations were fit to a two-compartment model with first-order input for the intracoelomic injection group. The enrofloxacin elimination half-life (t½), peak hemolymph concentration (CMAX), and area under the curve (AUC) were 38.82 hr, 90.92 μg/ml, and 1,199 hr·μg/ml, respectively. Enrofloxacin was modeled to a one-compartment model with first-order input for the immersion treatment. The enrofloxacin t½, CMAX, and AUC were 33.46 hr, 0.48 μg/ml, and 32.88 hr·μg/ml, respectively. Ciprofloxacin was detected in trace concentrations in all hemolymph samples, indicating minimal production of this metabolite. The concentrations of enrofloxacin achieved far exceeded minimum inhibitory concentrations reported for teleost pathogens. No adverse effects were associated with enrofloxacin administration by either treatment method or from hemolymph sampling.

Low temperature tolerance of a sea urchin pathogen: Implications for benthic community dynamics in a warming ocean

Disease outbreaks in the sea urchin Strongylocentrotus droebachiensis on the Atlantic coast of Nova Scotia have increased in frequency over the last 35years, in association with increasing sea temperatures and strong storm events. Previous studies suggest that the causative agent of disease, the pathogenic amoeba Paramoeba invadens, cannot withstand winter sea temperatures on this coast, and may be an alien species. Consequently, recurrent epizootics result from repeated introductions of this pathogen, possibly due to advection or vertical mixing associated with storm events. Here we test an alternative hypothesis that P. invadens has established resident populations, either through physiological adaptation to low winter temperatures, or because winter minima have increased since 1980. To test this hypothesis, we manipulated culture temperatures of P. invadens isolated from moribund sea urchins during a disease outbreak in 2012. Specific growth rates of P. invadens in culture at 0.5 to 18.0°C were similar to those observed in the early 1980s, providing no evidence of physiological adaptation. P. invadens declined to extinction in culture after ~1month at 0.5°C and 2.0°C, but survived for 3months at 3.5°C, as indicated by recovery of cultures following transfer to optimal temperature. These results indicate a lower temperature tolerance threshold around 2–3°C, which is consistent with observational data of recurrent mass mortalities of sea urchins in 1983 and 2012 in the absence of a storm but following relatively warm winters. A trend of increasing minimum temperatures in winter indicates that P. invadens could undergo a range expansion to establish a resident population in Nova Scotia within 1–2 decades. This is expected to increase the resilience of the kelp-bed state of the benthic ecosystem but result in the complete collapse of the sea urchin fishery.

Effects of sea urchin disease on coastal marine ecosystems

Outbreaks of disease in herbivorous sea urchins have led to ecosystem phase shifts from urchin barrens to kelp beds (forests) on temperate rocky reefs, and from coral to macroalgal-dominated reefs in the tropics. We analyzed temporal patterns in epizootics that cause mass mortality of sea urchins, and consequent phase shifts, based on published records over a 42-year period (1970–2012). We found no evidence for a general increase in disease outbreaks among seven species of ecologically important and intensively studied sea urchins. Periodic waves of recurrent amoebic disease of Strongylocentrotus droebachiensis in Nova Scotia coincide with periods when the system was in a barrens state and appear to have increased in frequency. In contrast, following a major epizootic that decimated Diadema antillarum throughout the Caribbean in 1983, subsequent outbreaks of disease were highly localized and none have been reported since 1991. Epizootics of Strongylocentrotus in the NW Atlantic and NE Pacific, and Paracentrotus and Diadema in the eastern Atlantic, have been linked to climate change and overfishing of sea urchin predators. The spatial extent of recurrent disease outbreaks in these species, and the frequency of phase shifts associated with these epizootics, has decreased over time due to the expansion of the macroalgal state and its stabilization through positive feedback mechanisms. Longitudinal studies to monitor disease outbreaks in sea urchin populations and improved techniques to identify causative agents are needed to assess changes in the frequency and extent of epizootics, which can profoundly affect the structure and functioning of coastal marine ecosystems.

Validating the identity of Paramoeba invadens, the causative agent of recurrent mass mortality of sea urchins in Nova Scotia, Canada

Green sea urchins Strongylocentrotus droebachiensis along the coast of Nova Scotia, Canada, suffer mass mortalities from infection by the pathogenic amoeba Paramoeba invadens Jones, 1985. It has been speculated that P. invadens could be a form of Neoparamoeba pemaquidensis, a species associated with disease in S. droebachiensis and lobsters in the northeast USA. During a disease outbreak in fall 2011, we isolated amoebae from moribund urchins collected from 4 locations along ~200 km of coastline. In laboratory infection trials, we found that timing and rate of morbidity corresponded to that of similar experiments conducted in the early 1980s, when P. invadens was first identified. All isolates had a similar size and morphology to the original description, including an absence of microscales. Sequences of nuclear SSU rDNA show that disease was caused by one 'species' of amoeba across the range sampled. Phylogenetic analyses prove that P. invadens is not conspecific with N. pemaquidensis, but is a distinct species most closely related to N. branchiphila, a suspected pathogen of sea urchins Diadema aff. antillarum in the Canary Islands, Spain. Morphology and closest phylogenetic affinities suggest that P. invadens would be assignable to the genus Neoparamoeba; however, nuclear SSU rDNA trees show that Neoparamoeba and Paramoeba are phylogenetically inseparable. Therefore, we treat Neoparamoeba as a junior synonym of Paramoeba, with P. invadens retaining that name, and N. pemaquidensis and N. aestuarina reverting to their original names (P. pemaquidensis and P. aestuarina), and with new combinations for N. branchiphila Dykova et al., 2005, and N. perurans Young et al., 2007, namely P. branchiphila comb. nov. and P. perurans comb. nov.

An outbreak of sea urchin disease associated with a recent hurricane: Support for the “killer storm hypothesis” on a local scale

The frequency of epizootics causing mass mortality of sea urchins (Strongylocentrotus droebachiensis) along the coast of Nova Scotia, Canada has increased over the past three decades. Laboratory and field studies show that outbreaks of disease are caused by the amoeba Paramoeba invadens, and are associated with hurricane activity and warm seawater temperatures. A statistical model indicates that the probability of a mass mortality event increases with the proximity of a hurricane to the coast and the maximum sustained wind speed, and is greater when post-storm seawater temperature is above a threshold for disease propagation. To assess the reliability of the model in predicting mass mortality events on an annual scale, and to examine spatial variability in mortality (in the event of a disease outbreak) on a local scale (meters to kilometers), we transplanted sea urchins into cages in kelp beds at 6 sites around St. Margarets Bay, Nova Scotia, where localized outbreaks of paramoebiasis have been observed following hurricanes in the past. On 4 September 2010 a Category 1 hurricane (Earl) made landfall 110km SSW of the experimental area. Based on the parameters of the storm, the model forecasted a 43% probability of a disease outbreak. Morbidity of caged animals was first documented on 6 September 2010, and morbidity and mortality in the cages was ~50% by the end of September and ~85% two months after the storm. Laboratory experiments indicated that the temperature-dependent transmission or induction of morbidity was consistent with paramoebiasis. Our findings provide support for the efficacy of the model to predict the occurrence of disease outbreaks, although the source population(s) of the pathogenic agent and oceanographic mechanisms affecting its introduction and spread along the coast of Nova Scotia remain poorly understood.

Invasion-mediated shifts in the macrobenthic assemblage of a rocky subtidal ecosystem

We tracked changes in community composition that occurred with state shifts in the rocky subtidal ecosystem in Nova Scotia, Canada, from 1992 to 2008. At the beginning of our study, a dense aggregation of sea urchins was destructively grazing a kelp bed, leaving coralline- algae- dominated barrens in its wake. In subsequent years, the system underwent a series of state shifts mediated by mass mortality of sea urchins due to amoebic disease, defoliation of kelp by an invasive bryozoan Membranipora membranacea, invasion of the green alga Codium fragile ssp. fragile, and finally decline of C. fragile and recolonisation by kelps. Using multidimensional scal- ing (MDS), we found 4 macroalgal assemblage types (dominated by kelp, coralline algae, C. frag- ile, or a transitional mixture of species), each associated with a distinct invertebrate assemblage. Sea stars and kelp-grazing gastropods were associated with the kelp-dominated state; sea urchins, chitons and scale worms were most abundant in the barrens state; and small bivalves and amphipods were most abundant in the C. fragile-dominated state. Invertebrate diversity remained relatively constant despite dramatic shifts in the algal assemblage. Although the system was vul- nerable to multiple perturbations between 1992 and 2002, the partial recovery of kelp beds and associated fauna by 2008 demonstrates some resilience in the longer term.

Killer storms: North Atlantic hurricanes and disease outbreaks in sea urchins

An increase in the incidence of disease in various marine organisms over the past few decades has been linked to ocean climate change. In Nova Scotia, Canada, mass mortalities of sea urchins, due to an amoebic disease, are associated with tropical cyclones of relatively high intensity that pass close to the coast when water temperature is above a threshold for disease propagation. These conditions increase the likelihood of introduction and spread of a nonindigenous water‐borne pathogen through turbulent mixing. Our analysis shows that the most deadly storms, in terms of the probability of a sea urchin mass mortality, have become more deadly over the past 30 years. We also found that storms have been tracking closer to the coast and that surface temperature has increased during the hurricane season. These trends are likely to continue with climate warming, resulting in a regional shift to a kelp bed ecosystem and the loss of the urchin fishery.

Disease outbreaks associated with recent hurricanes cause mass mortality of sea urchins in Nova Scotia

Field observations and laboratory experiments support the hypothesis that disease- induced mass mortality of the sea urchin Strongylocentrotus droebachiensis can be associated with hurricane events that introduce a pathogenic amoeba Paramoeba invadens to coastal waters. The temporal pattern of morbidity and mortality of sea urchins observed in a large embayment on the Atlantic coast of Nova Scotia following Hurricane Juan in September 2003 and Hurricane Bill in August 2009, and in laboratory infection experiments during the 2009 event, closely conformed to that expected based on the known temperature-dependent dynamics of this amoebic disease.

Physical disturbance and community organization on a subtidal cobble bed

This study examines the role of physical disturbance in regulating the development and maintenance of macroalgal community structure in a subtidal cobble bed on the Atlantic coast of Nova Scotia, Canada, after a mass mortality of sea urchins. Displacement by hydrodynamic forces of cobbles with and without attached macroalgae was measured in mark-recovery experiments. Cobbles without macroalgae experienced low levels of overturning or displacement by waves. In contrast, cobbles with large fleshy algae (such as kelps and fucoids) were displaced by drag forces along the seabed during periods of moderately strong wave action. Displacement of these “drifters” occurred once the attached alga reached a critical mass (relative to the weight of the anchoring cobble), which varied among species of differing morphology. The rate of displacement of drifters was an exponential function of the ratio of attached algal biomass to rock weight. The loss of larger erect forms from the cobble bed results in a patchwork of low-lying turfs. Clusters of small boulders, experimentally transplanted to the cobble bed, accumulated a greater biomass of large fleshy algae than surrounding cobbles after one year, likely because of the greater stability of boulders. Algal-mediated hydrodynamic displacement of cobbles appears to be an important form of physical disturbance in the cobble habitat, restricting the types and sizes of algae that can grow on cobbles following an urchin die-off.

Changes in subtidal habitat following mass mortality of sea urchins in Botany Bay, New South Wales

AbstractOn two occasions, between June and October 1986 and April/May 1988, mass mortality of the sea urchin Centrostephanus rodgersii occurred at Bare Island in Botany Bay, New South Wales. Following the widespread disappearance of C. rodgersii in August 1986, the abundance of filamentous and foliose algae increased and crustose corallines declined. There was also a large, but shortlived, increase in the density of limpets, particularly Patelloida alticostata, which then fell to nearly zero after 15 months. In October 1986, the habitat found at other sites in Botany Bay was consistent with a recent disappearance of C. rodgersii. At all sites a complex assemblage of non‐crustose algae developed, the species composition of which differed among sites. A large recruitment of C. rodgersii was observed at all affected sites in February 1987 and January 1988. These individuals grew quickly and reached a mean size of approximately 35 mm test diameter at the end of their first year.

Community dynamics on a subtidal cobble bed following mass mortalities of sea urchins

Community dynamics on a subtidal cobble bed following mass mortalities of sea urchins

Virulence ofParamoeba invadensJones (Amoebida, Paramoebidae) from Monoxenic and Polyxenic Culture1

Paramoeba invadens is a pathogenic marine amoeba responsible for mass mortalities of sea urchins (Strongylocentrotus droebachiensis) along the Atlantic coast of Nova Scotia in the early 1980's. The amoeba has been maintained in vivo in S. droebachiensis for five years in the laboratory without observable loss of virulence. Paramoeba invadens was cultured polyxenically (on mixed marine bacteria) and monoxenically (on a single strain of Pseudomonas nautica) on non-nutrient agar for 58 weeks and 19 weeks respectively. Pathogenicity tests showed some loss of virulence in monoxenic culture after 15 weeks and in polyxenic culture after 58 weeks. Polyxenic culture is recommended for long-term culture of P. invadens although periodic passage of the amoeba through the sea urchin host may be required to maintain virulence for periods exceeding one year.

Effect of Temperature and Prey Availability on Growth of Paramoeba invadens in Monoxenic Culture

Paramoeba invadens Jones 1985 is a pathogenic marine amoeba responsible for mass mortalities of sea urchins (Strongylocentrotus droebachiensis) of Nova Scotia between 1980 and 1983. A direct relationship between temperature and sea urchin paramoebiasis has been shown in previous laboratory and field studies. This study examined the effect of prey availability and temperature on the growth of P. invadens in monoxenic culture (with the marine bacterium Pseudomonas nautica). At 15 degrees C, the specific growth rate of P. invadens increased with bacterial prey concentration and was highest at 10 bacterial cells ml. Growth rate of P. invadens was maximal at 15 to 20 degrees C (which corresponds to annual sea temperature maxima in the natural environment) and the minimum generation time was 19.41 h at 20 degrees C. At 10 and 12 degrees C, generation times were 91.18 and 73.39 h, respectively; at 2 and 5 degrees C, there was no growth. P. invadens did not survive in monoxenic culture at 27 degrees C. Growth rates of P. invadens in vitro were positively correlated with time to morbidity of infected S. droebachiensis.

Herbivory on coral reefs: community structure following mass mortalities of sea urchins : Hughes, T.P., D.C. Reed and M.-J. Boyle, 1987. J. expl mar. Biol. Ecol., 113(1):39–59

Herbivory on coral reefs: community structure following mass mortalities of sea urchins : Hughes, T.P., D.C. Reed and M.-J. Boyle, 1987. J. expl mar. Biol. Ecol., 113(1):39–59

Herbivory on coral reefs: community structure following mass mortalities of sea urchins

The community structure of Jamaican coral reefs has undergone drastic change since mass mortalities of the long-spined black sea urchin Diadema antillarum Philippi occurred in 1983. In the absence of Diadema, algal abundance has increased enormously, up to a mean of 95% cover or 4.6 kg wet weight · m −2. Coral cover, which was already low on some reefs following Hurricane Allen in 1980, has been further reduced by as much as 60% since 1983 by competition with algae. Densities of D. antillarum at 10 sites in 1986 ranged from 0 to 12% of pre-1983 levels. Other echinoids, which might potentially compensate for the lack of herbivory from D. antillarum, have not increased significantly in density. Numbers of herbivorous scarids and acanthurids also remain at relatively low levels, because of overfishing. In the absence of high densities of fish and sea urchins, it is likely that recent changes in community structure will continue, resulting in further replacement of corals by algae in shallow water. The impact of the urchin mass mortalities is qualitatively similar to previous experimental removals of this species. In both cases, removal of echinoids resulted in substantial increases in macroalgae. However, quantitatively, the responses of algal and coral communities to the natural die-off were significantly greater, probably due to wide differences in spatial and temporal scales of the respective perturbations.