Self-recognition in corals facilitates deep-sea habitat engineering

The ability of coral reefs to engineer complex three-dimensional habitats is central to their success and the rich biodiversity they support. In tropical reefs, encrusting coralline algae bind together substrates and dead coral framework to make continuous reef structures, but beyond the photic zone, the cold-water coral Lophelia pertusa also forms large biogenic reefs, facilitated by skeletal fusion. Skeletal fusion in tropical corals can occur in closely related or juvenile individuals as a result of non-aggressive skeletal overgrowth or allogeneic tissue fusion, but contact reactions in many species result in mortality if there is no ‘self-recognition’ on a broad species level. This study reveals areas of ‘flawless’ skeletal fusion in Lophelia pertusa, potentially facilitated by allogeneic tissue fusion, are identified as having small aragonitic crystals or low levels of crystal organisation, and strong molecular bonding. Regardless of the mechanism, the recognition of ‘self’ between adjacent L. pertusa colonies leads to no observable mortality, facilitates ecosystem engineering and reduces aggression-related energetic expenditure in an environment where energy conservation is crucial. The potential for self-recognition at a species level, and subsequent skeletal fusion in framework-forming cold-water corals is an important first step in understanding their significance as ecological engineers in deep-seas worldwide

An Indo-Pacific coral spawning database.

The discovery of multi-species synchronous spawning of scleractinian corals on the Great Barrier Reef in the 1980s stimulated an extraordinary effort to document spawning times in other parts of the globe. Unfortunately, most of these data remain unpublished which limits our understanding of regional and global reproductive patterns. The Coral Spawning Database (CSD) collates much of these disparate data into a single place. The CSD includes 6178 observations (3085 of which were unpublished) of the time or day of spawning for over 300 scleractinian species in 61 genera from 101 sites in the Indo-Pacific. The goal of the CSD is to provide open access to coral spawning data to accelerate our understanding of coral reproductive biology and to provide a baseline against which to evaluate any future changes in reproductive phenology

Allorecognition maturation in the broadcast-spawning coral Acropora millepora

Many sessile marine invertebrates discriminate self from non-self with great precision, but maturation of allorecognition generally takes months to develop in juveniles. Here, we compare the development of allorecognition in full-sibling, half-sibling and non-sibling contact reactions between newly settled juveniles of the broadcast-spawning coral Acropora millepora on the Great Barrier Reef (Australia). Absence of a rejection response showed that A. millepora lacks a mature allorecognition system in the first 2 months post-settlement. From thereon, incompatibilities were observed between juveniles, their level of relatedness (i.e. full-, half- and non-sibling status) governing the rate of allorecognition maturation. All contact reactions between non-siblings resulted in rejections by 3 months post-settlement, whereas the expression of allorecognition took at least 5 months between half-siblings and longer than 13 months for some full-siblings. Approximately 74 % of fused full-siblings (n = 19) persisted as chimeras at 11 months, thus maturation of allorecognition in this spawning coral appeared to be slower (>13 months) than in brooding corals (∼4 months). We hypothesize that late maturation of allorecognition may contribute to flexibility in Symbiodinium uptake in corals with horizontal transmission, and could allow fusions and chimera formation in early ontogeny, which potentially enable rapid size increase through fusion

High potential for formation and persistence of chimeras following aggregated larval settlement in the broadcast spawning coral, Acropora millepora.

International audienceIn sessile modular marine invertebrates, chimeras can originate from fusions of closely settling larvae or of colonies that come into contact through growth or movement. While it has been shown that juveniles of brooding corals fuse under experimental conditions, chimera formation in broadcast spawning corals, the most abundant group of reef corals, has not been examined. This study explores the capacity of the broadcast spawning coral Acropora millepora to form chimeras under experimental conditions and to persist as chimeras in the field. Under experimental conditions, 1.5-fold more larvae settled in aggregations than solitarily, and analyses of nine microsatellite loci revealed that 50 per cent of juveniles tested harboured different genotypes within the same colony. Significantly, some chimeric colonies persisted for 23 months post-settlement, when the study ended. Genotypes within persisting chimeric colonies all showed a high level of relatedness, whereas rejecting colonies displayed variable levels of relatedness. The nearly threefold greater sizes of chimeras compared with solitary juveniles, from settlement through to at least three months, suggest that chimerism is likely to be an important strategy for maximizing survival of vulnerable early life-history stages of corals, although longer-term studies are required to more fully explore the potential benefits of chimerism

Crustose coralline algae that promote coral larval settlement harbor distinct surface bacterial communities

© 2020, Springer-Verlag GmbH Germany, part of Springer Nature. Most benthic invertebrates, including ecosystem engineers such as corals, sponges and bivalves, have a motile planktonic larval phase and rely on specific chemical cues to identify a suitable substrate to settle. Crustose coralline algae (CCA) can induce settlement and metamorphosis responses in many invertebrates including corals. We tested the respective coral settlement capacity of multiple CCA species in a choice experiment and investigated the composition of their microbiomes. Our findings revealed that coral larval settlement was drastically influenced by CCA genera and also suggest that bacterial communities on the CCA surface can potentially serve as a driver of coral larval settlement. The composition of the bacterial communities on the surface of the least attractive CCA genus, Neogoniolithon fosliei, was markedly different from the other genera, Porolithon gardineri and Titanoderma prototypum and was significantly enriched in Vibrio and Flammeovirgaceae. The activity of CCA-associated bacterial communities may contribute to some of the variability observed in settlement responses between CCA species. Specific bacterial ASVs assigned to the Neptuniibacter, Methylotrophic Group 3 and Cellvibrionaceae were positively correlated with coral settlement. Conversely, ASVs assigned as Vibrio and Flammeovirga were negatively correlated with coral settlement. This study identifies putative bacterial taxa involved in coral settlement, which is an essential step to understand the chemical cues involved in this process and to predict the ability of corals to recolonize damaged reefs following disturbances

Physiological diversity among sympatric, conspecific endosymbionts of coral (Cladocopium C1^acro) from the Great Barrier Reef

Most of the scleractinian corals living in the photic zone form an obligate symbiosis with dinoflagellates in the family Symbiodiniaceae that promotes reef accretion and niche diversification. However, sea surface temperature surpassing the normal summer average disrupts the symbioses, resulting in coral bleaching and mortality. Under climate warming, temperature anomalies and associated coral bleaching events will increase in frequency and severity. Therefore, it is imperative to better understand the variability in key phenotypic traits of the coral-Symbiodiniaceae association under such high temperature stress. Here, we describe the extent of genetically fixed differences in the in vitro acclimatory response of four conspecific strains of the common coral endosymbiont, Cladocopium C1acro. (formerly Symbiodinium type C1); these strains were isolated from Acropora corals from inshore sites on the Great Barrier Reef. We characterised algal growth and thylakoid membrane stability under different thermal scenarios and demonstrate previously undocumented physiological diversity among strains of a single Symbiodiniaceae species. Our results have important implications in terms of the perceived accuracy by which environmental stress tolerance of the coral holobiont can be predicted, potentially explaining patchiness in a coral community during bleaching based on the dominant Symbiodiniaceae genotype harboured by the host

Expression of putative immune response genes during early ontogeny in the coral Acropora millepora

Background: Corals, like many other marine invertebrates, lack a mature allorecognition system in early life history stages. Indeed, in early ontogeny, when corals acquire and establish associations with various surface microbiota and dinoflagellate endosymbionts, they do not efficiently distinguish between closely and distantly related individuals from the same population. However, very little is known about the molecular components that underpin allorecognition and immunity responses or how they change through early ontogeny in corals.\ud \ud Methodology/Principal Findings: Patterns in the expression of four putative immune response genes (apextrin, complement C3, and two CELIII type lectin genes) were examined in juvenile colonies of Acropora millepora throughout a six-month post-settlement period using quantitative real-time PCR (qPCR). Expression of a CELIII type lectin gene peaked in the fourth month for most of the coral juveniles sampled and was significantly higher at this time than at any other sampling time during the six months following settlement. The timing of this increase in expression levels of putative immune response genes may be linked to allorecognition maturation which occurs around this time in A.millepora. Alternatively, the increase may represent a response to immune challenges, such as would be involved in the recognition of symbionts (such as Symbiodinium spp. or bacteria) during winnowing processes as symbioses are fine-tuned.\ud \ud Conclusions/Significance: Our data, although preliminary, are consistent with the hypothesis that lectins may play an important role in the maturation of allorecognition responses in corals. The co-expression of lectins with apextrin during development of coral juveniles also raises the possibility that these proteins, which are components of innate immunity in other invertebrates, may influence the innate immune systems of corals through a common pathway or system. However, further studies investigating the expression of these genes in alloimmune-challenged corals are needed to further clarify emerging evidence of a complex innate immunity system in corals