CONSPECIFICITY AND INDO-PACIFIC DISTRIBUTION OF SYMBIODINIUM GENOTYPES (DINOPHYCEAE) FROM GIANT CLAMS

We previously reported the occurrence of genetically‐diverse symbiotic dinoflagellates (zooxanthellae) within and between 7 giant clam species (Tridacnidae) from the Philippines based on the algal isolates' allozyme and random amplified polymorphic DNA (RAPD) patterns. We also reported that these isolates all belong to clade A of the Symbiodinium phylogeny with identical 18S rDNA sequences. Here we extend the genetic characterization of Symbiodinium isolates from giant clams and propose that they are conspecific. We used the combined DNA sequences of the internal transcribed spacer (ITS)1, 5.8S rDNA, and ITS2 regions (rDNA‐ITS region) because the ITS1 and ITS2 regions evolve faster than 18S rDNA and have been shown to be useful in distinguishing strains of other dinoflagellates. DGGE of the most variable segment of the rDNA‐ITS region, ITS1, from clonal representatives of clades A, B, and C showed minimal intragenomic variation. The rDNA‐ITS region shows similar phylogenetic relationships between Symbiodinium isolates from symbiotic bivalves and some cnidarians as does 18S rDNA, and that there are not many different clade A species or strains among cultured zooxanthellae (CZ) from giant clams. The CZ from giant clams had virtually identical sequences, with only a single nucleotide difference in the ITS2 region separating two groups of isolates. These data suggest that there is one CZ species and perhaps two CZ strains, each CZ strain containing individuals that have diverse allozyme and RAPD genotypes. The CZ isolated from giant clams from different areas in the Philippines (21 isolates, 7 clam species), the Australian Great Barrier Reef (1 isolate, 1 clam species), Palau (8 isolates, 7 clam species), and Okinawa, Japan (1 isolate, 1 clam species) shared the same rDNA‐ITS sequences. Furthermore, analysis of fresh isolates from giant clams collected from these geographical areas shows that these bivalves also host indistinguishable clade C symbionts. These data demonstrate that conspecific Symbiodinium genotypes, particularly clade A symbionts, are distributed in giant clams throughout the Indo‐Pacific.     

IDENTITY OF ZOOXANTHELLAE ISOLATED FROM SOME PACIFIC TRIDACNIDAE

The taxonomy and nltrastructure of zooxanthellae from 6 species of Pacific giant clam have been studied from cultures. Comparison with type material shows that all of the isolates are Symbiodinium microadriaticum, a symbiotic dinoflagellate originally recorded from coelenterates in the Caribbean. Specific criteria for the identification of this organism are discussed, and their value to the comparative taxonomy of marine zooxanthellae is noted.     

Host hybridization as a potential mechanism of lateral symbiont transfer in deep‐sea vesicomyid clams

Deep‐sea vesicomyid clams live in mutualistic symbiosis with chemosynthetic bacteria that are inherited through the maternal germ line. On evolutionary timescales, strictly vertical transmission should lead to cospeciation of host mitochondrial and symbiont lineages; nonetheless, examples of incongruent phylogenies have been reported, suggesting that symbionts are occasionally horizontally transmitted between host species. The current paradigm for vesicomyid clams holds that direct transfers cause host shifts or mixtures of symbionts. An alternative hypothesis suggests that hybridization between host species might explain symbiont transfers. Two clam species, Archivesica gigas and Phreagena soyoae, frequently co‐occur at deep‐sea hydrocarbon seeps in the eastern Pacific Ocean. Although the two species typically host gammaproteobacterial symbiont lineages marked by divergent 16S rRNA phylotypes, we identified a number of clams with the A. gigas mitotype that hosted symbionts with the P. soyoae phylotype. Demographic inference models based on genome‐wide SNP data and three Sanger sequenced gene markers provided evidence that A. gigas and P. soyoae hybridized in the past, supporting the hypothesis that hybridization might be a viable mechanism of interspecific symbiont transfer. These findings provide new perspectives on the evolution of vertically transmitted symbionts and their hosts in deep‐sea chemosynthetic environments.     

Regulation and control of intracellular algae (= zooxanthellae) in hard corals

To examine algal (= zooxanthellae) regulation and control, and the factors determining algal densities in hard corals, the zooxanthellae mitotic index and release rates were regularly determined in branch tips from a colony of a staghorn coral, Acropora formosa, recovering from a coral ''bleaching'' event (the stress-related dissociation of the coral–algal symbiosis). Mathematical models based upon density-dependent decreases in the algal division frequency and increases in algal release rates during the post-bleaching recovery period accurately predict the observed recovery period (ca. 20 weeks). The models suggest that (i) the colony recovered its algal population from the division of the remaining zooxanthellae, and (ii) the continual loss of zooxanthellae significantly slowed the recovery of the coral. Possible reasons for the ''paradoxical'' loss of healthy zooxanthellae from the bleached coral are discussed in terms of endodermal processes occurring in the recovering coral and the redistribution of newly formed zooxanthellae to aposymbiotic host cells. At a steady-state algal density of 2.1 x 10⁶ zooxanthellae cm^-2 at the end of the recovery period, the zooxanthellae would have to form a double layer of cells in the coral tissues, consistent with microscopic observations. Neighbouring colonies of A. formosa with inherently higher algal densities possess proportionately smaller zooxanthellae. Results suggest that space availability and the size of the algal symbionts determines the algal densities in the coral colonies. The large increases in the algal densities reported in corals exposed to elevated nutrient concentrations (i.e between a two- and five-fold increase in the algal standing stock) are not consistent with this theory. We suggest that increases of this magnitude are a product of the experimental conditions: reasons for this statement are discussed. We propose that the stability of the coral–algal symbiosis under non-stress conditions, and the constancy of zooxanthellae densities in corals reported across growth form, depth and geographic range, are related to space availability limiting algal densities. However, at these densities, zooxanthellae have attributes consistent with nutrient limitation.     

Do clades of symbiotic dinoflagellates in scleractinian corals of the Gulf of Eilat (Red Sea) differ from those of other coral reefs?

The symbiotic association between corals and zooxanthellae has been a major contributing factor in the success of reef-building corals. Most of these endocellular microalgal symbionts belong to the dinoflagellate genus Symbiodinium. However, considerable genetic diversity was revealed within this taxon, as is evident in the several clades of Symbiodinium found in association with hermatypic corals all over the world. The coral reefs of Eilat (Aqaba), where winter temperature minima of 21 °C are close to threshold values that prevent reef development, are among the northernmost reefs in the world. Furthermore, due to the circulation patterns of the Gulf, the extremely high evaporation, and lack of any riverine inputs, the Gulf's waters are highly saline (40.5‰). In spite of the extreme location, a high diversity of coral species has been reported in this area. In this study, using PCR, we specifically amplified zooxanthellae 18S ribosomal DNA from symbionts of 11 coral species, and analyzed it with respect to RFLP and DNA sequence.Of the several clades described from the same coral hosts in other localities, only A and C were found in the present study. Symbiodinium populations in the host examined from Eilat were different relative to other parts of the world. This distribution is discussed in relation to reproduction strategy: broadcasting versus brooding. Based on our results, we suggest that clade A is transferred through a closed system. As mass bleaching in the Gulf has never been observed, we suggest that the adaptive mechanisms presumably favoring clade diversity were not yet significant in our relatively cool area.     

Development of coral and zooxanthella‐specific microsatellites in three species of Pocillopora (Cnidaria,Scleractinia) from French Polynesia

Since the building of coral reefs results from the association of corals and zooxanthellae, their intracellular algal symbionts, genetic markers for both organisms are essential for studying the contribution of their respective dispersal to the resilience of endangered reef ecosystems. Very few microsatellites have been obtained in corals thus far. Here we report the successful cloning of six polymorphic microsatellites (allele number: 5–15) from Pocillopora verrucosa, P. meandrina and P. damicornis. Four of them amplified coral, and two amplified zooxanthella DNA.     

Host-symbiont specificity during onset of symbiosis between the dinoflagellates Symbiodinium spp. and planula larvae of the scleractinian coral Fungia scutaria

Many corals which engage in symbioses with dinoflagellates from the genus Symbiodinium (zooxanthellae) produce offspring which initially lack zooxanthellae. These species must choose their symbionts from numerous genetically distinct strains of zooxanthellae co-occurring in the environment. In most cases, symbiosis onset results in an association between a specific host coral and a specific strain of algal symbiont. This is the first study to examine host-symbiont specificity during symbiosis onset in a larval cnidarian, and the first to examine such events in a scleractinian of any life stage. We infected planula larvae of the solitary Hawaiian scleractinian Fungia scutaria with both homologous zooxanthellae, freshly isolated from F. scutaria adults, and heterologous zooxanthellae, isolated from Montipora verrucosa, Porites compressa, and Pocillopora damicornis, three species of scleractinians which co-occur with F. scutaria. We found that homologous zooxanthellae were better able to establish symbioses with larval hosts than were heterologous isolates, by two separate measures: percent of a larval population infected, and densities of zooxanthellae per larva. We also measured algal densities in larvae over a 4-day period until the onset of settlement and metamorphosis. We found no changes in zooxanthella population densities, regardless of zooxanthella type or the light environment in which they were incubated. Strong infection of host larvae with homologous algae compared to heterologous algae suggests that there is a specificity process which occurs sometime during the early stages of infection between the partners, and which results in the establishment of a specific symbiosis.     

Thrips tabaci Population Genetic Structure and Polyploidy in Relation to Competency as a Vector of Tomato Spotted Wilt Virus

Knowledge of population-level genetic differences can help explain variation among populations of insect vectors in their role in the epidemiology of specific viruses. Variation in competency to transmit Tomato spotted wilt virus (TSWV) that exists among populations of Thrips tabaci has been associated with the presence of cryptic species that exhibit different modes of reproduction and host ranges. However, recent findings suggest that vector competency of T. tabaci at any given location depends on the thrips and virus populations that are present. This study characterizes the population genetic structure of T. tabaci collected from four locations in North Carolina and examines the relationship between population genetic structure and variation in TSWV transmission by T. tabaci. Mitochondrial COI sequence analysis revealed the presence of two genetically distinct groups with one characterized by thelytokous, parthenogenetic reproduction and the other by arrhenotokous, sexual reproduction. Using a set of 11 microsatellite markers that we developed to investigate T. tabaci population genetic structure, we identified 17 clonal groups and found significant genetic structuring among the four NC populations that corresponded to the geographic locations where the populations were collected. Application of microsatellite markers also led to the discovery of polyploidy in this species. All four populations contained tetraploid individuals, and three contained both diploid and tetraploid individuals. Analysis of variation in transmission ofTSWV among isofemale lines initiated with individuals used in this study revealed that ‘clone assignment,’ ‘virus isolate’ and their interaction significantly influenced vector competency. These results highlight the importance of interactions between specific T. tabaci clonal types and specific TSWV isolates underlying transmission of TSWV by T. tabaci.     

Population Genetic Analysis Infers Migration Pathways of Phytophthora ramorum in US Nurseries

Recently introduced, exotic plant pathogens may exhibit low genetic diversity and be limited to clonal reproduction. However, rapidly mutating molecular markers such as microsatellites can reveal genetic variation within these populations and be used to model putative migration patterns. Phytophthora ramorum is the exotic pathogen, discovered in the late 1990s, that is responsible for sudden oak death in California forests and ramorum blight of common ornamentals. The nursery trade has moved this pathogen from source populations on the West Coast to locations across the United States, thus risking introduction to other native forests. We examined the genetic diversity of P. ramorum in United States nurseries by microsatellite genotyping 279 isolates collected from 19 states between 2004 and 2007. Of the three known P. ramorum clonal lineages, the most common and genetically diverse lineage in the sample was NA1. Two eastward migration pathways were revealed in the clustering of NA1 isolates into two groups, one containing isolates from Connecticut, Oregon, and Washington and the other isolates from California and the remaining states. This finding is consistent with trace forward analyses conducted by the US Department of Agriculture''s Animal and Plant Health Inspection Service. At the same time, genetic diversities in several states equaled those observed in California, Oregon, and Washington and two-thirds of multilocus genotypes exhibited limited geographic distributions, indicating that mutation was common during or subsequent to migration. Together, these data suggest that migration, rapid mutation, and genetic drift all play a role in structuring the genetic diversity of P. ramorum in US nurseries. This work demonstrates that fast-evolving genetic markers can be used to examine the evolutionary processes acting on recently introduced pathogens and to infer their putative migration patterns, thus showing promise for the application of forensics to plant pathogens.     

Coupling of Bacterial Endosymbiont and Host Mitochondrial Genomes in the Hydrothermal Vent Clam Calyptogena magnifica

The hydrothermal vent clam Calyptogena magnifica (Bivalvia: Vesicomyidae) depends for its nutrition on sulfur-oxidizing symbiotic bacteria housed in its gill tissues. This symbiont is transmitted vertically between generations via the clam's eggs; however, it remains uncertain whether occasionally symbionts are horizontally transmitted or acquired from the environment. If symbionts are transmitted strictly vertically through the egg cytoplasm, inheritance of symbiont lineages should behave as if coupled to the host's maternally inherited mitochondrial DNA. This coupling would be obscured, however, with low rates of horizontal or environmental transfers, the equivalent of recombination between host lineages. Population genetic analyses of C. magnifica clams and associated symbionts from eastern Pacific hydrothermal vents clearly supported the hypothesis of strictly maternal cotransmission. Host mitochondrial and symbiont DNA sequences were coupled in a clam population that was polymorphic for both genetic markers. These markers were not similarly coupled with sequence variation at a nuclear gene locus, as expected for a randomly mating sexual population. Phylogenetic analysis of the two cytoplasmic genes also revealed no evidence for recombination. The tight association between vesicomyid clams and their vertically transmitted bacterial endosymbionts is phylogenetically very young (<50 million years) and may serve as a model for the origin and evolution of eukaryotic organelles.     

The extent of clonality and genetic diversity in the rare Caldesia grandis (Alismataceae): Comparative results for RAPD and ISSR markers

Genetic variation and clonal diversity of three natural populations of the rare, highly clonal marsh herb Caldesia grandis Samuelsson were investigated using random amplified polymorphic DNA (RAPD) and inter-simple sequence repeat (ISSR) markers. Both of the markers worked effectively in clone identification of C. grandis. RAPD markers detected more diversity than ISSR markers in the three populations examined. Of the 60 RAPD primers screened, seven produced highly reproducible bands. Using these primers, a total of 61 DNA fragments were generated with 52 (85.25%) being polymorphic indicating considerable genetic variation at the species level. Analysis of molecular variance (AMOVA) showed that a large proportion of genetic variation (81.5%) resided within populations, while only a small proportion (18.5%) resided among populations. With the use of 52 polymorphic RAPD markers, we were able to identify 127 genets among 342 samples from three populations. The proportion of distinguishable genets (PD: mean 0.37), Simpson's diversity index (D: mean 0.91), and evenness (E: mean 0.78) exhibited high levels of clonal diversity compared to other clonal plants. These results imply that sexual reproduction has played an important role at some time during the history of these populations. Nevertheless, the high level of diversity could have been also partially generated from somatic mutations, although this is unlikely to account for the high diversity generally found among C. grandis genets.     

Fairy, tadpole, and clam shrimps (Branchiopoda) in seasonally inundated clay pans in the western Mojave Desert and effect on primary producers

Background

Fairy shrimps (Anostraca), tadpole shrimps (Notostraca), clam shrimps (Spinicaudata), algae (primarily filamentous blue-green algae [cyanobacteria]), and suspended organic particulates are dominant food web components of the seasonally inundated pans and playas of the western Mojave Desert in California. We examined the extent to which these branchiopods controlled algal abundance and species composition in clay pans between Rosamond and Rogers Dry Lakes. We surveyed branchiopods during the wet season to estimate abundances and then conducted a laboratory microcosm experiment, in which dried sediment containing cysts and the overlying algal crust were inundated and cultured. Microcosm trials were run with and without shrimps; each type of trial was run for two lengths of time: 30 and 60 days. We estimated the effect of shrimps on algae by measuring chlorophyll content and the relative abundance of algal species.

Results

We found two species of fairy shrimps (Branchinecta mackini and B. gigas), one tadpole shrimp (Lepidurus lemmoni), and a clam shrimp (Cyzicus setosa) in our wet-season field survey. We collected Branchinecta lindahli in a pilot study, but not subsequently. The dominant taxa were C. setosa and B. mackini, but abundances and species composition varied greatly among playas. The same species found in field surveys also occurred in the microcosm experiment. There were no significant differences as a function of experimental treatments for either chlorophyll content or algal species composition (Microcoleus vaginatus dominated all treatments).

Conclusions

The results suggest that there was no direct effect of shrimps on algae. Although the pans harbored an apparently high abundance of branchiopods, these animals had little role in regulating primary producers in this environment.     

Recombination between Clonal Lineages of the Asexual Fungus Verticillium dahliae Detected by Genotyping by Sequencing

Most asexual species of fungi have either lost sexuality recently, or they experience recombination by cryptic sexual reproduction. Verticillium dahliae is a plant-pathogenic, ascomycete fungus with no known sexual stage, even though related genera have well-described sexual reproduction. V. dahliae reproduces mitotically and its population structure is highly clonal. However, previously described discrepancies in phylogenetic relationships among clonal lineages may be explained more parsimoniously by recombination than mutation; therefore, we looked for evidence of recombination within and between clonal lineages. Genotyping by sequencing was performed on 141 V. dahliae isolates from diverse geographic and host origins, resulting in 26,748 single-nucleotide polymorphisms (SNPs). We found a strongly clonal population structure with the same lineages as described previously by vegetative compatibility groups (VCGs) and molecular markers. We detected 443 recombination events, evenly distributed throughout the genome. Most recombination events detected were between clonal lineages, with relatively few recombinant haplotypes detected within lineages. The only three isolates with mating type MAT1-1 had recombinant SNP haplotypes; all other isolates had mating type MAT1-2. We found homologs of eight meiosis-specific genes in the V. dahliae genome, all with conserved or partially conserved protein domains. The extent of recombination and molecular signs of sex in (mating-type and meiosis-specific genes) suggest that V. dahliae clonal lineages arose by recombination, even though the current population structure is markedly clonal. Moreover, the detection of new lineages may be evidence that sexual reproduction has occurred recently and may potentially occur under some circumstances. We speculate that the current clonal population structure, despite the sexual origin of lineages, has arisen, in part, as a consequence of agriculture and selection for adaptation to agricultural cropping systems.     

Cheaper is not always worse: strongly protective isolates of a defensive symbiont are less costly to the aphid host

Defences against parasites are typically associated with costs to the host that contribute to the maintenance of variation in resistance. This also applies to the defence provided by the facultative bacterial endosymbiont Hamiltonella defensa, which protects its aphid hosts against parasitoid wasps while imposing life-history costs. To investigate the cost–benefit relationship within protected hosts, we introduced multiple isolates of H. defensa to the same genetic backgrounds of black bean aphids, Aphis fabae, and we quantified the protection against their parasitoid Lysiphlebus fabarum as well as the costs to the host (reduced lifespan and reproduction) in the absence of parasitoids. Surprisingly, we observed the opposite of a trade-off. Strongly protective isolates of H. defensa reduced lifespan and lifetime reproduction of unparasitized aphids to a lesser extent than weakly protective isolates. This finding has important implications for the evolution of defensive symbiosis and highlights the need for a better understanding of how strain variation in protective symbionts is maintained.     

The effects of elevated temperature on the photosynthetic efficiency of zooxanthellae in hospite from four different species of reef coral: a novel approach

Bleaching of reef corals is a phenomenon linked to temperature stress which involves loss of the symbiotic algae of the coral, which are known as zooxanthellae, and/or loss of algal pigments. The photosynthetic efficiency of zooxanthellae within the corals Montastrea annularis, Agaricia lamarki, Agaricia agaricites and Siderastrea radians was examined by pulse-amplitude modulation fluorometry (PAM) during exposure to elevated temperatures (30–36°C). Zooxanthellae within M. annularis and A. lamarki were found to be more sensitive to elevated temperature, virtually complete disruption of photosynthesis being noted during exposure to temperatures of 32 and 34°C. The photosynthetic efficiency of zooxanthellae within S. radians and A. agaricites decreased to a lesser extent. Differences in the loss of algal cells on an aerial basis and in the cellular chlorophyll concentration were also found between these species. By combining the non-invasive PAM technique with whole-cell fluorescence of freshly isolated zooxanthellae, we have identified fundamental differences in the physiology of the symbionts within different species of coral. Zooxanthellae within M. annularis appear to be more susceptible to heat-induced damage at or near the reaction centre of Photosystem II, while zooxanthellae living in S. radians remain capable of dissipating excess excitation energy through non-photochemical pathways, thereby protecting the photosystem from damage during heat exposure.     

Phylogenetic analyses of Bradyrhizobium symbionts associated with invasive Crotalaria zanzibarica and its coexisting legumes in Taiwan

In this study, the genetic diversity and identification of Bradyrhizobium symbionts of Crotalaria zanzibarica, the most widely-distributed invasive legume in Taiwan, and other sympatric legume species growing along riverbanks of Taiwan were evaluated for the first time. In total, 59 and 54 Bradyrhizobium isolates were obtained from C. zanzibarica and its coexisting legume species, respectively. Based on the multilocus sequence analysis (MLSA) of concatenated four housekeeping genes (dnaK-glnII-recA-rpoB gene sequences, 1901 bp), the 113 isolates displayed 53 unique haplotypes and grouped into 21 clades. Of these clades, 11 were found to be congruent to already defined Bradyrhizobium species, while the other 10 clades were found to not be congruent to any defined species. In particular, the C. zanzibarica isolates belong to 14 MLSA clades, six of which overlapped with the isolates of coexisting legumes. According to the nodA gene sequences (555 bp) obtained from the 105 isolates, these isolates were classified into three known nodA clades, III.2, III.3 and VII and were further clustered into 10 groups. Furthermore, the C. zanzibarica isolates were clustered into 8 nodA groups, five of which overlapped with the isolates from coexisting legumes. Additionally, the nodA genes of the isolates from native species were dominated by Asian origin, while those from C. zanzibarica were dominated by American origin. In conclusion, C. zanzibarica is a promiscuous host capable of recruiting diverse Bradyrhizobium symbionts, some of which are phylogenetically similar to the symbionts of coexisting legumes in Taiwan.     

Nutrition of the giant clam Tridacna gigas (L.). II. Relative contributions of filter-feeding and the ammonium-nitrogen acquired and recycled by symbiotic alga towards total nitrogen requirements for tissue growth and metabolism

We compare the relative contributions of filter-feeding and ammonium-nitrogen translocated from algal symbionts (zooxanthellae) towards the total nitrogen requirements for tissue growth and metabolism in the giant clam Tridacna gigas. Isotope enrichments suggest that zooxanthellae effected most if not all primary assimilation from high concentrations ( 15 μM) of ¹⁵N-ammonium. There was a net daily uptake of ammonium from natural seawater, which was modulated according to nutritional history, light history, ammonium concentration, and possible biological rhythmicity. Zooxanthellae also assimilated nitrogen excreted by host tissues, including ¹⁵N absorbed from ingested alga. Nearly 100% of the ¹⁵N subsequently released from zooxanthellae was incorporated in host tissues, with no significant loss from the clam over at least 10 days. Zooxanthellae therefore conserve and recycle essentially all nitrogenous end-products within T. gigas, affording giant clams a nutritional advantage over non-symbiotic bivalves. In further contrast with heterotrophic bivalves from particle-rich environments, T. gigas show longer gut passage times and high net and gross efficiencies (78.1 ± 9.4% and 99.2 ± 0.4%, respectively) with which organic nitrogen is absorbed from ingested particles. Filter-feeding and/or other non-autotrophic processes were essential as a source of nitrogen to maintain observed rates of juvenile growth, but declined in significance with increasing clam size. Even under cloudy conditions of limited irradiance, the total contribution of nitrogen acquired as ammonium by zooxanthellae exceeded the contribution from ingested particles in T. gigas larger than about 0.1 g, increasing to at least 45% of total nitrogen requirements in clams of 10 g. Ammonium-nitrogen translocated from zooxanthellae also satisfied all basal requirements for nitrogen in particle-starved clams larger than 0.1 g. Irrespective of size, ingested particles and ammonium-nitrogen acquired by zooxanthellae together supplied about 70% of the total observed nitrogen requirements. We stress the need to determine effects of irradiance on uptake of dissolved inorganic nitrogen, and to quantify dissolved organic matter as a potential additional significant source of nitrogen, that might further explain the high rates of growth in this clam species.     

Response of sponges with autotrophic endosymbionts during the coral-bleaching episode in Puerto Rico

An updated list of sponges with algal endosymbionts including new records for Puerto Rico and the Caribbean, indicates that thirty-five species of common Caribbean sponges possess photosynthetic endosymbionts. Of these, 23 (67.6%) species in seven orders, were found with unicellular chroococcoid cyanobacteria (Aphanocapsa-like) and 5 (14.7%) hadromerid species were found with zooxanthellae. Sponges with other algae as symbionts occur less frequently (6%). Thirty-one common sponge species were inspected for bleaching during coral-bleaching months (July-September 1987; January 1988) in Puerto Rico. Anthosigmella varians, Xestospongia muta and Petrosia pellasarca bleached partially, but only few individuals within any given population became bleached and the bleaching of sponges was very localized. Adaptations between cyanobacterial symbionts and sponges, acquired during the long evolutionary history of these two taxa may explain the paucity of bleached sponges when compared to the high incidence of bleached corals reported.     

Fine-scale spatial genetic structure and clonal distribution of the cold-water coral Lophelia pertusa

Determining the spatial genetic structure within and among cold-water coral populations is crucial to understanding population dynamics, assessing the resilience of cold-water coral communities and estimating genetic effects of habitat fragmentation for conservation. The spatial distribution of genetic diversity in natural populations depends on the species’ mode of reproduction, and coral species often have a mixed strategy of sexual and asexual reproduction. We describe the clonal architecture of a cold-water coral reef and the fine-scale population genetic structure (<35 km) of five reef localities in the NE Skagerrak. This study represents the first of this type of analysis from deep waters. We used thirteen microsatellite loci to estimate gene flow and genotypic diversity and to describe the fine-scale spatial distribution of clonal individuals of Lophelia pertusa. Within-population genetic diversity was high in four of the five reef localities. These four reefs constitute a genetic cluster with asymmetric gene flow that indicates metapopulation dynamics. One locality, the Säcken reef, was genetically isolated and depauperate. Asexual reproduction was found to be a highly important mode of reproduction for L. pertusa: 35 genetic individuals were found on the largest reef, with the largest clone covering an area of nearly 300 m².