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Suspension feeders, including ascidians (Phylum Chordata, Class Ascidiacea), experience a dilute prey field composed of extremely small particles. The filtration apparatus of ascidians is based on a mucous-mesh that is continuously secreted and ingested. The rate and metabolic cost of this mesh secretion has not been quantified to date. We used video boroscopy to quantify the mucous-mesh production rate of the solitary ascidian Herdmania momus under different food and temperature treatments. H. momus individuals with an average (±95% CI) biomass of 30.7 ± 1.1 mg and a branchial sac area of 10.3 ± 1.2 cm2 produced an average of 276 ± 33.5 cm2 of mucous-mesh h-1, corresponding to a median turnover rate of 625 ± 82 mesh d-1. Since the mean mesh mass was 2.44 ± 0.58 mg, this production rate corresponds to roughly 50 ± 8 times the individual’s biomass per day. Food concentration had no detectable effect on mesh production rate, whereas a temperature difference of ~9°C (20 vs. 29°C) moderately increased mesh production by 30-50%. The current study reveals that the feeding process of H. momus involves a high expenditure on mucous-mesh synthesis that, combined with low food availability, may limit its growth in oligotrophic waters and under changing climate regimes.

Introductions of non-indigenous ascidian species are highly common nowadays due to their ability to establish successfully on artificial substrates. Didemnid species are of particular concern because of their rapid a-sexual growth and high fecundity. The colonial ascidian Didemnum perlucidum Monniot F., 1983 was first described from Guadeloupe Island in the Caribbean, although its native range remains unknown. To date, it has been recorded from numerous sites across the Indo-Pacific and Atlantic oceans. Here, having employed both classic taxonomy and genetic tools to verify its identification, we document for the first time its occurrence in the Mediterranean Sea – on two artificial substrates along the Israeli coast. The ability of D. pelucidum to establish reproductive populations despite the harsh environmental conditions of this region, with temperature fluctuations between 16-31°C and a salinity of 38-39 ppt, raises concern regarding this species’ potential for introductions at numerous sites across the Mediterranean.

The disproportionately low presence of marine species in the list of invasive alien species (IAS) of Union concern of the European Union (EU) Regulation 1143/2014 does not fully acknowledge the threat they pose to the EU marine environment. In this study, the first EU-scale Horizon Scanning (HS) focusing on marine alien species was performed, aiming to deliver a ranked list of species that should be of high priority for risk assessment (Article 5 of the EU IAS Regulation). Species absent from or with a limited distribution in EU marine waters were targeted. In total, 363 alien species were initially screened for HS by a panel of experts, including a broad range of taxonomic groups. Species were scored for their likelihood of arrival, establishment, spread, and impact in EU waters. A consensus workshop ranked 267 species, including a subset of 26 prioritized species. These species are considered to be mainly introduced by shipping (fouling and ballast water), via the Suez Canal, and aquaculture activities. The 26 priority species were also scrutinized in terms of feasibility of their management; 18 of them were suggested for performing risk assessments on the basis of the EU IAS Regulation. Since biological invasions are dynamic and connected with accelerated globalization and diversified human activities, we recommend HS to be repeated periodically to review the species already listed and assess new ones.

The increasing use of pharmaceuticals in human and veterinary medicine, along with their poor removal rates at wastewater treatment facilities is resulting in the chronic release of pharmaceutically-active compounds (PhACs) into the marine environment, where they pose a threat to non-target organisms. A useful approach, as applied in the current study for assessing the effects of PhACs on non-target organisms, is the proteomic approach: the large-scale study of an organism's proteins. Using ‘shotgun’ proteomics, we identified differentially-expressed proteins based on peptide fragments in the solitary ascidian, Herdmania momus, following a 14-day laboratory experimental exposure to the PhAC carbamazepine (CBZ), an anticonvulsant and antidepressant medication, frequently detected in the aquatic environment. Individuals were exposed to environmentally relevant concentrations: 5 or 10 µg/L of CBZ, in addition to a control treatment. Out of 199 identified proteins, 24 were differentially expressed (12%) between the treatment groups, and thus can potentially be developed as biomarkers for CBZ contamination. Ascidians’ phylogenetic position within the closest sister group to vertebrates presents an advantage in examining the pathological effects of PhACs on vertebrate-related organs and systems. Together with the world-wide distribution of some model ascidian species, and their ability to flourish in pristine and polluted sites, they provide a promising tool through which to study the extent and effects of PhAC contamination on marine organisms.

The solitary ascidian Ciona intestinalis is among the most damaging of invasive fouling species in the world. It is mostly known from cold-water or temperate environments, where it is able to form dense aggregations. We present the first record of C. intestinalis type-A, now recognized as C. robusta, from the Eilat marina, Israeli Red Sea coast. Several individuals found fouling the bottom of the floating docks in March 2015 were identified using both mitochondrial sequencing and morphological tools. Periodical surveys revealed some of them with full gonoducts, and small individuals appeared two months later, indicating a reproductive population. The extremely high salinity and temperature at the Eilat marina indicates the wide environmental tolerance of this species. Since the only other report from this region dates to 100 years ago, from a dredging vessel at the entrance to the Suez Canal, and it has not since been recorded from any natural substrates in the region, it should be considered a non-indigenous species in the Red Sea.

Heavy metal (HM) inputs into marine environments and their effect on marine organisms are of major concern. Here, we examined the potential use of two invasive ascidian species, Phallusia nigra and Microcosmus exasperatus, as bio-indicators of 11 HMs in the Mediterranean and Red Sea coasts of Israel. Individuals were collected on a seasonal basis from three sites over 1 year, and analysis was carried out separately for the tunic and the body. Both species accumulated high levels of HMs, which varied seasonally and spatially. In M. exasperatus the majority of HMs were found in the tunic, and in P. nigra in the body, suggesting the need to analyze total individuals in future studies. Hepato-Somatic Index values for M. exasperatus were significantly lower at the polluted site. Investigation of a popular public beach revealed high levels of certain dissolved HMs in both the water and in the ascidians. The wide geographic distribution and high filtration capacity of invasive ascidians offer great potential for their use in monitoring metal pollution in marine environments.

The solitary ascidian Microcosmus exasperatus (Class: Ascidiacea, Family: Pyuridae) is found in both tropical and sub-tropical waters. In the Mediterranean it has been recorded on the coasts of Israel, Lebanon, Turkey, and Tunisia. Here we report its first record from Cyprus, where it was found forming aggregations on ropes in a marina. It was also recorded there on the hull of a 7-meter pleasure craft. We provide both molecular and morphological diagnostics to facilitate the accurate identification of M. exasperatus, as its external appearance resembles that of M. squamiger, which is a widely-distributed species in the western Mediterranean. The occurrence of M. exasperatus on ship hulls and its ability to form large aggregations on artificial substrata may accelerate introduction of the non-native epifaunal organisms that cover its tunic, consequently altering local biodiversity. Thus, we strongly recommend monitoring the progression of this invasive species in the eastern Mediterranean.

Environmental factors, such as temperature and salinity, are known to influence distribution patterns and invasion success in ascidians. The solitary ascidian Microcosmus exasperatus Heller, 1878 has a wide global distribution and can be found in both tropical and sub-tropical waters. In the Mediterranean Sea, it is considered to be an invasive species introduced through the Suez Canal, with a restricted distribution in the eastern Mediterranean. Despite its global distribution, the environmental tolerances of this species are poorly known. We examined the effect of varying temperature and salinity on the survival of adult individuals of M. exasperatus in a laboratory setting to partially determine its environmental tolerance range. In addition, it’s global and local distribution as well as the seasonal abundance in ‘Akko Bay (northern Mediterranean coast of Israel) were examined. Field observations and laboratory experiments show that M. exasperatus is able to tolerate a temperature range of 12– 30 oC, and salinity of 37– 45, but it survived poorly in salinity of 33– 35 and temperatures > 32 oC. Considering this relatively wide tolerance range of M. exasperatus to temperature and salinity together with the anticipated rise in anthropogenic disturbances, we expect this species to further spread into new locations along the Mediterranean coast.

Global increase in the use of pharmaceutically-active compounds (PhACs), and their insufficient removal in wastewater treatment plants, have resulted in their continuous release into the marine environment. We investigated the use of the solitary ascidians Herdmania momus, Microcosmus exasperatus, and Styela plicata as bioindicators of three common PhACs in the Israeli coastal waters: Bezafibrate, carbamazepine and diclofenac. Both the Mediterranean and the Red-Sea coasts were found contaminated with PhACs, detected at all 11 sampling sites, with four sites contaminated with all three studied PhACs. Diclofenac was most frequent, present in nine of the 11 sites with concentrations reaching 51.9 ng/g of dry weight sample (dw). Bezafibrate and carbamazepine reached concentrations of 47.8 ng/g dw and 14.3 ng/g dw, respectively. The alarming detection of such high concentrations of PhACs in ascidians along Israel's coasts demonstrates both the extent of PhACs contamination in the region, and the potential of ascidians as bioindicators, and emphasizes the urgent need for additional research into PhAC contamination sources and effects.

The interaction of microplastic (MP) with marine organisms is crucial for understanding the significant effect that MP and its additives may have on marine environments. However, knowledge regarding the magnitude of these pollutants in the Eastern Mediterranean Sea and the tropical Red Sea is still scarce. Here we examined the levels of phthalate acid esters (PAEs) and MP in Herdmania momus and Microcosmus exasperatus sampled along the Mediterranean and Red Sea coasts of Israel. High levels of dibutyl phthalate (DBP) and bis (2‑ethylhexyl) phthalate (DEHP) were found in ascidians at the majority of sampling sites, and MP particles were found in ascidians at all sites. As efficient filter-feeders and being widely-distributed, invasive ascidians present fundamental opportunities for the environmental monitoring of microplastic and its additives worldwide. The high levels of pollutants revealed emphasize the need for further research into the magnitude and effects of MP and PAEs in these regions.

Coral-reef community structure has generally changed following declines in coverage by reef-building organisms. Ascidians as fast-growing, filter-feeding organisms, are of concern due to their invasiveness potential, but few studies have examined their life-history traits in tropical environments. The present study examined the distribution, settlement patterns, and reproductive cycle of the solitary, hermaphroditic ascidian Halocynthia spinosa in Eilat, Red Sea (29°30′N, 34°55′E), from August 2013 to August 2015. This species has a native distribution in the western Indian Ocean, with a continuous presence along the southern and eastern coasts of Africa. In Eilat, the field survey found that H. spinosa was prevalent in shallow depths, on cryptic, shaded substrates. Histological analysis of spermatogenesis and oogenesis, and a determination of the gonad index in monthly samples revealed a seasonal reproductive cycle that was significantly correlated with seawater temperature, associated with photoperiod and food availability. All measures of reproduction showed the highest activity in the summer (June–August), and a short resting period in the winter months (December–March). H. spinosa individuals were rarely observed on artificial structures. Thus, the introduction of H. spinosa into new regions as part of a fouling community is relatively unlikely. These findings contribute to understanding ecological interactions of non-reef-building organisms such as ascidians, and add to the knowledge of ascidian reproductive patterns.

Invasive ascidians (Chordata, Tunicata) are dominant nuisance organisms. The current study investigated the role of marine vessels in their dispersal and introduction. An examination of 45 dry-docked marine vessels, comprising recreational, commercial, and military craft, in five Israeli shipyards along the Mediterranean coast, revealed non-indigenous ascidians (NIA) on every second vessel investigated. Military vessels featured the highest ascidian abundance and richness, potentially related to their maintenance routine. Niche areas on the vessels such as sea chests and the propeller exhibited the highest occurrence of ascidians. Overall, these findings provide strong evidence that marine vessels play an acute role in NIA introduction and dispersal, with military vessels and niche areas on all the vessels being more susceptible to serving as vectors. A discovery of a new introduced species during the surveys suggests that the monitoring of marine vessels can serve as an effective tool for the early detection of NIA.

Este artículo contiene 17 páginas, 2 figuras, 2 tablas., The meticulous revision by taxonomic experts of established alien species in the Mediterranean resulted in a major revision of the list proposed by Galil et al. (2016), with 73 species to be excluded (35 species categorised as non-established and 37 as not true aliens), and 72 species added to the list. Consequently, by year 2016 the total number of established alien species in the Mediterranean reached 613, which is a 28% increase over the preceding four years. If we also consider casual species (208 species), the total number of alien species in the Mediterranean is 821. This is attributed to: new findings, change in establishment status of species previously known on the basis of few and scattered records, and results of phylogenetic studies in some cosmopolitan species. However, the true number of alien species reported here is considered to be an underestimation, as it does not include phytoplanktonic organisms, Foraminifera, cryptogenic and species known on the basis of questionable records that might turn out to be true aliens. EASIN and INVASIVESNET can play a major role in the future revision/update of the present list, which currently serves for assessing indicators that are necessary for policy, and for management of alien species in the Mediterranean Sea. An increasing trend in new arrivals since 1950, which culminated in the 2001e2010 period, appeared to decline after 2010. Whether this negative trend is an indication of improvement, or is an artefact, remains to be seen. The current list provides a reliable updated database from which to continue monitoring the arrival and spread of invasive species in the Mediterranean, as well as to provide counsel to governmental agencies with respect to management and control. Current geographical, taxonomical and impact data gaps can be reduced only by instituting harmonised standards and methodologies for monitoring alien populations in all countries bordering the Mediterranean Sea., This work was initiated during the DEVOTES (DEVelopment Of innovative Tools for understanding marine biodiversity and assessing good Environmental Status) project, funded by the European Union under the 7th Framework Program (grant agreement no. 30839). The work of FC and AZ was partly funded by The East and South European Network for Invasive Alien Species - a tool to support the management of alien species in Bulgaria (ESENIASTOOLS) (European Economic Area funded - Contract No. D-33-51/ 30.06.2015). GS and FC were supported by STSM contracts in the framework of Alien Challenge COST Action (TD1209) funded by the European Commission.

This is the first recording on the genus Nikoides Paulson, 1875 in the Mediterranean Sea, with remarks on taxonomy and ecology.

Owing to their impressive regeneration abilities and close phylogenetic relationship to vertebrates, ascidians (phylum: Chordata) are a key model system for regenerative and developmental studies. Nevertheless, ascidian regeneration research has been limited, to date, to only a few model species. In the present work, we investigated the ability of four solitary ascidians of the order Stolidobranchia, Polycarpa mytiligera, Herdmania momus, Microcosmus exasperatus, and Styela plicata, to regenerate their neural complex and siphons. Four treatments were used: a) amputation of the oral siphon, b) amputation of the atrial siphon, c) amputation of both siphons and the neural complex located between the siphons, and d) a control treatment with no amputations. All four species initiated regeneration of the amputated organs, with P. mytiligera demonstrating the most powerful regenerative ability, as it was the only species to survive all amputation procedures and have its neural complex fully regenerated. P. mytiligera's phylogenetic proximity to colonial species suggests that its remarkable regeneration abilities have been maintained as an ancestral feature, and that this species may constitute a unique phylogenetic transition between coloniality and a solitary life style. P. mytiligera's distinctive regeneration traits may have also provided this species with an ecological advantage, as evidenced by its abundance in the Red Sea. Our findings highlight P. mytiligera's potential to serve as a most valid model organism for research into the evolution of regeneration within chordates and into regenerative biology in general.

Although the Red Sea is considered as a center of endemism and a ‘hotspot’ of biodiversity, the ascidian fauna (Chordata, Ascidiacea) of the region has been poorly studied. The current study provides a review of the literature concerning the ascidian fauna of the Red Sea, including the Gulf of Aqaba and the Gulf of Suez, an updated species list based on extensive field sampling and museum data, and an analysis of the diversity and rates of endemism of the Ascidiacea of the Red Sea. The inventory lists 73 ascidian species, belonging to 13 families. The majority of species records are of colonial species, as typical of tropical regions. Representatives of 30 species are available at the National Collections of Natural History at Tel Aviv University. Only 12 species (17 %) of the Red Sea ascidians were found to be endemic to the region, whereas the majority have an extra Indo-Pacific distribution. Eight species have been recorded for the first time from this region. The Gulf of Aqaba and the Gulf of Suez represent the northern boundary of the natural distribution for the majority of tropical species recorded in this study. However, several species have successfully entered the Mediterranean Sea through the Suez Canal. This list is certainly incomplete and emphasizes the need to increase the sampling effort for this group, and for additional taxonomic studies of the ascidian fauna of this highly diverse and rich region.

The small brooding ascidian Molgula citrina Alder and Hancock, 1848 has long been known as a common inhabitant of shallow waters on both sides of the north Atlantic on subtidal natural hard substrates and also as a fouler of floating docks. There are published records from the White Sea (NW Russia), but none from the north Pacific. In May and August 2008, a number of adult brooding Molgula sp. specimens were collected from floating docks at the small fishing village of Seldovia on the Kenai Peninsula at Kachemak Bay, Alaska. Morphologically these individuals exactly match Atlantic specimens of Molgula citrina in all characters that were examined. The unique constellation of characters for this species differentiates it from all other Molgula species, as noted by Van Name (1945). In addition, the full-length 18S and 28S rDNA sequences are identical for both the Alaska specimens and New England M. citrina. Is this a new invasion, or is M. citrina a northern circumpolar species that was somehow overlooked? If this new record indicates a natural distribution, why has it not been collected before? If it is a recent introduction, it is unlikely that M. citrina, an inhabitant of cold waters, could survive in sea chests of ships from the N. Atlantic arriving in Alaska via the Panama Canal, but the intriguing idea exists of transport from Atlantic to Pacific through the Northwest or Northeast Passages. As global warming diminishes the ice cover in this region, more and more ships are traversing the Northwest Passage across northern Canada as well as the NE Passage across northern Russia, representing significant new routes for anthropogenic transport of marine species. The tadpoles of this small brooder are retained for some time after hatching, resulting in an extremely short free larval life, but could survive as metamorphosed juveniles attached in sea chests or free-floating in ballast water. They have a wide temperature tolerance and once they metamorphose can live free-floating in sea water for some time. They become very sticky and will ultimately stick to whatever they contact. Thus they could conceivably live for many generations in sea chests and sustain a viable population from which to invade new habitats.

In the coral reefs of Eilat, a newly described colonial ascidian, Botryllus eilatensis, has been observed overgrowing dead coral skeletons and rapidly colonizing artificial substrates. To study overgrowth dynamics and the relationship between coral morphology, size, and orientation and ascidian overgrowth we monitored (monthly) the interaction of 50 to 70 tagged coral colonies with B. eilatensis. The rate of B. eilatensis overgrowth on corals was influenced by the coral's morphology and orientation combined with a seasonal appearance of the ascidian. During spring, the average ascidian cover was 27 ± 4.6 % (mean ± SE), decreasing rapidly during the summer months when sex- ual reproduction took place. This seasonality is linked to the vertical mixing phenomenon that occurs in Eilat every winter. Moreover, fragments of B. eilatensis translocated to a nutrient-rich site increased 3-fold in size and exhibited higher survivorship rates in comparison to the fragments at the original site. Increased anthropogenic activity, particularly the eutrophication occurring along the Israeli coasts, creates favorable conditions for filter-feeding organisms such as ascidians, providing them with an advantage in competition for space over reef-building corals and other organisms.

Although ascidians are conspicuous members of the fouling community not much is known regarding their recruitment patterns in coral reefs. A 1-year study was carried out along the Red Sea coast of Israel to examine the effects of season and spatial distribution on ascidian recruitment to artificial marine structures. In general, autumn and spring were characterized by higher coverage with a significantly higher percentage of cover of Didemnum granulatum in autumn and higher numbers of Herdmania momus in spring. These species contributed the most to similarity between treatments consequently setting the pattern for each group (colonial and solitary). Halocynthia spinosa had significantly higher numbers during winter and Phallusia nigra was absent in spring and winter. H. momus showed a preference for horizontal surfaces. P. nigra and Ascidia cannelata showed a preference for floating units. It is concluded that the ascidian recruitment patterns are species-specific and vary between seasons, orientation and position on the substrata and in the water column.

Each summer 80–90% of the colonies of Oculina patagonica undergo bleaching off the Mediterranean coast of Israel. To investigate fluctuations through a yearly bleaching cycle, monthly measurements of zooxanthella density, mitotic index and chlorophyll-a concentration were conducted. Results showed (1) a significant negative correlation between sea surface temperature (SST) and zooxanthella density; (2) both significantly lower zooxanthella mitotic index and higher chlorophyll-a per zooxanthella content during the bleaching season compared with the non-bleaching period; (3) prior to bleaching, a lag between the peak of zooxanthella density and chlorophyll-a concentration followed by a similar lag during recovery. Zooxanthella density declined significantly between March and May while chlorophyll-a concentration peaked in April, and then declined. Zooxanthella density increased significantly in November while chlorophyll-a concentration increased significantly in January. We conclude that during bacterial bleaching events, zooxanthellae are severely damaged. However, by the time of the following bleaching event the coral tissues regain their “normal” (pre-bleaching) zooxanthella population density.

Styela plicata (Lesueur, 1823) (Tunicata; Stolidobranchia; Styelidae) is a solitary ascidian with a global distribution. Until recently it had not been observed along the Mediterranean coast of Israel, but is now to be found in many harbors attached to suspended ropes and other artificial structures. In order to monitor the populations of S. plicata, we surveyed eight harbors along the Israeli Mediterranean coast. Our results show that the condition and maintenance of each harbor significantly affected the presence of S. plicata. We also characterized the microbial communities in the tunic of three individuals using 16S rRNA gene tag pyrosequencing and compared them to those in the surrounding seawater, in order to determine whether the symbiotic bacteria may contribute to the successful establishment of this ascidian species. We found 15 bacterial phyla in total, of which 14 were present in the animal hosts: 6 were present in all of the individuals and 4 in two of the individuals. Three of the 15 phyla observed were absent in the surrounding seawater. The high variability among the three hosts suggests a mechanism of horizontal transmission and may play a role in the process of invading new and less tolerant niches.

9 páginas, 4 tablas, 4 figuras., Although marine biological invasions via the Suez Canal have been extensively documented, little is known about the introduction of non-indigenous ascidians (Chordata, Ascidiacea), a group containing particularly aggressive invasive species. Here, we used a multidisciplinary approach to study the introduction of the ascidianHerdmania momus into the Mediterranean Sea. We reviewed its taxonomy and global distribution, and analyzed how genetic variation is partitioned between sides of the Suez Canal. The taxonomic revision showed that H. momus currently has a wide Indo-Pacific distribution. Genetic data indicated two well-differentiated colonization histories across the eastern Mediterranean. Our findings suggest that the range expansion of H. momus has been greatly facilitated by the combined effect of human-mediated transport and the species’ ability to adapt to different environments. The integrative approach presented here is critical to attain a holistic understanding of marine biological invasions, especially when studying groups with a poorly resolved taxonomy., The research was funded by the Dan-David Foundation, the Steinhardt National Collections of Natural History (Zoological Museum, Tel Aviv University) the Israeli Taxonomy Initiative fellowship to N.S., and the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. PIOF-GA-2009-254634 to M.R.

One of the most extensively documented marine bioinvasion phenomena is the invasion of Red Sea species to the Mediterranean Sea through the Suez Canal. The present study reports the occurrence of seven non-indigenous ascidian species along the Mediterranean coast of Israel: Ecteinascidia thurstoni, Ascidia cannelata, Phallusia nigra, Rhodosoma turcicum, Symplegma brakenhielmi, Microcosmus exasperatus and Herdmania momus. Five of these species (excluding P. nigra, and R. turcicum) probably reached the Mediterranean via the Suez Canal, since they have an extra-tropical Indo-Pacific distribution and a restricted distribution in the eastern Mediterranean. This is the first record of E. thurstoni in the Mediterranean. The accumulating evidence for the negative impact of non-indigenous ascidians on local species and habitats raises the necessity for long-term studies and monitoring of this group.

A 2-yr continuous photographic monitoring of a tagged population of the encrusting coral Oculina patagonica in the Mediterranean was conducted to study intra-colonial bleaching dynamics and the relationship between bleaching, mortality, and colony size. Surveys of non-tagged colonies showed that during the peak bleaching season (August, sea surface temperature = 31°C), non-bleached colonies were frequently found to be small colonies averaging 4.6 ± 2.3 cm in diameter. Within tagged colonies, percent bleached surface area was correlated to water temperature. In colonies that underwent bleaching, the perimeter of the colony was affected first, and, as water tem- peratures increased, bleaching progressed toward the colony center. During the summer months, partial mortality occurred in the perimeter region of bleached colonies in 22% of the tagged colonies and 25% of the tagged colonies died; 40% of the colonies that died belonged to the largest size group. This partial mortality caused an average decline of 46 ± 27% in the average colony size, resulting in a shift to a smaller size group within the monitored population. Since in this species, colonies as small as 2 cm in diameter are reproductive, bleaching may have a less significant effect on the reproductive fitness of the small size groups in the population. The high mortality of large colonies, high survivorship of the small colonies, and the decline in colony size, due to partial mortality, suggest that, in the case of bleaching in populations of O. patagonica, small colony size is advantageous.