The research, conducted over multiple spawning events in 2023 and 2024, is the first to field test the effects of underwater sound on fish and coral larvae simultaneously, alongside other emerging restoration technologies including living surface materials and 3D-printed settlement surfaces. The work formed part of Rapid Resilient Reefs for Coastal Defense, or R3D, a consortium project led in collaboration with the University of Hawai’i and focused on nature-based strategies to reduce wave energy, protect coastlines and improve coral resilience. The studies and the broader R3D effort were funded by the Defense Advanced Research Projects Agency, DARPA. The newest study, published October 8 in Communications Biology, examines how acoustic enrichment affects coral larval settlement, the critical stage when free-drifting larvae search for a place to attach and begin building a reef.

“We have demonstrated in a single set of field studies that acoustic enrichment works in increasing the presence of both fish larvae and coral larvae on artificial reef structures,” said Aaron Thode, a Scripps Oceanography researcher who heads the Scripps Environmental Acoustics Lab and served as lead author of the coral larvae study. According to Thode, the findings suggest that drifting organisms use reef sound as an informational cue about habitat quality. “Our data indicate that when these drifting organisms detect a lot of sound from a reef, it signals to them that this is a good place to settle,” he said. The analysis also produced strong evidence that synthetic chemical cues can attract these organisms to reefs, pointing to a potential multi-sensory approach to restoration.

The experimental design began with sound collection. The team deployed a recorder at a healthy reef environment off O’ahu, Hawaii, capturing the acoustic signature of the reef across a full lunar cycle. The recordings contained the sounds of fish as well as numerous other organisms, including shrimp and crustaceans that produce distinctive snapping noises. These recordings were then broadcast from an underwater speaker at the study site, a flat, sandy area off the small island of Moku o Lo’e. For two weeks, the speaker played the reef soundscape continuously from sunset to sunrise, mimicking the natural rhythm of reef activity during the hours when many larvae are on the move.

Around the speaker, the researchers placed 37 artificial structures on the seafloor at a depth of 4.5 meters, roughly 15 feet, at distances ranging from 1 to 42 meters, about 3 to 138 feet, from the sound source. The structures varied in design and surface properties and included engineered microhabitats developed in the Coral Reef Ecophysiology and Engineering Lab of Scripps researcher Daniel Wangpraseurt. These microhabitats were enhanced in two distinct ways. Their physical architecture created complex, protected settlement spaces with crevices where larvae could attach, and their surfaces were coated with a living material called BRINK, a bioactive “reef ink” containing living bacteria, developed by former Scripps postdoctoral researcher Natalie Levy and colleagues in Wangpraseurt’s lab.

Across three experiments conducted over two years, scientific divers measured coral larval settlement one and two weeks after the new moon, when spawning occurs. Using a handheld blue light and yellow filter, a technique that makes settled coral larvae fluoresce, they counted the individual larvae that had attached to each structure. The results showed a clear spatial pattern: structures closest to the speaker had the highest levels of coral settlement, and among those, the structures treated with BRINK performed best. “The acoustics help, and with the living biofilm, it’s a lot better,” said Thode. “When combined with structures that had crevices, acoustic enrichment worked very well for the coral larvae settlement.” The study also noted that specialized 3D-printed structures developed by colleagues at the Hawai’i Institute of Marine Biology performed well, though those were tested with sound alone and not in combination with BRINK. For future experiments, the team recommends pairing acoustic enrichment with the more complex structures coated with the living biofilm to maximize settlement success.

“We now have evidence that several of these technologies can work in the field, which is a major step forward,” said Wangpraseurt, who is UC San Diego’s lead principal investigator for the R3D project and a co-author of the study. “It brings us closer to the vision of hybrid reefs, a new class of living coastal infrastructure that combines engineering and biology to protect our shores while supporting the growth and recovery of reef ecosystems.” The hybrid reef concept represents a shift away from purely hard engineering solutions such as concrete seawalls, toward structures that actively support biological communities while still performing coastal defense functions.

The companion study, published in Scientific Reports and led by Scripps PhD candidate Océane Boulais, addressed the fish side of the equation. Fish are important components of healthy coral reefs because certain species feed on smothering microalgae that would otherwise make it difficult for coral larvae to settle and grow. Monitoring young fish, however, is notoriously difficult, since their behavior is easily disturbed by human presence. Thode likened the problem of counting fish while scuba diving to “Godzilla trampling through a city and trying to get an accurate count of all the humans scurrying away.” To solve it, Boulais developed an array of low-power autonomous cameras capable of continuously detecting and counting fish larvae for up to three weeks without any diver in the water.

Boulais positioned the cameras near the entrances of complex, 3D-printed structures that functioned as “fish hotels,” documenting fish as they entered and left while also tracking the presence of larvae. “By developing these non-invasive cameras, we can essentially spy on the fish and observe a lot of their natural behavior,” she said. “The autonomy and long-term monitoring design of these cameras enable us to observe which animals show up, and how long they stay.” The cameras were deployed at two sites: one near the underwater speaker providing acoustic enrichment, and one at a control site where an identical speaker emitted no sound. Larval counts at both sites peaked around the new moon, but the acoustically enriched site attracted 4 to 14 times more fish larvae overall, a promising indication that the added sound helped draw young fish to the structures. The results held even when the team swapped the locations of the active speaker and the control speaker, ruling out site-specific effects. “The cameras are relatively new, but they’re already helping us learn so much about the early life stages of reef fish, and how sound might enhance their presence on a reef,” Boulais said.

The findings arrive at a moment when coral reefs worldwide face mounting pressure from warming waters, pollution and coastal development, and restoration scientists are increasingly looking to combine biological, chemical and engineering approaches. The next major test of the technologies validated in Kāne’ohe Bay is already scheduled. DARPA plans to install a $22 million hybrid reef structure off O’ahu this fall, with corals likely to be outplanted in late 2026 or early 2027. The Kalaeloa Hybrid Reef will span 50 meters, about 164 feet, and will incorporate technologies tested through the R3D program, including the Scripps-developed microstructures and an acoustic enrichment system. After installation, DARPA will transition ownership of the prototype living breakwater to the Hawai’i Department of Transportation, a transition partner for the R3D program. More than 50 researchers and R3D consortium members contributed to the studies as co-authors, and samples of the reef sounds used in the experiments are featured at Birch Aquarium’s Living Seas Tropical Pacific exhibit, offering the public a chance to hear the soundscapes that may soon help rebuild reefs along Hawaii’s shores.

Subject of Research: Acoustic enrichment to enhance coral and fish larval settlement for coral reef restoration

Article Title: Healthy reef sounds can boost coral and fish recovery efforts

Article References: Healthy reef sounds can boost coral and fish recovery efforts. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: coral reefs, acoustic enrichment, larval settlement, reef restoration, Kāne'ohe Bay, Scripps Oceanography, 3D-printed structures, living materials, fish larvae, hybrid reefs, DARPA, marine acoustics

The study, published in Discover Oceans, represents the first experimental investigation of bryozoans as biofoulers in the Gulf of Mannar and Palk Bay region. Between June 2020 and June 2021, the team deployed eight static immersion panels, two at each of four locations: Pamban Island, Vedalai, Manouli Island and Ervadi. Each panel carried twelve-inch square plates of four different materials, positioned equidistant from one another to prevent interference. One panel at each site was retrieved after six months, and the experiment was repeated with fresh plates in a second six-month window, while a parallel long-term panel remained submerged for the entire year. This design allowed the researchers to track both seasonal patterns of settlement and the slower dynamics of ecological succession.

The results were strikingly selective. Ten species of encrusting bryozoans were identified over the course of the study, but two dominated almost everywhere: Parasmittina projecta and Celleporaria aperta. These pioneer species appeared on ceramic and terracotta plates at every stage of the experiment, regardless of location or season. Rarer species, including Parasmittina tubula, Parasmittina egyptiaca, Hippopodina feegeensis and Pleurocodonellina signata, showed up only as small colonies or patches. Electra pilosa, a species known for its preference for rough surfaces, colonised the coarse reverse side of a terracotta plate at Pamban Island, while the long-term panels revealed two latecomers, Parasmittina triangularis and Rhynchozoon compactum, that never appeared on the shorter deployments.

Just as telling was what the bryozoans refused to colonise. Metal plates, left unpainted and exposed to seawater, corroded heavily over the study period, rusting and in some cases crumbling apart. Not a single bryozoan colony established itself on metal during the first six-month study, and the researchers attribute this to corrosion-induced surface instability and the release of iron oxides, both known to interfere with larval settlement in sessile invertebrates. Wood fared little better: it degraded rapidly, became infested with serpulid worms and bivalves, and hosted bryozoans only in the crevices of rotten timber during the long-term deployment. Ceramic and terracotta, by contrast, offered the stable, wettable surfaces that bryozoan larvae clearly favour, with post-hoc statistical comparisons confirming significantly greater cover on these two substrates than on wood or metal.

Texture and orientation mattered as much as material. The researchers analysed each retrieved plate using a quadrat method, dividing the surface into a four-by-four grid, and complemented colony counts with photographic point-count analysis in ImageJ to estimate total percentage cover. Across the board, the reverse sides of the plates, which faced away from the prevailing swell and sat in shadow, carried more bryozoan cover than the front-facing surfaces, a difference confirmed by a Mann-Whitney U test. Many encrusting bryozoans are sciaphilic, meaning their larvae preferentially settle in shaded, sheltered microhabitats where algal competition and physical disturbance are reduced. The forty-five-degree inclination of the panels also created a gradient of water movement, with the upper portions experiencing greater flow that delivered larvae and food particles without burying the feeding apparatus, or lophophore, in sediment.

Seasonality left its own fingerprint. The first six-month study, spanning the monsoon months when rainfall was highest and temperatures remained within the optimal range for reef-building organisms, yielded seven bryozoan species. The second window, covering the drier, hotter months when water temperatures in 2020 climbed to between 32 and 35.1 degrees Celsius and salinity rose through evaporation and upwelling, produced only four. The researchers suggest that the favourable calcium concentrations and moderate conditions of the monsoon period supported the calcification that bryozoans, like corals, depend upon, while the hotter, saltier conditions of the second period favoured corrosion over colonisation. Notably, P. delicatula appeared only in the second window, consistent with its known salt tolerance.

Perhaps the most vivid finding came from the year-long panels, which transformed into miniature ecosystems. By the time they were retrieved, they harboured hydroids, serpulid worms, barnacles, thickets of seaweed and even juvenile crabs and other crustaceans sheltering among the growth. Scanning electron micrographs captured Celleporaria aperta overgrowing less competitive species, illustrating the competitive displacement that unfolds as succession progresses. The bryozoans, by secreting an adhesive polysaccharide that rapidly mineralises into a solid crust, had created the foundational substrate on which the entire assemblage depended. In ecological terms, they acted as pioneer species initiating facilitative succession, paving the way for a climax community of the kind long described in port fouling studies but never before documented experimentally in this biosphere reserve.

The study’s authors are careful about its limits. Each site hosted a single short-term and single long-term panel, so the deployments served as parallel observations rather than true statistical replicates, and the analyses were deliberately descriptive and comparative. Several plates and panels were lost to monsoonal currents, corrosion and occasional human disturbance, and environmental parameters such as temperature, salinity and chlorophyll-a were not measured in situ. These constraints, common to open-water colonisation experiments in protected reserves, mean the findings should be read as robust baseline evidence rather than definitive inference. Even so, the consistency of the substrate-driven pattern, reinforced by principal component analysis that cleanly separated high-colonisation ceramic and terracotta samples from wood and metal, gives the conclusions considerable weight.

Why does any of this matter beyond the taxonomy of small colonial animals? Bryozoans have long been cast as villains in the biofouling story, blamed for dragging down ship hulls, clogging cooling pipes and ferrying invasive species across oceans. This study reframes them as ecological engineers whose crusts stabilise reef structures, facilitate the settlement of macro-foulers and sustain biodiversity. Artificial substrates colonised in this way can support fisheries, diving tourism and ecotourism without touching endangered natural reefs, and dense fouling communities may even help engineered structures absorb wave energy during storms and tsunamis. The discovery that Paralicornia obtecta, previously recorded from India’s southwest coast, Sri Lanka and the wider Indian Ocean, now appears in the Gulf of Mannar, adds a biogeographical data point to a region whose fouling fauna had been studied almost exclusively in harbours.

The broader lesson is one of hidden complexity in plain sight. A square metre of submerged ceramic, left alone for a year, becomes a stage on which larvae discriminate between textures, pioneers build skeletons, competitors wage slow-motion overgrowth battles and juvenile crustaceans find refuge. Understanding these dynamics matters for anyone designing artificial reefs, managing port infrastructure or predicting how tropical reef communities will respond as seas warm and salinities shift. The Gulf of Mannar’s bryozoans, once dismissed as ecologically insignificant, have now been shown to be among the quiet architects of their underwater world, and the researchers hope their baseline data will guide future studies that pair replicated deployments with continuous environmental sensing to unravel the mechanisms behind the patterns they have revealed.

Subject of Research: Biofouling bryozoan settlement and succession on artificial substrates in the Gulf of Mannar

Article Title: Experimental investigation on biofouling marine bryozoans in reefs

Article References: Sanjay, M. S., Venkatraman, C., Sen, A., & Yogesh Kumar, J. S. (2026). Experimental investigation on biofouling marine bryozoans in reefs. Discover Oceans, 3(1), Article 27. https://doi.org/10.1007/s44289-026-00138-0

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00138-0

Keywords: bryozoa, marine biofouling, Gulf of Mannar, Parasmittina projecta, reef formation, larval settlement, substrate preference, ecological succession, artificial reefs, marine biodiversity, coral reefs, India

Crown-of-thorns outbreaks are not a new phenomenon. Four major outbreaks have swept the Great Barrier Reef since the 1960s, and rising densities detected near Lizard Island suggest a fifth is now underway. What drives these population explosions has long divided researchers, with three broad hypotheses on the table: bottom-up boosts in larval food supply, top-down release from predation on juveniles and adults, and natural population fluctuations. None of these mechanisms is mutually exclusive, but the new study, led by Frances Patel of the Australian Institute of Marine Science together with colleagues at the University of Otago, homes in on the first of them with unusual precision—asking not just whether food matters, but when and for how long.

The researchers conducted five experiments in total, using separate larval cohorts spawned from broodstock collected from mid-shelf reefs near Townsville between 2022 and 2025. All rearing was performed at the National Sea Simulator in Townsville in an automated flow-through feeding system that held food concentrations at tightly controlled levels, a marked improvement over daily manual feeding setups used in most previous crown-of-thorns larval studies. Larvae were fed mixtures of the algae Dunaliella sp. and Tisochrysis lutea at either satiating high food levels—0.6 to 1.7 micrograms of chlorophyll a per litre, corresponding to 2,205 to 3,405 algal cells per millilitre—or limiting low food levels of 0.12 to 0.44 micrograms chlorophyll a per litre. Crucially, these concentrations mirror real conditions: the high food levels match those measured on mid-shelf reefs after nutrient pulses from upwelling or river runoff, while the low levels resemble typical surface waters on the Great Barrier Reef outside flood events.

The first set of experiments tested a long-standing but unverified idea: that crown-of-thorns larvae might harbor photosynthetic microbial symbionts capable of supplementing their nutrition through light-driven metabolite transfer. Such associations have been documented in coral larvae and in larvae of the tropical sea star Mithrodia clavigera, and a microbial study had suggested that crown-of-thorns larvae contain bacteria potentially capable of photosynthesis. If true, light could act as a nutritional lifeline for larvae drifting through the food-poor waters that characterize much of the reef. The result was emphatically negative. Larvae raised under a 12-hour light and 12-hour dark cycle performed no better—or worse—than larvae raised in complete darkness, at both food levels. Food concentration alone drove development: 57 to 62 percent of high-food larvae had reached the brachiolaria stage by day seven, compared with just 5 to 9 percent under low food, and by day fifteen roughly 80 percent of high-food larvae had progressed to mid or late brachiolaria versus about 1 percent of low-food larvae. Settlement success under high food reached 46 to 50 percent, with no influence of light, while no larvae settled at all under low food conditions.

The second set of experiments manipulated the timing of food availability. Larvae were switched between high and low food regimes five days after the onset of feeding, at seven days post-fertilisation. Larvae given high food early and then switched to low food still developed far better than larvae given low food early and then switched to high food. By day eleven, 88 percent of larvae kept on constant high food had reached mid to late brachiolaria stages, compared with 67 percent of the high-to-low group, 26 percent of the low-to-high group, and only 1 percent of larvae on constant low food. This asymmetry reveals a striking developmental plasticity with a clear message: food encountered early in larval life matters far more than food delivered later. Larvae that started well partially recovered when conditions improved, but they never caught up with siblings that had a strong start.

The third experiment delivered the study’s most consequential result by quantifying exactly how many days of high food are needed. Larvae exposed to one to three days of high food showed modest benefits, with 17 to 37 percent reaching advanced stages. But at four days of high food—a threshold response—more than 70 percent of larvae reached mid to late brachiolaria, essentially matching larvae raised on high food for the entire nine-day experiment. Settlement, however, required a slightly longer window: fewer than 1 percent of larvae exposed to only one or two days of high food settled, around 6 percent settled after three to four days, and 34 to 69 percent settled after five to nine days of high food exposure. In other words, a bloom lasting roughly four to five days at the start of the larval phase is sufficient to push most larvae onto a fast developmental track and to lift settlement rates to levels comparable with permanent abundance.

The physiological logic behind this sensitivity lies in the energetics of early development. Crown-of-thorns larvae at about two days old develop a functional gut and begin feeding, but maternal resources in the egg are depleted by the late bipinnaria stage. Marine invertebrate larvae carry lipids as their dominant energy reserve, yet accumulating carbohydrates during the bipinnaria stages may fuel the protein-rich brachiolaria phase and metamorphosis that follow. Previous work on related sea stars has shown that well-fed larvae establish higher carbohydrate concentrations during early development, and that later starvation does not necessarily compromise settlement if early nutrition was adequate. The authors suggest two possible outcomes from the pulse-feeding experiments: larvae that received high food in their first five days accumulated sufficient energy reserves to fuel complete development regardless of later nutrition, while larvae that started on low food accrued inadequate reserves that later abundance could not repay.

These results mesh tightly with what is known about phytoplankton dynamics on the Great Barrier Reef during the seastar’s spawning season. Intense rainfall events push nutrients into coastal waters through river runoff, while intrusions of nutrient-rich upwelled water fertilize deeper layers where larvae have recently been found at depths of up to 30 metres. Phytoplankton can double in abundance one to two times per day, allowing blooms to develop rapidly and persist for days to weeks. One recorded bloom lasting five days reached the reef corridor between Cairns and Lizard Island—the so-called outbreak initiation zone. That duration sits squarely within the four-to-five-day window the new experiments identify as sufficient to dramatically boost larval development, suggesting that even a single episodic bloom coinciding with spawning could amplify the number of larvae surviving to settlement.

The study also carries implications for how reef managers think about water quality. Nutrient enrichment from catchment runoff has long been suspected of feeding crown-of-thorns outbreaks, but the mechanistic link has been difficult to demonstrate because larvae are notoriously hard to sample in the wild. By showing that the critical nutritional window is narrow, early, and quantitatively defined, the research makes the bottom-up hypothesis testable against environmental monitoring data: blooms of sufficient concentration and duration overlapping the spawning season become a concrete, observable trigger to look for. Shorter larval development times may also increase larval retention near source reefs, a process previously proposed to contribute to primary outbreak initiation.

Important caveats remain. Laboratory settlement rates cannot be translated directly into recruitment success, because newly settled juveniles face heavy predation and other mortality in the field, and nutritional carryover effects into the juvenile stage may not be visible at settlement. The light experiments were also conducted under aquarium conditions that may lack the diverse natural microbial communities needed to establish photosynthetic symbionts, so the authors caution that the symbiont question warrants further study with larvae from natural environments. Still, the core conclusion stands: light does not rescue food-limited larvae, but a brief, well-timed pulse of phytoplankton abundance can transform larval fortunes. In the seasonal rhythm of the Great Barrier Reef, where floods and upwelling punctuate an otherwise nutrient-poor sea, those transient windows of plenty may be precisely what turns an ordinary spawning season into the beginning of an outbreak.

Subject of Research: Effects of light and short-term phytoplankton food pulses on larval development and settlement of the coral-eating crown-of-thorns seastar (Acanthaster cf. solaris)

Article Title: Does light and short-term high food availability enhance larval success in the coral eating crown-of-thorns seastar (Acanthaster cf. solaris)?

Article References: Patel, F., McDowell, E., Bastin, L., Gomez Cabrera, M., Lamare, M., & Uthicke, S. (2026). Does light and short-term high food availability enhance larval success in the coral eating crown-of-thorns seastar (Acanthaster cf. solaris)?. Coral Reefs. https://doi.org/10.1007/s00338-026-02962-4

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02962-4

Keywords: crown-of-thorns seastar, Acanthaster cf. solaris, Great Barrier Reef, phytoplankton blooms, larval development, larval settlement, nutrient enrichment, upwelling, river runoff, coral reef decline, population outbreaks, developmental plasticity