Mass mortality in Northwestern Mediterranean rocky benthic communities: effects of the 2003 heat wave

Late in summer 2003, extensive mass mortality of at least 25 rocky benthic macro-invertebrate species (mainly gorgonians and sponges) was observed in the entire Northwestern (NW) Mediterranean region, affecting several thousand kilometers of coastline. We were able to characterize the mortality event by studying six areas covering the main regions of the NW Mediterranean basin. The degree of impact on each study area was quantified at 49 sites by estimating the proportion of colonies affected in populations of several gorgonian species compared with reference data obtained in years without mortality signs. According to these data, the western areas (Catalan coast and Balearic Islands) were the least affected, while the central areas (Provence coast and Corsica-Sardinia) showed a moderate impact. The northernmost and eastern areas (Gulf of Genoa and Gulf of Naples) displayed the highest impact, with almost 80% of gorgonian colonies affected. The heat wave of 2003 in Europe caused an anomalous warming of seawater, which reached the highest temperatures ever recorded in the studied regions, between 1 and 3 °C above the climatic values (mean and maximum). Because this exceptional warming was observed in the depth ranges most affected by the mortality, it seems likely that the 2003 anomalous temperature played a key role in the observed mortality event. A correlation analysis between temperature conditions and degree of impact seems to support this hypothesis. Under the present climate warming trend, new mass mortality events may occur in the near future, possibly driving a major biodiversity crisis in the Mediterranean Sea.     

Mitochondrial Haplogroup U5b3: A Distant Echo of the Epipaleolithic in Italy and the Legacy of the Early Sardinians

There are extensive data indicating that some glacial refuge zones of southern Europe (Franco-Cantabria, Balkans, and Ukraine) were major genetic sources for the human recolonization of the continent at the beginning of the Holocene. Intriguingly, there is no genetic evidence that the refuge area located in the Italian Peninsula contributed to this process. Here we show, through phylogeographic analyses of mitochondrial DNA (mtDNA) variation performed at the highest level of molecular resolution (52 entire mitochondrial genomes), that the most likely homeland for U5b3—a haplogroup present at a very low frequency across Europe—was the Italian Peninsula. In contrast to mtDNA haplogroups that expanded from other refugia, the Holocene expansion of haplogroup U5b3 toward the North was restricted by the Alps and occurred only along the Mediterranean coasts, mainly toward nearby Provence (southern France). From there, ∼7,000–9,000 years ago, a subclade of this haplogroup moved to Sardinia, possibly as a result of the obsidian trade that linked the two regions, leaving a distinctive signature in the modern people of the island. This scenario strikingly matches the age, distribution, and postulated geographic source of a Sardinian Y chromosome haplogroup (I2a2-M26), a paradigmatic case in the European context of a founder event marking both female and male lineages.     

Epidemic Mortality of the Sponge Ircinia variabilis (Schmidt, 1862) Associated to Proliferation of a Vibrio Bacterium

In recent years, several episodes of mass mortality of sessile epibenthic invertebrates, including sponges, have been recorded worldwide. In the present study, we report a disease event on Ircinia variabilis recorded in September 2009 along the southern Adriatic and Ionian seas (Apulian coast), with the aim to quantify the mortality incidence on the sponge population, to investigate the effect of the disease on the sponge tissues and to assess whether the disease is associated with vibrios proliferation. The injured sponges showed wide necrotic areas on the surface or disruption of the body in several portions. Necrotic areas were whitish and often were covered with a thin mucous coat formed by bacteria. In the most affected specimens, sponge organisation resulted partial or complete loss, with the final exposure of the dense skeletal network of spongine fibres to the environment. The results of microbiological cultural analysis using in parallel Marine Agar 2216 and thiosulphate/citrate/bile salts/sucrose agar demonstrated that, in affected specimens, vibrios represented 15.8 % of the total I. variabilis surface culturable bacteria. Moreover, all the isolated vibrios, grown from the wide whitish areas that characterize the surface of the diseased sponges, were identified, and their assignment to the Vibrio rotiferianus was consistent with phylogenetic analysis and data of morphological, cultural and biochemical tests. Studies on V. rotiferianus have shown that its pathogenicity, with respect to various aquatic organisms, is higher than that of Vibrio harveyi. The factors triggering the disease outbreak in Ircinia variabilis populations remain unclear. At present, we can hypothesize the involvement in the disease of a synergetic mechanism that, under stressful physiological conditions (high temperature, elevated nutrients and reduced water flow), induces sponge pathogens, in our case V. rotiferanius, to become virulent, making sponges unable to control their proliferation. Additional studies are needed to understand the etiological processes as well as the factors involved in sponges recovering from this epidemic event allowing them to face mass mortality. A drastic reduction of sponge-specific representatives could have marked a negative impact on the environmental health on account of their role in the sea remediation processes as filter-feeding organisms.     

Seasonal energetic constraints in Mediterranean benthic suspension feeders: effects at different levels of ecological organization

The aim of this work is to examine the role of food as a constraining factor at different levels of ecological organization in benthic littoral ecosystems. In the search for patterns in ecological systems, it has recently been documented that seasonality in the dynamics of benthic suspension feeders (BSF) in the Mediterranean Sea is characterized by summer dormancy. We review recent studies on seston availability in the water column and feeding and respiration by BSF in the Mediterranean. The objective is to assess whether a pattern across particular studies exists that could provide evidence of food as a constraining factor. A pattern emerges from these organism‐level studies, one which indicates the seasonal occurrence of an energy shortage in the taxa exhibiting summer dormancy. This energy shortage is closely related to low food availability and suggests that an energetic constraint underlie the summer dormancy phenomenon. The seasonal occurrence of summer energy shortage also appears to affect the dynamics of BSF at population and community level. In this sense, in late summer 1999, a mass mortality event of BSF affected several hundreds of kilometers in the Ligurian Sea (NW Mediterranean). The fact that the mass mortality event occurred in late summer and especially affected the taxa that exhibit energy shortage – such as anthozoans and sponges – suggests, that energetic constraints may contribute to the understanding of the mass mortality event. The energy shortage phenomenon may provide a mechanism to understand how the occurrence of anomalous climatic conditions may have induced the mass mortality of some BSF taxa. This review points out the existence of a common energy shortage phenomenon mainly related to low food availability as an important determinant of the dynamics of most BSF taxa at organism level in the Mediterranean. But this determinant also affects the dynamics of BSF at population and community level. Therefore, the extant data point up the crucial role of food as a constraining factor for benthic littoral ecosystems in oligotrophic areas like the Mediterranean, with important seasonal variations in seston abundance and composition.     

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.     

Presence and histopathological effects of the Parvatrema sp. (Digenea, Gymnophallidae) in the stout razor clam Tagelus plebeius (Bivalvia, Psammobiidae)

The stout razor clam Tagelus plebeius (Bivalvia, Psammobiidae) has a wide geographic distribution range, including the Brazilian coasts from the northeast (Alagoas) to the south (Santa Catarina). In March 2008, an episode of mass T. plebeius mortality (70%) occurred in an intertidal bed at The Pontal da Daniela, State of Santa Catarina, Brazil. We report here high prevalences (to 100%) of the trematode parasite Parvatrema sp. Cable, 1953 (Digenea, Gymnophallidae) infecting T. plebeius at high intensities. We describe the gymnophalid, echinostomatid and unidentified metacercariae parasites infecting the clam and the host reactions elicited by them. The use of special diagnostic techniques such as Ray’s fluid thioglycollate medium (RFTM) and PCR assays to detect Perkinsus sp. pathogens, hemolymph cytology, and histopathological examinations did not show Perkinsus sp. infections, microcell infections, or neoplastic conditions. However, neither infections or pathology caused by trematode parasites; nor any other pathological condition could be uniquely correlated with the mortality event. A coincident flash flood might have contributed to cause the mortality episode. This is the first report of the Parvatrema sp. metacercariae larvae infecting the stout razor clam T. plebeius from Brazil.     

Le Protoaurignacien : première culture de l'homme moderne de Provence et Ligurie

G. Laplace defined the Protoaurignacian in Liguria (“abri Mochi”) as the earliest occurrences of Upper Paleolithic in Italy. Stratigraphic sequences in Var exemplify that the Protoaurignacian is the first sequence of Upper Paleolithic in Provence and widely different from classic Aurignacian defined in southwestern France. Recorded in Languedoc-Roussillon, that one is documented in some other stratigraphic sequences from Monaco to southern Spain (“cueva del boquete de Zafarraya”). The Protoaurignacian argue that first modern humans arrived and occupied all along the Mediterranean coasts from Gibraltar to Toscana.     

Pyrosequencing Characterization of the Microbiota from Atlantic Intertidal Marine Sponges Reveals High Microbial Diversity and the Lack of Co-Occurrence Patterns

Sponges are ancient metazoans that host diverse and complex microbial communities. Sponge-associated microbial diversity has been studied from wide oceans across the globe, particularly in subtidal regions, but the microbial communities from intertidal sponges have remained mostly unexplored. Here we used pyrosequencing to characterize the microbial communities in 12 different co-occurring intertidal marine sponge species sampled from the Atlantic coast, revealing a total of 686 operational taxonomic units (OTUs) at 97% sequence similarity. Taxonomic assignment of 16S ribosomal RNA tag sequences estimated altogether 26 microbial groups, represented by bacterial (75.5%) and archaeal (22%) domains. Proteobacteria (43.4%) and Crenarchaeota (20.6%) were the most dominant microbial groups detected in all the 12 marine sponge species and ambient seawater. The Crenarchaeota microbes detected in three Atlantic Ocean sponges had a close similarity with Crenarchaeota from geographically separated subtidal Red Sea sponges. Our study showed that most of the microbial communities observed in sponges (73%) were also found in the surrounding ambient seawater suggesting possible environmental acquisition and/or horizontal transfer of microbes. Beyond the microbial diversity and community structure assessments (NMDS, ADONIS, ANOSIM), we explored the interactions between the microbial communities coexisting in sponges using the checkerboard score (C-score). Analyses of the microbial association pattern (co-occurrence) among intertidal sympatric sponges revealed the random association of microbes, favoring the hypothesis that the sponge-inhabiting microbes are recruited from the habitat mostly by chance or influenced by environmental factors to benefit the hosts.     

Species of the planktonic diatom genus Pseudo-nitzschia of the Pacific coasts of Mexico

Species of the diatom genus Pseudo-nitzschia are common in the marine phytoplankton world-wide. Some species of this genus have been proved to be source of domoic acid (DA), a powerful toxin causing Amnesic Shellfish Poisoning (ASP) in humans and probably mass mortality in sea birds and mammals. Net plankton samples obtained during several cruises and seasons from the Pacific coasts of Mexico: western coasts of Baja California, Gulf of California, coasts of the tropical Pacific of Mexico (including the Gulf of Tehuantepec), were analyzed to study the species of the diatom genus Pseudo-nitzschia. Four species ( P. australis, a presumed toxic species, P. fraudulenta, P. lineola, P. pungens) and one former species of the genus, Nitzschia americana were recorded and studied by light and electron microscopy. The most common species was P. pungens, widely distributed along the Pacific coasts of Mexico. All other species appeared occasionally and in low relative abundances. The probable misidentification of P. australis as P. seriata is discussed, as well as the presence of another potentially toxic species, P. delicatissima, in the Gulf of California. No case of toxicity (ASP) has been fully documented and therefore related to toxic Pseudo-nitzschia species in the Gulf of California. This revised version was published online in July 2006 with corrections to the Cover Date.     

RECOVERY AND GENETIC DIVERSITY OF THE INTERTIDAL KELP LESSONIA NIGRESCENS (PHAEOPHYCEAE) 20 YEARS AFTER EL NIÑO 1982/831

Massive mortality in kelp beds of the Pacific coasts of North and South America was caused by the rise in surface seawater temperature during the El Niño Southern Oscillation (ENSO) event of 1982/83, the strongest in the four and half previous centuries. In northern Chile a stretch of 600 km of coastline showed massive mortality of the intertidal kelp species Lessonia nigrescens Bory, of which only a few individuals managed to survive. Kelps and their associated biodiversity recovered but kelp beds re-colonization in general was variable in time and space seemingly very slow along northern Chilean coasts. Here we show, effectively, that northward re-colonization advanced less than 60 km in 20 years. Conversely, kelp beds of the Northern Hemisphere recovered 300 km in only six months after the same ENSO event. Genetic diversity in the two most affected populations of L. nigrescens shows half of the heterozygosity and polymorphism with respect to that observed in six non affected populations. In addition, geographically separated populations seem highly isolated as evidenced by high and significant fixation indices (all F_ST values over 0.4).     

Tolerance of Sponge Assemblages to Temperature Anomalies: Resilience and Proliferation of Sponges following the 1997–8 El-Ni?o Southern Oscillation

Coral reefs across the world are under threat from a range of stressors, and while there has been considerable focus on the impacts of these stressors on corals, far less is known about their effect on other reef organisms. The 1997–8 El-Niño Southern Oscillation (ENSO) had notable and severe impacts on coral reefs worldwide, but not all reef organisms were negatively impacted by this large-scale event. Here we describe how the sponge fauna at Bahia, Brazil was influenced by the 1997–8 ENSO event. Sponge assemblages from three contrasting reef habitats (reef tops, walls and shallow banks) at four sites were assessed annually from 1995 to 2011. The within-habitat sponge diversity did not vary significantly across the study period; however, there was a significant increase in density in all habitats. Multivariate analyses revealed no significant difference in sponge assemblage composition (ANOSIM) between pre- and post-ENSO years for any of the habitats, suggesting that neither the 1997–8 nor any subsequent smaller ENSO events have had any measurable impact on the reef sponge assemblage. Importantly, this is in marked contrast to the results previously reported for a suite of other taxa (including corals, echinoderms, bryozoans, and ascidians), which all suffered mass mortalities as a result of the ENSO event. Our results suggest that of all reef taxa, sponges have the potential to be resilient to large-scale thermal stress events and we hypothesize that sponges might be less affected by projected increases in sea surface temperature compared to other major groups of reef organisms.     

Biogeography of sponge chemical ecology: comparisons of tropical and temperate defenses

Examples from both marine and terrestrial systems have supported the hypothesis that predation is higher in tropical than in temperate habitats and that, as a consequence, tropical species have evolved more effective defenses to deter predators. Although this hypothesis was first proposed for marine sponges over 25 years ago, our study provides the first experimental test of latitudinal differences in the effectiveness of sponge chemical defenses. We collected 20 common sponge species belonging to 14 genera from tropical Guam and temperate Northeast Spanish coasts (Indo-Pacific and Mediterranean biogeographic areas) and conducted field-based feeding experiments with large and small fish predators in both geographic areas. We use the term global deterrence to describe the deterrent activity of a sponge extract against all of the predators used in our experiments and to test the hypothesis that sponges from Guam are chemically better defended than their Mediterranean counterparts. Sympatric and allopatric deterrence refer to the average deterrent activity of a sponge against sympatric or allopatric predators. All of the sponges investigated in this study showed deterrent properties against some predators. However, 35% of the sponge species were deterrent in at least one but not in all the experiments, supporting the idea that predators can respond to chemical defenses in a species-specific manner. Tropical and temperate sponges have comparable global, sympatric, and allopatric deterrence, suggesting not only that chemical defenses from tropical and temperate sponges are equally strong but also that they are equally effective against sympatric and allopatric predators. Rather than supporting geographic trends in the production of chemical defenses, our data suggest a recurrent selection for chemical defenses in sponges as a general life-history strategy.     

Abundance of Diadema antillarum (Echinodermata: Echinoidea) in the coasts of Venezuela

Diadema antillarum is a shallow-water sea-urchin from the tropical Atlantic whose populations almost disappeared in 1983-84 because of widespread mortalities which reached 87-100 %. In Venezuela, urchin population densities before the mortality event were comparable to those of other Caribbean regions; however, later abundancies remain unknown. The objectives of this study were to evaluate the recent densities of certain D. antillarum populations along the Venezuelan coasts and compare the densities at the Parque Nacional Mochima before and after the mortality. At each location urchin densities were determined by means of transects using 1 m2-quadrats as sampling units. The highest mean densities were observed at the sites on the central coast: Ensenada de Oricao, 0.28 ind/m2 (2002) and 1.05 ind/m2 (2003), and Chichiriviche de la Costa, 0.84 ind/m2 (2002) and 0.74 ind/m2 (2003). In Mochima, the mean density before the mortality for D. antillarum oscillated between 0.28 and 4 ind/m2, after the mortality event the mean density varied between 0.15 ind/m2 (2000) and 0.47 ind/m2 (2000). The populations of D. antillarum studied at Parque Nacional Morrocoy and Refugio de Fauna Silvestre Cuare showed highest densities at Playuela (0.43 ind/m2) and Cayo Sur (0.95 ind/m2) respectively, whereas other sites showed densities below 0.1 ind/m2. The density registered at Playuela in 2003 is lower than that reported before the mortality event (0.58-3.64 ind/m2). The density for Parque Nacional Archipiélago de Los Roques, specifically for the Arrecife de Herradura remained constant between 2002 and 2003 with values between 0.22-0.23 ind/m2 respectively. To conclude, the sea urchin abundancies observed at most of the Venezuelan coastal sites that we studied were higher than those reported for other areas of the northern Caribbean, even though the values have not yet returned to those preceding the 1984 mass-mortality event, due to the slow recovery of the populations.     

Vibrio carchariae,a pathogen of the abalone Haliotis tuberculata

Since 1997, mass mortality of the abalone Haliotis tuberculata L. has occurred in the natural environment along the French coast. The outbreak of disease started on the south coast of Brittany near Concarneau in 1997, then spread to the north of Brittany (in 1998) and the west coast of Normandy (Golfe de St. Malo in 1999). Between 60 and 80% of the abalone died. In 1999, mortality also affected a land-based abalone farm in Normandy during the summer. At this farm, a Vibrio sp. was isolated in abundance from abalone that had just died. The disease was experimentally reproduced by inoculation or by introducing the pathogen into the surrounding water. This vibrio, identified by genotypic and phenotypic characters, is related to V carchariae. It is similar to the V carchariae, responsible for mortality in the Japanese abalone Sulculus diversicolor supratexta, but some phenotypic characters differentiate both strains. In 2000, healthy abalone placed in 2 sites on the north and south coasts of Brittany died, and the pathogen V carchariae could be isolated from dead individuals, demonstrating that the pathogen was probably the cause of the abalone disease that has been occurring since 1997 in Brittany.     

Antifouling activities against colonizer marine bacteria of extracts from marine invertebrates collected in the Colombian Caribbean Sea and on the Brazilian coast (Santa Catarina)

The growth inhibition of 12 native marine bacteria isolated from Aplysina sponge surfaces, the shell of a bivalve, and Phytagel immersed for 48 h in sea water were used as indicator of the antifouling activity of the extracts of 39 marine organisms (octocorals, sponges, algae, and zoanthid) collected in the Colombian Caribbean Sea and on the Brazilian coast (Santa Catarina). Gram-negative bacteria represented 75% of the isolates; identified strains belonged to Oceanobacillus iheyensis, Ochrobactrum pseudogrignonense, Vibrio campbellii, Vibrio harveyi, and Bacillus megaterium species and seven strains were classified at genus level by the 16S rRNA sequencing method. The extracts of the octocorals Pseudopterogorgia elisabethae, four Eunicea octocorals, and the sponges Topsentia ophiraphidites, Agelas citrina, Neopetrosia carbonaria, Monanchora arbuscula, Cliona tenuis, Iotrochota imminuta, and Ptilocaulis walpersii were the most active, thus suggesting those species as antifoulant producers. This is the first study of natural antifoulants from marine organisms collected on the Colombian and Brazilian coasts.     

Epithelia, an evolutionary novelty of metazoans

At the point in animal evolution when cells began to adhere to each other they presumably initially functioned as colonies. The formation of an epithelium that enclosed and controlled an internal milieu would have been the first event to distinguish an individual animal from a colony. To better understand when the first epithelium arose and what its characteristics were, we evaluate the morphological, functional, and molecular characters of epithelia in sponges, considered here the extant representatives of the first metazoans. In particular, we show new claudin-like sequences from sponges align most closely with sequences from Drosophila that have a barrier function in septate junctions. We also show that type IV collagen, the main component of the basement membrane (BM), is present in calcareous sponges, and we confirm the presence of type IV-like collagen (spongin short chain collagen) in other sponges. Though in sponges as in other metazoans the epithelium has grades of specialization with varying complexity of junctions and the BM, the main character of a functional epithelium, the ability to seal and control the ionic composition of the internal milieu, is a property of even the simplest sponge epithelium, and therefore the first metazoans likely also had epithelia with these characteristics, which we consider a "true" epithelium.     

Cascading effects of mass mortality events in Arctic marine communities

Mass mortality events caused by pulse anthropogenic or environmental perturbations (e.g., extreme weather, toxic spills or epizootics) severely reduce the abundance of a population in a short time. The frequency and impact of these events are likely to increase across the globe. Studies on how such events may affect ecological communities of interacting species are scarce. By combining a multispecies Gompertz model with a Bayesian state‐space framework, we quantify community‐level effects of a mass mortality event in a single species. We present a case study on a community of fish and zooplankton in the Barents Sea to illustrate how a mass mortality event of different intensities affecting the lower trophic level (krill) may propagate to higher trophic levels (capelin and cod). This approach is especially valuable for assessing community‐level effects of potential anthropogenic‐driven mass mortality events, owing to the ability to account for uncertainty in the assessed impact due to uncertainty about the ecological dynamics. We hence quantify how the assessed impact of a mass mortality event depends on the degree of precaution considered. We suggest that this approach can be useful for assessing the possible detrimental outcomes of toxic spills, for example oil spills, in relatively simple communities such as often found in the Arctic, a region under increasing influence of human activities due to increased land and sea use.     

What makes a species common? No evidence of density-dependent recruitment or mortality of the sea urchin Diadema antillarum after the 1983–1984 mass mortality

A potential consequence of individuals compensating for density-dependent processes is that rare or infrequent events can produce profound and long-term shifts in species abundance. In 1983–1984 a mass mortality event reduced the numbers of the abundant sea urchin Diadema antillarum by 95–99 % throughout the Caribbean and western Atlantic. Following this event, the abundance of macroalgae increased and the few surviving D. antillarum responded by increasing in body size and fecundity. These initial observations suggested that populations of D. antillarum could recover rapidly following release from food limitation. In contrast, published studies of field manipulations indicate that this species had traits making it resistant to density-dependent effects on offspring production and adult mortality; this evidence raises the possibility that density-independent processes might keep populations at a diminished level. Decadal-scale (1983–2011) monitoring of recruitment, mortality, population density and size structure of D. antillarum from St John, US Virgin Islands, indicates that population density has remained relatively stable and more than an order of magnitude lower than that before the mortality event of 1983–1984. We detected no evidence of density-dependent mortality or recruitment since this mortality event. In this location, model estimates of equilibrium population density, assuming density-independent processes and based on parameters generated over the first decade following the mortality event, accurately predict the low population density 20 years later (2011). We find no evidence to support the notion that this historically dominant species will rebound from this temporally brief, but spatially widespread, perturbation.     

Unexpected response of the seagrass Posidonia oceanica to a warm-water episode in the north western Mediterranean Sea

The response of Posidonia oceanica (Linnaeus) Delile to the warm-water episode of summer 1999 was studied by means of the technique of lepidochronology. Study sites include three sites affected by the mass mortality event of benthic invertebrates and one not affected. The results showed a significant decline in some parameters (number of leaves and/or rhizome growth) for the three sites affected by the mass mortality event for the year following the warm-water episode (1999-2000). A similar decline was not observed for the unaffected site. The fact that high temperatures could have a negative impact on deep Posidonia oceanica near its cold limit of distribution is an unexpected result.