Metabolic plasticity of the corals Porites lutea and Diploastrea heliopora exposed to large amplitude internal waves

The metabolic plasticity of the two mounding coral species Porites lutea (Milne-Edwards and Haime, 1860) and Diploastrea heliopora (Lamarck, 1816) was investigated in the Similan Islands (Thailand), an offshore Andaman Sea island group subjected to large amplitude internal waves (LAIW). Nutrient concentrations were highly correlated with LAIW intensity and contributed to 3- and 10-fold higher symbiont densities in P. lutea and D. heliopora, respectively, along with elevated pigment concentrations, protein content, host tissue, and symbiont biomass. The comparison of LAIW-exposed and LAIW-sheltered island faces, and LAIW-intense and LAIW-weak years suggests a species-specific metabolic plasticity to LAIW, where D. heliopora benefits more from increased nutrient and organic matter availability than P. lutea. The ubiquitous LAIW in Southeast Asia and beyond may provide so far unexplored clues to coral acclimatization to disturbances on various scales, and hence, a potential key to coral resilience to climate change.     

Interseasonal and interspecies diversities of <Emphasis Type="Italic">Symbiodinium</Emphasis> density and effective photochemical efficiency in five dominant reef coral species from Luhuitou fringing reef,northern South China Sea

Although it is well established that different coral species have different susceptibilities to thermal stress, the reasons behind this variation are still unclear. In this study, 384 samples across five dominant coral species were collected seasonally between September 2013 and August 2014 at Luhuitou fringing reef in Sanya, Hainan Island, northern South China Sea, and their algal symbiont density and effective photochemical efficiency (Φ _PSII) were measured. The results indicated that both the Symbiodinium density and Φ _PSII of corals were subject to significant interspecies and seasonal variations. Stress-tolerant coral species, including massive Porites lutea and plating Pavona decussata, had higher symbiont densities but lower Φ _PSII compared to the vulnerable branching species of Acropora over the course of all four seasons. Seasonally, coral symbiont densities were the lowest during winter, while during the same period, Φ _PSII of corals was at the highest point. Further analysis suggested that dissolved inorganic nutrients and upwelling in the reef area were probably responsible for the observed seasonal variations in symbiont density. The fact that Porites lutea has the lowest Φ _PSII during all four seasons is likely related to their symbionts’ lower capacity to provide required photosynthates for calcification. These results suggest that a coral’s thermal tolerance is primarily and positively dependent on its symbiont density and is less related to its effective photochemical efficiency.     

A multi-trait systems approach reveals a response cascade to bleaching in corals

Background

Climate change causes the breakdown of the symbiotic relationships between reef-building corals and their photosynthetic symbionts (genus Symbiodinium), with thermal anomalies in 2015–2016 triggering the most widespread mass coral bleaching on record and unprecedented mortality on the Great Barrier Reef. Targeted studies using specific coral stress indicators have highlighted the complexity of the physiological processes occurring during thermal stress, but have been unable to provide a clear mechanistic understanding of coral bleaching.

Results

Here, we present an extensive multi-trait-based study in which we compare the thermal stress responses of two phylogenetically distinct and widely distributed coral species, Acropora millepora and Stylophora pistillata, integrating 14 individual stress indicators over time across a simulated thermal anomaly. We found that key stress responses were conserved across both taxa, with the loss of symbionts and the activation of antioxidant mechanisms occurring well before collapse of the physiological parameters, including gross oxygen production and chlorophyll a. Our study also revealed species-specific traits, including differences in the timing of antioxidant regulation, as well as drastic differences in the production of the sulfur compound dimethylsulfoniopropionate during bleaching. Indeed, the concentration of this antioxidant increased two-fold in A. millepora after the corals started to bleach, while it decreased 70% in S. pistillata.

Conclusions

We identify a well-defined cascading response to thermal stress, demarking clear pathophysiological reactions conserved across the two species, which might be central to fully understanding the mechanisms triggering thermally induced coral bleaching. These results highlight that bleaching is a conserved mechanism, but specific adaptations linked to the coral’s antioxidant capacity drive differences in the sensitivity and thus tolerance of each coral species to thermal stress.

     

Can heterotrophic uptake of dissolved organic carbon and zooplankton mitigate carbon budget deficits in annually bleached corals?

Annual coral bleaching events due to increasing sea surface temperatures are predicted to occur globally by the mid-century and as early as 2025 in the Caribbean, and severely impact coral reefs. We hypothesize that heterotrophic carbon (C) in the form of zooplankton and dissolved organic carbon (DOC) is a significant source of C to bleached corals. Thus, the ability to utilize multiple pools of fixed carbon and/or increase the amount of fixed carbon acquired from one or more pools of fixed carbon (defined here as heterotrophic plasticity) could underlie coral acclimatization and persistence under future ocean-warming scenarios. Here, three species of Caribbean coral—Porites divaricata, P. astreoides, and Orbicella faveolata—were experimentally bleached for 2.5 weeks in two successive years and allowed to recover in the field. Zooplankton feeding was assessed after single and repeat bleaching, while DOC fluxes and the contribution of DOC to the total C budget were determined after single bleaching, 11 months on the reef, and repeat bleaching. Zooplankton was a large C source for P. astreoides, but only following single bleaching. DOC was a source of C for single-bleached corals and accounted for 11–36 % of daily metabolic demand (CHAR_DOC), but represented a net loss of C in repeat-bleached corals. In repeat-bleached corals, DOC loss exacerbated the negative C budgets in all three species. Thus, the capacity for heterotrophic plasticity in corals is compromised under annual bleaching, and heterotrophic uptake of DOC and zooplankton does not mitigate C budget deficits in annually bleached corals. Overall, these findings suggest that some Caribbean corals may be more susceptible to repeat bleaching than to single bleaching due to a lack of heterotrophic plasticity, and coral persistence under increasing bleaching frequency may ultimately depend on other factors such as energy reserves and symbiont shuffling.     

Protection of Coral Larvae from Thermally Induced Oxidative Stress by Redox Nanoparticles

Coral reefs are one of the most biologically diverse and economically important ecosystems on earth. However, the destruction of coral reefs has been reported worldwide owing to rising seawater temperature associated with global warming. In this study, we investigated the potential of a redox nanoparticle (RNP^O) to scavenge reactive oxygen species (ROS), which are overproduced under heat stress and play a crucial role in causing coral mortality. When reef-building coral (Acropora tenuis) larvae, without algal symbionts, were exposed to thermal stress at 33 °C, RNP^O treatment significantly increased the survival rate. Proteome analysis of coral larvae was performed using nano-liquid chromatography-tandem mass spectrometry for the first time. The results revealed that several proteins related to ROS-induced oxidative stress were specifically identified in A. tenuis larvae without RNP^O treatment, whereas these proteins were absent in RNP^O-treated larvae, which suggested that RNP^O effectively scavenged ROS from A. tenuis larvae. Results from this study indicate that RNP^O treatment can reduce ROS in aposymbiotic coral larvae and would be a promising approach for protecting corals from thermal stress.     

Skeletal dissolution kinetics and mechanical tests in response to morphology among coral genera

Ocean acidification is widely accepted as a primary threat to coral reef populations. Negative physiological effects include decreased calcification rates, heightened metabolic energy expenditure, and increased dissolution of coral skeletons. However, studies on the dissolution of coral skeletons structures under ocean acidification conditions and their implications on sediments remain scarce. In this work, we examined skeletal dissolution kinetics from four of the most representative hermatypic corals of the Eastern Pacific coasts (Pocillopora, Porites, Pavona, and Psammocora). Samples were treated with a highly acidic solution for defined periods of time, and measurements of dissolved calcium ([Ca⁺²]) were used to evaluate the kinetics of coral skeleton dissolution. All genera tests except Porites showed a zero reaction rate. Porites exhibited a first-order reaction and a faster reaction rate than other genera. Compression strength tests and skeletal density did not correlate with reaction rate. Pavona showed greater structural strength. Porites were the most susceptible to acidic dissolution compared to other genera tested due to their morphology, i.e., possession of the largest surface area, suggesting a high vulnerability under low-pH conditions. The hierarchical response in dissolution kinetics among coral genera tested suggests that the most soluble coral might act as a buffer under ocean acidification conditions.     

Bleaching response of coral species in the context of assemblage response

Caribbean coral reefs are declining due to a mosaic of local and global stresses, including climate change-induced thermal stress. Species and assemblage responses differ due to factors that are not easily identifiable or quantifiable. We calculated a novel species-specific metric of coral bleaching response, taxon-α and -β, which relates the response of a species to that of its assemblages for 16 species over 18 assemblages. By contextualizing species responses within the response of their assemblages, the effects of environmental factors are removed and intrinsic differences among taxa are revealed. Most corals experience either a saturation response, overly sensitive to weak stress (α > 0) but under-responsive compared to assemblage bleaching (β < 1), or a threshold response, insensitive to weak stress (α < 0) but over-responsive compared to assemblage bleaching (β > 1). This metric may help reveal key factors of bleaching susceptibility and identify species as targets for conservation.     

Large-amplitude internal waves benefit corals during thermal stress

Tropical scleractinian corals are particularly vulnerable to global warming as elevated sea surface temperatures (SSTs) disrupt the delicate balance between the coral host and their algal endosymbionts, leading to symbiont expulsion, mass bleaching and mortality. While satellite sensing of SST has proved a reliable predictor of coral bleaching at the regional scale, there are large deviations in bleaching severity and mortality on the local scale that are poorly understood. Here, we show that internal waves play a major role in explaining local coral bleaching and mortality patterns in the Andaman Sea. Despite a severe region-wide SST anomaly in May 2010, frequent upslope intrusions of cold sub-pycnocline waters due to breaking large-amplitude internal waves (LAIW) mitigated coral bleaching and mortality in shallow waters. In LAIW-sheltered waters, by contrast, bleaching-susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW benefit coral reefs during thermal stress and provide local refugia for bleaching-susceptible corals. LAIW are ubiquitous in tropical stratified waters and their swash zones may thus be important conservation areas for the maintenance of coral diversity in a warming climate. Taking LAIW into account can significantly improve coral bleaching predictions and provide a valuable tool for coral reef conservation and management.     

Near-future ocean acidification enhances the feeding rate and development of the herbivorous juveniles of the crown-of-thorns starfish, <Emphasis Type="Italic">Acanthaster planci</Emphasis>

Population outbreaks of the corallivorous crown-of-thorns starfish, Acanthaster planci, are a major contributor to the decline in coral reef across the Indo-Pacific. The success of A. planci and other reef species in a changing ocean will be influenced by juvenile performance because the naturally high mortality experienced at this sensitive life history stage maybe exacerbated by ocean warming and acidification. We investigated the effects of increased temperature and acidification on growth of newly metamorphosed juvenile A. planci and their feeding rates on crustose coralline algae (CCA) during the initial herbivorous phase of their life history. The juveniles were exposed to three temperature (26, 28, 30 °C) and three pH (NIST scale: 8.1, 7.8, 7.6) levels in a flow-through cross-factorial experiment. There were positive but independent effects of warming and acidification on juvenile growth and feeding. Early juveniles were highly tolerant to moderate increases in temperature (+2 °C above ambient) with the highest growth at 30 °C. Growth and feeding rates of A. planci on CCA were highest at pH 7.6. Thus, ocean warming and acidification may enhance the success of A. planci juveniles. In contrast to its coral prey, at this vulnerable developmental stage, A. planci appears to be highly resilient to future ocean change. Success of juveniles in a future ocean may have carry-over effects into the coral-eating life stage, increasing the threat to coral reef systems.     

Temporal consistency in background mortality of four dominant coral taxa along Australia’s Great Barrier Reef

Studies on the population and community dynamics of scleractinian corals typically focus on catastrophic mortality associated with acute disturbances (e.g., coral bleaching and outbreaks of crown-of-thorns starfish), though corals are subject to high levels of background mortality and injuries caused by routine and chronic processes. This study quantified prevalence (proportion of colonies with injuries) and severity (areal extent of injuries on individual colonies) of background mortality and injuries for four common coral taxa (massive Porites, encrusting Montipora, Acropora hyacinthus and branching Pocillopora) on the Great Barrier Reef, Australia. Sampling was conducted over three consecutive years during which there were no major acute disturbances. A total of 2276 adult colonies were surveyed across 27 sites, within nine reefs and three distinct latitudinal sectors. The prevalence of injuries was very high (>83%) across all four taxa, but highest for Porites (91%) and Montipora (85%). For these taxa (Montipora and Pocillopora), there was also significant temporal and spatial variation in prevalence of partial mortality. The severity of injuries ranged from 3% to more than 80% and varied among coral taxa, but was fairly constant spatially and temporally. This shows that some injuries have considerable longevity and that corals may invest relatively little in regenerating tissue over sites of previous injuries. Inter-colony variation in the severity of injury also had no apparent effect on the realized growth of individual colonies, suggesting that energy diverted to regeneration has a limited bearing on overall energetic allocation, or impacts on other life-history processes (e.g., reproduction) rather than growth. Establishing background levels of injury and regeneration is important for understanding energy investment and life-history consequences for reef-building corals as well as for predicting susceptibility to, and capacity to recover from, acute disturbances.     

Upper Pleistocene cold-water corals from the Inner Sea of the Maldives: taphonomy and environment

Cold-water corals of the Late Pleistocene (21,400–22,500 BP) are recorded from the sea-bottom of two inter-atoll channels (Kardiva Channel at 457-m depth and Malé Vaadhoo Channel at 443-m depth) of the eastern row of the Maldives archipelago. Coral assemblages are composed mainly by Lophelia pertusa and secondarily by Madrepora oculata and Enallopsammia rostrata. These cold-water coral patches are places where the benthic life, mainly sessile, is concentrated, which is clearly absent off-rubble patches. The main epibionts are tube-dwelling polychaetes (mainly Spirorbis and Serpula), bryozoans, siliceous sponges, barnacles, gorgonids, solitary corals, encrusting foraminifera, and microbial mats. The analysis of epibionts assemblages shows different biocoenoses between both studied sites as well as a dependency of the epibiont coverage with regard to the coral genus. Some living benthic organisms such as brachiopods, bivalves, gastropods, barnacles, and ophiuroids find refuge among coral branches. The common record of juvenile specimens of vagile organisms such as small ophiuroids, is probably related to the nursery function of the cold-water corals in spite they are fossils. Environmental requirements of Recent cold-water corals (Lophelia, Madrepora and Enallopsammia) differ of conditions at both sampling sites with sensibly lower oxygenation degree and density of waters than needed for cold-water corals. Therefore, it is proposed that the present-day oxygen and density conditions are the factors which inhibit modern cold-water coral growth in the inter-atoll channels.     

Book review of Littler DM. Littler MM (2000) Caribbean Reef Plants An Identification Guide to the Reef Plants of the Caribbean,Bahamas, Florida and Gulf of Mexico

Information on spatial variability and distribution patterns of organisms in coral reef environments is necessary to evaluate the increasing anthropogenic disturbance of marine environments (Richmond 1993; Wilkinson 1993; Dayton 1994). Therefore different types of subtidal, reef-associated hard substrata (reef flats, reef slopes, coral carpets, coral patches, rock grounds), each with different coral associations, were investigated to determine the distribution pattern of molluscs and their life habits (feeding strategies and substrate relations). The molluscs were strongly dominated by taxa with distinct relations to corals, and five assemblages were differentiated. The Dendropoma maxima assemblage on reef flats is a discrete entity, strongly dominated by this encrusting and suspension-feeding gastropod. All other assemblages are arranged along a substrate gradient of changing coral associations and potential molluscan habitats. The Coralliophila neritoidea–Barbatia foliata assemblage depends on the presence of Porites and shows a dominance of gastropods feeding on corals and of bivalves associated with living corals. The Chamoidea–Cerithium spp. assemblage on rock grounds is strongly dominated by encrusting bivalves. The Drupella cornus–Pteriidae assemblage occurs on Millepora–Acropora reef slopes and is strongly dominated by bivalves associated with living corals. The Barbatia setigera–Ctenoides annulata assemblage includes a broad variety of taxa, molluscan life habits and bottom types, but occurs mainly on faviid carpets and is transitional among the other three assemblages. A predicted degradation of coral coverage to rock bottoms due to increasing eutrophication and physical damage in the study area (Riegl and Piller 2000) will result in a loss of coral-associated molluscs in favor of bivalve crevice dwellers in dead coral heads and of encrusters on dead hard substrata.     

Long-term dynamics of the brown macroalga <Emphasis Type="Italic">Lobophora variegata</Emphasis> on deep reefs in Curaçao

Lobophora variegata occurs in the eulittoral zone and in deep water on coral reefs in Curaçao. An analysis of the long-term (1979–2006) changes in the vertical distribution of the macroalga in permanent quadrats indicated a significant increase in cover of the deepwater community. In 1998, Lobophora covered 1 and 5% of the quadrats at 20 and 30 m, respectively. By 2006, these values had risen to 25 and 18%, precipitating a shift in abundance of corals and macroalgae at both depths. This increase coincided with losses in coral cover, possibly linked to bleaching, disease and storm-related mortality in deep water plating Agaricia corals. In contrast, macroalgae remained relatively rare (<6% cover) on shallower (10 m) and deeper (40 m) reefs despite declines in coral cover also occurring at these depths, illustrating the depth-dependent dynamics of coral reefs. Several hypotheses are suggested to explain these changes.     

Contrasting physiological plasticity in response to environmental stress within different cnidarians and their respective symbionts

Given concerns surrounding coral bleaching and ocean acidification, there is renewed interest in characterizing the physiological differences across the multiple host–algal symbiont combinations commonly found on coral reefs. Elevated temperature and CO₂ were used to compare physiological responses within the scleractinian corals Montipora hirsuta (Symbiodinium C15) and Pocillopora damicornis (Symbiodinium D1), as well as the corallimorph (a non-calcifying anthozoan closely related to scleractinians) Discosoma nummiforme (Symbiodinium C3). Several physiological proxies were affected more by temperature than CO₂, including photochemistry, algal number and cellular chlorophyll a. Marked differences in symbiont number, chlorophyll and volume contributed to distinctive patterns of chlorophyll absorption among these animals. In contrast, carbon fixation either did not change or increased under elevated temperature. Also, the rate of photosynthetically fixed carbon translocated to each host did not change, and the percent of carbon translocated to the host increased in the corallimorph. Comparing all data revealed a significant negative correlation between photosynthetic rate and symbiont density that corroborates previous hypotheses about carbon limitation in these symbioses. The ratio of symbiont-normalized photosynthetic rate relative to the rate of symbiont-normalized carbon translocation (P:T) was compared in these organisms as well as the anemone, Exaiptasia pallida hosting Symbiodinium minutum, and revealed a P:T close to unity (D. nummiforme) to a range of 2.0–4.5, with the lowest carbon translocation in the sea anemone. Major differences in the thermal responses across these organisms provide further evidence of a range of acclimation potential and physiological plasticity that highlights the need for continued study of these symbioses across a larger group of host taxa.     

Influence of the coral reef assemblages on the spatial distribution of echinoderms in a gradient of human impacts along the tropical Mexican Pacific

Fourteen species of echinoderms and their relationships to the benthic structure of the coral reefs were assessed at 27 sites—with different levels of human disturbances—along the coast of the Mexican Central Pacific. Diadema mexicanum and Phataria unifascialis were the most abundant species. The spatial variation of the echinoderm assemblages showed that D. mexicanum, Eucidaris thouarsii, P. unifascialis, Centrostephanus coronatus, Toxopneustes roseus, Holothuria fuscocinerea, Cucumaria flamma, and Echinometra vanbrunti accounted for the dissimilarities among the sites. The spatial variation among the sites was mainly explained by the cover of the hard corals (Porites, Pocillopora, Pavona, Psammocora), different macroalgae species (turf, encrusting calcareous algae, articulated calcareous algae, fleshy macroalgae), sponges, bryozoans, rocky, coral rubble, sand, soft corals (hydrocorals and octocorals), Tubastrea coccinea coral, Balanus spp., and water depth. The coverage of Porites, Pavona, and Pocillopora corals, soft coral, rock, and Balanos shows a positive relationship with the sampling sites included within the natural protected area with low human disturbances. Contrary, fleshy macroalgae, sponges, and soft coral show a positive relationship with higher disturbance sites. The results presented here show the importance of protecting the structural heterogeneity of coral reef habitats because it is a significant factor for the distribution of echinoderm species and can contribute to the design of conservation programs for the coral reef ecosystem.     

<Emphasis Type="Italic">Megaviridae</Emphasis>-like particles associated with <Emphasis Type="Italic">Symbiodinium</Emphasis> spp. from the endemic coral <Emphasis Type="Italic">Mussismilia braziliensis</Emphasis>

Coral reefs are one of the most dynamic and productive marine ecosystems. The coral holobiont consists of the coral animal and a variety of associated microorganisms that include symbiotic dinoflagellates of the genus Symbiodinium, bacteria, archaea, fungi and viruses. The interactions among these components are crucial for coral health and, consequently, to the coral reef resilience to disturbance. Environmental stressors such as elevated temperature, high irradiance and ultraviolet (UV) radiation can lead to the breakdown of the coral-Symbiodinium symbiosis in a phenomenon known as “coral bleaching”. The present study provides evidence for virus-like particles (VLPs) induced in UV-irradiated Symbiodinium spp. cultures (clades A and C) that were isolated from the coral Mussismilia braziliensis, suggesting a latent viral infection in these strains. Scanning and transmission electron microscopy images of the UV stressed cultures revealed the presence of giant (ca. 450 nm) and small (ca. 40 nm) VLPs. Morphological features link the giant VLPs to the family Megaviridae. Symbiodinium spp. Megaviridae giant viruses and other associated viruses may represent dynamic forces driving and influencing health of the coral holobiont.     

Microhabitat characteristics of <Emphasis Type="Italic">Stegastes planifrons</Emphasis> and <Emphasis Type="Italic">S. adustus</Emphasis> territories

Stegastes adustus and Stegastes planifrons are two species of damselfishes commonly found in the Caribbean. These territorial fishes have been widely studied due to their major ecological role on coral reef in controlling the growth of macroalgae that compete with corals for space and, inversely, on their deleterious role in destroying coral tissues to impulse the development of algae. However, few studies were conducted on the biotic and abiotic components of their territories. In the present study, territory size and surfaces of benthic components (macroalgae, algal turf, massive corals, branching corals, Milleporidae, sponges, sand and rubbles) were estimated for the two species at two contrasted sites. At Ilet Pigeon site (IP), the two damselfishes were found at different depth and exhibited different territory sizes. S. adustus defended a larger territory characterized by massive corals, sand and Milleporidae, while S. planifrons territories were smaller, deeper and characterized by branching corals, sponges and rubble. At Passe-à-Colas site (PC), the two fish species coexisted in the same depth range and defended territories of similar size. Their territories presented higher proportions of macroalgae, but smaller surfaces of Milleporidae than at IP. At PC, the main difference between the two species was a higher surface of massive corals inside S. planifrons territories than S. adustus territories. Differences in microhabitat characteristics between the two Stegastes seemed mostly site related. This resulted from the high plasticity of two species, allowing them to persist on Caribbean coral reefs after the decline of most branching acroporids, their former favorite habitats.     

<Emphasis Type="Italic">Euphyllia paradivisa</Emphasis>, a successful mesophotic coral in the northern Gulf of Eilat/Aqaba,Red Sea

Mesophotic coral ecosystems (MCEs) host a thriving community of biota that has remained virtually unexplored. Here we report for the first time on a large population of the endangered coral species Euphyllia paradivisa from the MCEs of the Gulf of Eilat/Aqaba (GOE/A), Red Sea. The mesophotic zone in some parts of the study site harbors a specialized coral community predominantly comprising E. paradivisa (73 % of the total coral cover), distributed from 36 to 72 m depth. Here we sought to elucidate the strict distribution but high abundance of E. paradivisa in the MCEs at the GOE/A. We present 4 yr of observations and experiments that provide insight into the physiological plasticity of E. paradivisa: its low mortality rates at high light intensities, high competitive abilities, successful symbiont adaptation to the shallow-water environment, and tolerance to bleaching conditions or survival during prolonged bleaching. Despite its ability to survive under high irradiance in shallow water, E. paradivisa is not found in the shallow reef of the GOE/A. We suggest several factors that may explain the high abundance and exclusivity of E. paradivisa in the MCE: its heterotrophic capabilities; its high competition abilities; the possibility of it finding a deep-reef refuge there from fish predation; and its concomitant adaptation to this environment.     

Genetic diversity of free-living <Emphasis Type="Italic">Symbiodinium</Emphasis> in the Caribbean: the importance of habitats and seasons

Although reef corals are dependent of the dinoflagellate Symbiodinium, the large majority of corals spawn gametes that do not contain their vital symbiont. This suggests the existence of a pool of Symbiodinium in the environment, of which surprisingly little is known. Reefs around Curaçao (Caribbean) were sampled for free-living Symbiodinium at three time periods (summer 2009, summer 2010, and winter 2010) to characterize different habitats (water column, coral rubble, sediment, the macroalgae Halimeda spp., Dictyota spp., and Lobophora variegata, and the seagrass Thalassia testudinum) that could serve as environmental sources of symbionts for corals. We detected the common clades of Symbiodinium that engage in symbiosis with Caribbean coral hosts A, B, and C using Symbiodinium-specific primers of the hypervariable region of the chloroplast 23S ribosomal DNA gene. We also discovered clade G and, for the first time in the Caribbean, the presence of free-living Symbiodinium clades F and H. Additionally, this study expands the habitat range of free-living Symbiodinium as environmental Symbiodinium was detected in T. testudinum seagrass beds. The patterns of association between free-living Symbiodinium types and habitats were shown to be complex. An interesting, strong association was seen between some clade A sequence types and sediment, suggesting that sediment could be a niche where clade A radiated from a free-living ancestor. Other interesting relationships were seen between sequence types of Symbiodinium clade C with Halimeda spp. and clades B and F with T. testudinium. These relationships highlight the importance of some macroalgae and seagrasses in hosting free-living Symbiodinium. Finally, studies spanning beyond a 1-yr cycle are needed to further expand on our results in order to better understand the variation of Symbiodinium in the environment through time. All together, results presented here showed that the great diversity of free-living Symbiodinium has a dynamic distribution across habitats and time.