Removing environmental sources of variation to gain insight on symbionts vs. transient microbes in high and low microbial abundance sponges.

In this study, we pursue unravelling the bacterial communities of 26 sponges, belonging to several taxonomical orders, and comprising low microbial abundance (LMA) and high microbial abundance (HMA) representatives. Particularly, we searched for species-specific bacteria, which could be considered as symbionts. To reduce temporal and spatial environmentally caused differences between host species, we sampled all the sponge species present in an isolated small rocky area in a single dive. The bacterial communities identified by pyrosequencing the 16S rRNA gene showed that all HMA species clustered separated from LMA sponges and seawater. HMA sponges often had highest diversity, but some LMA sponges had also very diverse bacterial communities. Network analyses indicated that no core bacterial community seemed to exist for the studied sponges, not even for such a space and time-restricted sampling. Most sequences, particularly the most abundant ones in each species, were species-specific for both HMA and LMA sponges. The bacterial sequences retrieved from LMA sponges, despite being phylogenetically more similar to seawater, did not represent transient seawater bacteria. We conclude that sponge bacterial communities depend more on the host affiliation to the HMA or LMA groups than on host phylogeny.

"Living together apart": the hidden genetic diversity of sponge populations.

Intraorganism genetic stability is assumed in most organisms. However, here we show for the first time intraorganism genetic heterogeneity in natural populations of marine sponges. A total of 36 different multilocus genotypes (MLGs) were detected in 13 individuals of Scopalina lophyropoda sampled at 4 distant points within each sponge. All genotypes (showing a mosaic distribution), were transmitted to the progeny, thus contributing to the high genetic diversity and low clonality reported for this species, although its populations are small and structured and show high fission rates. There did not seem to be intraindividual genotype conflicts; on the contrary, chimeric individuals are expected to show low mortality thanks to the differential mortality of their different MLGs. This novel mechanism may also counterbalance genetic constraints in other benthic invertebrate species. The presence of sponge chimerism also suggests that results from previous population genetics studies could have been misinterpreted.

Population genetics at three spatial scales of a rare sponge living in fragmented habitats.

BACKGROUND: Rare species have seldom been studied in marine habitats, mainly because it is difficult to formally assess the status of rare species, especially in patchy benthic organisms, for which samplings are often assumed to be incomplete and, thus, inappropriate for establishing the real abundance of the species. However, many marine benthic invertebrates can be considered rare, due to the fragmentation and rarity of suitable habitats. Consequently, studies on the genetic connectivity of rare species in fragmented habitats are basic for assessing their risk of extinction, especially in the context of increased habitat fragmentation by human activities. Sponges are suitable models for studying the intra- and inter-population genetic variation of rare invertebrates, as they produce lecitotrophic larvae and are often found in fragmented habitats. RESULTS: We investigated the genetic structure of a Mediterranean sponge, Scopalina lophyropoda (Schmidt), using the allelic size variation of seven specific microsatellite loci. The species can be classified as "rare" because of its strict habitat requirements, the low number of individuals per population, and the relatively small size of its distribution range. It also presents a strong patchy distribution, philopatric larval dispersal, and both sexual and asexual reproduction. Classical genetic-variance-based methods (AMOVA) and differentiation statistics revealed that the genetic diversity of S. lophyropoda was structured at the three spatial scales studied: within populations, between populations of a geographic region, and between isolated geographic regions, although some stochastic gene flow might occur among populations within a region. The genetic structure followed an isolation-by-distance pattern according to the Mantel test. However, despite philopatric larval dispersal and fission events in the species, no single population showed inbreeding, and the contribution of clonality to the population makeup was minor (only ca. 4%). CONCLUSIONS: The structure of the S. lophyropoda populations at all spatial scales examined confirms the philopatric larval dispersal that has been reported. Asexual reproduction does not seem to play a relevant role in the populations. The heterozygote excess and the lack of inbreeding could be interpreted as a hitherto unknown outcrossing strategy of the species. The envisaged causes for this strategy are sperm dispersal, a strong selection against the mating of genetically related individuals to avoid inbreeding depression or high longevity of genets combined with stochastic recruitment events by larvae from other populations. It should be investigated whether this strategy could also explain the genetic diversity of many other patchy marine invertebrates whose populations remain healthy over time, despite their apparent rarity.

Contrasting biological features in morphologically cryptic Mediterranean sponges.

Sponges are key organisms in the marine benthos where they play essential roles in ecological processes such as creating new niches, competition for resources, and organic matter recycling. Despite the increasing number of taxonomical studies, many sponge species remain hidden, whether unnoticed or cryptic. The occurrence of cryptic species may confound ecological studies by underestimating biodiversity. In this study, we monitored photographically growth, fusions, fissions, and survival of two morphologically cryptic species Hemimycale mediterranea Uriz, Garate & Agell, 2017 and H. columella (Bowerbank, 1874). Additionally, we characterized the main environmental factors of the corresponding species habitats, trying to ascertain whether some abiotic factors were correlated with the distribution of these species. Sponge monitoring was performed monthly. Seawater samples were collected the same monitoring days in the vicinity of the target sponges. Results showed contrasting growth and survival patterns for each species: H. mediterranea totally disappeared after larval release while 64% of individuals of H. columella survived the entire two years we monitored. The species also differed in the number of fissions and fusions. These events were evenly distributed throughout the year in the H. mediterranea population but concentrated in cold months in H. columella. No measured environmental factor correlated with H. mediterranea growth rates, while temperature and dissolved organic nitrogen were negatively correlated with H. columella growth rates. The strong differences in depth distribution, survival, growth, fusions, and fissions found between these two cryptic species, highlights the importance of untangling cryptic species before ecological studies are performed in particular when these species share geographical distribution.

Snapshot of a Bacterial Microbiome Shift during the Early Symptoms of a Massive Sponge Die-Off in the Western Mediterranean.

Ocean warming is affecting marine benthic ecosystems through mass mortality events that involve marine invertebrates, in particular bivalves, corals, and sponges. Among these events, extensive die-offs of Ircinia fasciculata sponges have been recurrently reported in western Mediterranean. The goal of our study was to test whether the temperature-related mass sponge die-offs were associated with or preceded by an early unbalanced bacterial microbiome in the sponge tissues. We took advantage of the early detection of disease and compared the microbiomes of healthy vs. early diseased I. fasciculata tissues. Our results showed a microbiome shift in early diseased tissues. The abundance of Gammaproteobacteria and Acidobacteria increased and that of Deltaproteobacteria decreased in diseased vs. healthy tissues. The change in community composition was also noticeable at the operational taxonomic unit (OTU) level. Diseased tissues contained more bacterial sequences previously identified in injured or stressed sponges and corals than healthy tissues. Bacterial diversity increased significantly in diseased tissues, which contained a higher number of low abundance OTUs. Our results do not support the hypothesis of one particular pathogen, whether a Vibrio or any other bacteria, triggering the Northwestern Mediterranean mass mortalities of I. fasciculata. Our data rather suggest an early disruption of the bacterial microbiome balance in healthy sponges through a shift in OTU abundances, and the purported consequent decline of the sponge fitness and resistance to infections. Opportunistic bacteria could colonize the sponge tissues, taking benefit of the sponge weakness, before one or more virulent pathogens might proliferate ending in the mass sponge die-off.

Endosymbiotic calcifying bacteria: a new cue to the origin of calcification in metazoa?

Sponges show the highest diversity of associated bacteria among marine invertebrates. Immunological evidence traces the origin of the sponge bacterial symbioses to the Precambrian era. Hence, sponges appear to be ideally suited for studying the evolutionary origins of prokaryote-metazoan associations. Sponges produce either calcareous or siliceous skeletons, which only coexist in a relict group of demosponges, the sclerosponges. We report here, for the first time, intensive calcification in nonsclerosponge siliceous demosponges. Calcification is mediated by endosymbiotic bacteria (calcibacteria) located in archeocyte-like sponge cells. These calcibacteria are devoid of bacterial walls and divide within sponge cells until they became surrounded by a calcitic sheet, being subsequently extruded to the sponge subectosomal (subepithelial) zone. Thousands of bacteria-produced calcitic spherules cover the surface of the host sponges, forming a cortex-like structure that mimics a rudimentary peripheral skeleton. Calcibacteria are vertically transferred to the sponge larvae during embryogenesis. Calcium detoxification may have generated this symbiotic association, with some additional benefits for the sponges, such as skeletal formation and deterrence from predation. This unique symbiosis holds implications for sponge biology and may advance discussions on the role of bacteria in early biocalcification processes in metazoans.