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Abstract The global degradation of coral reefs is steadily increasing with ongoing climate change. Yet coral larvae settlement, a key mechanism of coral population rejuvenation and recovery, is largely understudied. Here, we show how the lipophilic, settlement-inducing bacterial pigment cycloprodigiosin (CYPRO) is actively harvested and subsequently enriched along the ectoderm of larvae of the scleractinian coral Leptastrea purpura. A light-dependent reaction transforms the CYPRO molecules through photolytic decomposition and provides a constant supply of hydrogen peroxide (H2O2), leading to attachment on the substrate and metamorphosis into a coral recruit. Micromolar concentrations of H2O2 in seawater also resulted in rapid metamorphosis, but without prior larval attachment. We propose that the morphogen CYPRO is responsible for initiating attachment while simultaneously acting as a molecular generator for the comprehensive metamorphosis of pelagic larvae. Ultimately, our approach opens a novel mechanistic dimension to the study of chemical signaling in coral settlement and provides unprecedented insights into the role of infochemicals in cross-kingdom interactions.

Sexually produced juvenile scleractinian corals play a key role in the adaptation process of coral reefs, as they are considered to possess an innate plasticity and thus can adjust to changing environmental parameters within a certain range. In this study we investigated in detail the early life stages of the brooding species Leptastrea purpurea to identify, categorize and visualize the critical steps of the complex transformation process from a swimming coral larva to a sessile coral recruit and later to a coral colony. For that, we performed settlement experiments using previously known cues: cycloprodigiosin (CYPRO) and crustose coralline algae (CCA) as well as novel cues: crude extracts of Pseudoalteromonas espejiana and P. piscicida to identify a general, cue-independent settlement pathway. We monitored the development of L. purpurea over 12 months using bright field and fluorescence microscopy. Also we identified the fluorescence signals of L. purpurea with confocal microscopy at four crucial development steps: (A) swimming larva, (B) metamorphosing larva, (C) coral recruit and (D) adult coral. Our methodological approach allowed us to observe an ontogenetic shift of fluorescence signals which provokes the hypothesis that certain fluorescence patterns might be connected to distinct sequential functions in the early life cycle of scleractinian corals. Our observations showed great similarities to the early development of other brooding and spawning corals, making L. purpurea a prospective candidate to be used as a model organism for coral research. Furthermore, our in-depth picture series provides a robust monitoring reference for coral nurseries or field applications and demonstrates the potential of fluorescence as an indicator to instantly determine the growth stage of a developing coral recruit.

Coral reefs are globally declining due to various anthropogenic stressors. Amongst those, chemical pollutants, such as pesticides from agricultural runoff, sewage or an overabundance of personal care products in coastal waters due to intense tourism, may be considered as a local stressor for reef-building corals. The extent to which such chemicals exhibit toxic effects towards corals at environmentally relevant concentrations is currently controversially discussed and existing studies are often based on varying and sometimes deficient test methods. To address this uncertainty, we adapted available methods into a reliable and comprehensive acute coral toxicity test method for the reef-building coral Montipora digitata. The toxicities of the four substances benzophenone-3 (BP-3), Diuron (DCMU), copper (Cu2+ as CuCl2, positive control) and dimethylformamide (DMF, solvent) were assessed in a 96 h semi-static test design. Endpoints such as maximum quantum yield, bleaching, tissue loss and mortality were evaluated with respect to their suitability for regulatory purposes. Overall, the endpoints bleaching and mortality yielded sensitive and robust results for the four tested substances. As the test method follows the principles of internationally standardized testing methods (ISO, OECD), it can be considered suitable for further validation and standardization. Once validated, a standardized test method will help to obtain reproducible toxicity results useful for marine hazard and risk assessment and regulatory decision making.

Organic solvents are often used in aquatic toxicity tests to facilitate the testing of hydrophobic or poorly water-soluble substances such as ultraviolet (UV) filters, pesticides, or polycyclic aromatic hydrocarbons (PAHs). Knowledge of intrinsic effects (i.e., measured as standardized and non-standardized endpoints) of such carrier solvents in non-standardized organisms (i.e., corals), is critical to regulatory processes. Therefore, we exposed the reef-building coral Montipora digitata to the most commonly used carrier solvents ethanol, methanol, dimethyl sulfoxide, and dimethylformamide in the range of 10–100 µL L−1 for 16 days. The effects on mortality, photobiological, morphological, and oxidative stress markers were evaluated. In our study, all solvents resulted in significant morphological and/or oxidative stress responses, but not in mortality. Moreover, ethanol led to a rapid increase in turbidity, thus questioning its suitability as a carrier solvent in aquatic studies in general. Based on our observations, we could rank the solvent effects as follows: dimethylformamide < dimethyl sulfoxide ≈ methanol ≤ ethanol, with dimethylformamide showing the least and ethanol the most pronounced effects. We conclude that the use of solvents in toxicity studies with corals, particularly by examining non-standardized (e.g., morphological, physiological) endpoints, should be taken with caution and requires further elaboration.

The survival of coral reefs largely depends among other factors on the recruitment of a new generation of coral individuals that are more adapted to a rapidly changing climate and other anthropogenic stressors (e.g., pollution, sedimentation). Therefore, a better understanding of the coral settlement process, the molecules involved as well as crucial environmental drivers that control settlement success are needed. In this study, we identified a novel settlement inducer for the brooding scleractinian coral Leptastrea purpurea and highlight the importance of light for the settlement process. Crude extract of the red-pigmented bacterium Pseudoalteromonas rubra reliably triggered attachment and metamorphosis in L. purpurea larvae in less than 24 h. Prodigiosin (II) and the two derivatives, cycloprodigiosin (I) and 2-methyl-3-hexyl prodiginine (III) were isolated and structurally elucidated from the crude extract of P. rubra. We demonstrated that the photosensitive pigment cycloprodigiosin (I) was the responsible compound for attachment and metamorphosis in L. purpurea larvae. Under the tested light regimes (i.e., darkness, constant light and a dark-light alternation), cycloprodigiosin (I) triggered approximately 90% settlement at a concentration of 0.2 μg mL–1 under a 12 h alternating dark-light cycle, mimicking the light-flooded coral reef environment. Our findings enable for the first time a mechanistic understanding of the light-dependent larva to polyp transformation by discovering the novel bacterial settlement cue cycloprodigiosin and its photosensitivity as a determining factor for coral settlement.

Coral reefs are highly endangered ecosystems. The identification and quantification of potential stress factors are essential to protect them. UV filters from sunscreens that are introduced to coral reef areas are considered as one of these stressors and their impact on corals needs to be further investigated. Even though UV filters are functionally similar, their structural features are very diverse. Their impact on limnic organisms have also been shown to be highly variable ranging from no or low to high toxicity. It is therefore to be expected that their effect on corals also differs significantly and that each compound has to be evaluated individually. The demand for conclusive benchmarks and guidelines from policy makers and the public over the past years shows the necessity for an objective literature review on the effects of various UV filters on scleractinian corals. Here, we review the present literature, summarize the data on the different UV filters and discuss the different approaches, advantages and limitations of the studies. However, the methods used in the latter studies vary greatly. They differ in many aspects such as species and life stage used, field and laboratory approaches, with exposure times ranging from hours to weeks. Some studies include analytics and measure the actual test concentration, others only provide nominal concentrations. The lack of standardized methods renders comparisons between studies futile. Additionally, most UV filters have only been investigated in a single or a few studies of different quality. Reliable thresholds are therefore impossible to draw on the basis of currently available studies. Nevertheless, certain UV filters repeatedly showed comparable toxicity in both freshwater and marine species tested. Yet, existing differences in results from coral tests emphasize the need for a standardized testing method comparable to those established for other aquatic organisms in order to allow for a more conclusive assessment. In this review, we describe what a scientifically sound testing proposal should include in order to obtain reliable and reproducible data, which ultimately should result in an internationally organized standardized ring test trial. Such standardized toxicity tests would enable validation of coral toxicity data related to UV filters, but also testing of other types of compounds that are known to be introduced and effect coral reefs, thus helping to identify significant stressors and enabling objective policy decisions.

Coral reefs have been declining globally at a historically unprecedented rate. Ultraviolet (UV) filters used in sunscreens may contribute to this decline at local scales, which has already led to bans on various organic UV filters in some regions. However, the underlying studies for these bans demonstrated significant flaws in the experimental design due to a lack of validated and standardized testing methods for corals. This study aimed to investigate options for the development of a standard acute toxicity test for the larval stage of scleractinian corals. Planula larvae of two brooding (Leptastrea purpurea and Tubastraea faulkneri) and two spawning (Acropora digitifera and A. millepora) species were exposed to the organic UV filter benzophenone-3 (BP3) for 48 h under static conditions. We observed interspecific variations in toxicity, with A. digitifera being the most sensitive (LC50 = 0.75 µg L−1) and T. faulkneri the least sensitive (LC50 = 2951.24 µg L−1) species. Inhibition of settlement was found to be a useful endpoint leading to an EC50 of 1.84 µg L−1 in L. purpurea larvae. Although the analytical challenges of measuring lipophilic substances in small volume test setups remain, the here applied test design and selected endpoints are suitable for further validation and subsequent standardization.

Phyllodesmium longicirrum is the largest aeolidoidean species known to date, and extremely rich in terpenoid chemistry. Herein we report the isolation of a total of 19 secondary metabolites from a single specimen of this species, i.e., steroids 1–4, cembranoid diterpenes 5–13, complex biscembranoids 14 and 15, and the chatancin-type diterpenes 16–19. These compounds resemble those from soft corals of the genus Sarcophyton, of which to date, however, only S. trocheliophorum is described as a food source for P. longicirrum. Fish feeding deterrent activity was determined using the tropical puffer fish Canthigaster solandri, and showed activity for (2S)-isosarcophytoxide (10), cembranoid bisepoxide 12 and 4-oxochatancin (16). Determining the metabolome of P. longicirrum and its bioactivity, makes it evident that this seemingly vulnerable soft bodied animal is well protected from fish by its chemical arsenal.

Sexually produced larvae are used in various fields of coral research. Because the vast majority of scleractinians reproduces only on one or few occasions per year through simultaneous release of gametes, and because an ex situ spawning induction is still very hard to achieve, great efforts are required to obtain planula larvae. Brooding corals have been used to harvest planulae although their larvae often differ in various traits from most spawning corals, e.g., in settlement behavior. Other cnidarians, such as Aiptasia spp., have been substituting for scleractinians in many aspects of coral research. However, organisms such as Aiptasia differ strongly from scleractinians, thus limiting the transferability of obtained results. This study examines the potential of Leptastrea purpurea as a reliable source of larvae for coral research. Larval output as well as settlement behavior of planulae was investigated. Our results show that colonies of L. purpurea released a daily average of 3.7 (±0.2) larvae during a period of 65 days, thus allowing continual access to planula larvae. We collected a total of 58127 larvae from our broodstock of 243 colonies. Larval settlement is induced by the same and/or similar cues as in many spawning species which increases the transferability of conclusions. We discuss the utility of L. purpurea for research on scleractinian physiology, ecology and larval settlement and conclude that L. purpurea is a well-suited organism to accelerate progress in many fields of coral research.

Sponges and other sessile invertebrates are lacking behavioural escape or defense mechanisms and rely therefore on morphological or chemical defenses. Studies from terrestrial systems and marine algae demonstrated facultative defenses like induction and activation to be common, suggesting that sessile marine organisms also evolved mechanisms to increase the efficiency of their chemical defense. However, inducible defenses in sponges have not been investigated so far and studies on activated defenses are rare. We investigated whether tropical sponge species induce defenses in response to artificial predation and whether wounding triggers defense activation. Additionally, we tested if these mechanisms are also used to boost antimicrobial activity to avoid bacterial infection. Laboratory experiments with eight pacific sponge species showed that 87% of the tested species were chemically defended. Two species, Stylissa massa and Melophlus sarasinorum, induced defenses in response to simulated predation, which is the first demonstration of induced antipredatory defenses in marine sponges. One species, M. sarasinorum, also showed activated defense in response to wounding. Interestingly, 50% of the tested sponge species demonstrated induced antimicrobial defense. Simulated predation increased the antimicrobial defenses in Aplysinella sp., Cacospongia sp., M. sarasinorum, and S. massa. Our results suggest that wounding selects for induced antimicrobial defenses to protect sponges from pathogens that could otherwise invade the sponge tissue via feeding scars.