Variable food alters responses of larval crown-of-thorns starfish to ocean warming but not acidification.

Phytoplankton abundance is decreasing and becoming more variable as the ocean climate changes. We examine how low, high, and variable phytoplankton food supply affected the survival, development, and growth of larval crown-of-thorns starfish, Acanthaster sp. exposed to combined warming (26, 30 °C) and acidification (pH 8.0, 7.6). Larvae fed a low food ration are smaller, and develop slower and with more abnormalities than larvae fed a high ration. Larvae fed a variable food supply (low, followed by high ration) overcome the negative effects of low food on development rate and occurrence of abnormalities, but are 16-17% smaller than larvae fed the high ration continuously. Acidification (pH 7.6) slows growth and development and increases abnormalities regardless of the food regime. Warming slows growth and development, but these effects are mitigated by high food availability. As tropical oceans warm, the success of crown-of-thorns starfish larvae may depend on the abundance of their phytoplankton prey.

Range expansion of a widespread Indo-Pacific haemulid, the barred javelin Pomadasys kaakan (Cuvier, 1830), in a climate change hotspot.

The authors report a first sighting of a euryhaline fish in the climate change hotspot along Australia's south-eastern coast. The barred javelin, Pomadasys kaakan (Cuvier, 1830) was found in the Nambucca River in New South Wales, Australia, during 2021 and 2022. Specimens were adult, suggesting they may not be transitory vagrants. The new southernmost location recorded here represents a c. 200 km out-of-range sighting compared to previous records, and is c. 380 km south of the southernmost Australian stronghold of the species in Moreton Bay, Queensland.

Alkalinity of diverse water samples can be altered by mercury preservation and borosilicate vial storage.

We compared the effects of preservation and storage methods on total alkalinity (AT) of seawater, estuarine water, freshwater, and groundwater samples stored for 0-6 months. Water samples, untreated or treated with HgCl2, 0.45 µm filtration, or filtration plus HgCl2, were stored in polypropylene or borosilicate glass vials for 0, 1, or 6 months. Mean AT of samples treated with HgCl2 was reduced by as much as 49.1 µmol kg-1 (1.3%). Borosilicate glass elevated AT, possibly due to dissolving silicates. There was little change in AT of control and filtered samples stored in polypropylene, except for untreated groundwater (~ 4.1% reduction at 6 months). HgCl2 concentrations of 0.02-0.05% reduced the AT of fresh, estuarine, and ground water samples by as much as 35.5 µmol kg-1 after 1 month, but had little effect on the AT of seawater. Adding glucose as a carbon source for microbial growth resulted in no AT changes in 0.45 µm-filtered samples. We suggest water samples intended for AT analyses can be filtered to 0.45 µm, and stored in polypropylene vials at 4 °C for at least 6 months. Borosilicate glassware and HgCl2 can be avoided to prevent analytical uncertainties and reduce risks related to use of Hg2+.

Echidnas of the Sea: The Defensive Behavior of Juvenile and Adult Crown-of-Thorns Sea Stars.

AbstractCrown-of-thorns sea stars are one of the most ecologically important tropical marine invertebrates, with boom-bust population dynamics that influence the community structure of coral reefs. Although predation is likely to influence the development of population outbreaks, little is known about the defensive behavior of crown-of-thorns sea stars. Righting behavior after being overturned, a key defensive response in echinoderms, was investigated for the newly settled herbivorous juvenile, the corallivorous juvenile, and adult stages of crown-of-thorns sea stars. The average righting time of the newly settled juveniles (0.3-1.0-mm diameter) was 2.74 minutes. For the coral-eating juveniles (15-55-mm diameter), the righting time (mean = 6.24 min) was faster in larger juveniles, and the mean righting time of the adults was 6.28 minutes. During righting and in response to being lifted off of the substrate, the juveniles and adults exhibited an arm curling response, during which their arms closed over their oral side, often forming a spine ball, a feature not known for other asteroids. The righting and curling responses of the corallivorous juveniles were influenced by the presence of a natural enemy, a coral guard crab, which caused the juveniles to spend more time with their arms curled. These behaviors indicate that crown-of-thorns sea stars use their spines to protect the soft tissue of their oral side. The highly defended morphology and behavioral adaptations of crown-of-thorns sea stars are likely to have evolved as antipredator mechanisms. This points to the potential importance of predators in regulating their populations, which may have decreased in recent times due to fishing, a factor that may contribute to outbreaks.

Knowledge Gaps in the Biology, Ecology, and Management of the Pacific Crown-of-Thorns Sea Star Acanthaster sp. on Australia's Great Barrier Reef.

AbstractCrown-of-thorns sea stars (Acanthaster sp.) are among the most studied coral reef organisms, owing to their propensity to undergo major population irruptions, which contribute to significant coral loss and reef degradation throughout the Indo-Pacific. However, there are still important knowledge gaps pertaining to the biology, ecology, and management of Acanthaster sp. Renewed efforts to advance understanding and management of Pacific crown-of-thorns sea stars (Acanthaster sp.) on Australia's Great Barrier Reef require explicit consideration of relevant and tractable knowledge gaps. Drawing on established horizon scanning methodologies, this study identified contemporary knowledge gaps by asking active and/or established crown-of-thorns sea star researchers to pose critical research questions that they believe should be addressed to improve the understanding and management of crown-of-thorns sea stars on the Great Barrier Reef. A total of 38 participants proposed 246 independent research questions, organized into 7 themes: feeding ecology, demography, distribution and abundance, predation, settlement, management, and environmental change. Questions were further assigned to 48 specific topics nested within the 7 themes. During this process, redundant questions were removed, which reduced the total number of distinct research questions to 172. Research questions posed were mostly related to themes of demography (46 questions) and management (48 questions). The dominant topics, meanwhile, were the incidence of population irruptions (16 questions), feeding ecology of larval sea stars (15 questions), effects of elevated water temperature on crown-of-thorns sea stars (13 questions), and predation on juveniles (12 questions). While the breadth of questions suggests that there is considerable research needed to improve understanding and management of crown-of-thorns sea stars on the Great Barrier Reef, the predominance of certain themes and topics suggests a major focus for new research while also providing a roadmap to guide future research efforts.

Diet flexibility and growth of the early herbivorous juvenile crown-of-thorns sea star, implications for its boom-bust population dynamics.

The ecology of the early herbivorous juvenile stage of the crown-of-thorns sea star (COTS, Acanthaster spp.) is poorly understood, yet the success of this life stage is key to generating population outbreaks that devastate coral reefs. Crustose coralline algae (CCA) has been considered to be the main diet of herbivorous juveniles. In this study, we show that COTS can avail of a range of algal food. Juveniles were reared on CCA, Amphiroa sp., and biofilm, and survived for 10 months on all three diets. The juveniles fed CCA and Amphiroa sp. reached 15-16.5 mm diameter at ~ 6 months and maintained this size for the rest the experiment (an additional ~4 months). Juveniles fed biofilm grew more slowly and to a smaller maximum size (~3 mm diameter). However, when juveniles were switched from biofilm to CCA they resumed growth to a new asymptotic size (~13.5 mm, 13-20 months). In diet choice experiments, juveniles did not show a preference between Amphiroa sp. and CCA, but generally avoided biofilm. Our results show that juvenile COTS grew equally well on CCA and Amphiroa sp. and can subsist on biofilm for months. Some juveniles, mostly from the biofilm diet treatment, decreased in size for a time and this was followed by recovery. Flexibility in diet, growth, and prolonged maintenance of asymptotic size indicates capacity for growth plasticity in herbivorous juvenile COTS. There is potential for juvenile COTS to persist for longer than anticipated and increase in number as they wait for the opportunity to avail of coral prey. These findings complicate our ability to predict recruitment to the corallivorous stage and population outbreaks following larval settlement and the ability to understand the age structure of COTS populations.

Training fish for restocking: refuge and predator training in the hatchery has limited benefits for a marine fish.

Large numbers of hatchery-reared fish are released in stocking programmes; however, success is limited by high mortality. Predation is seen as the main cause of deaths but might be reduced by training fish before release to avoid predators and/or use refuge. In this study on a potential restocking species, yellowfin bream Acanthopagrus australis, the effects of predator training and refuge on the behaviour of fish in the hatchery were tested. In the first experiment, juvenile bream were exposed to predatory mangrove jack (Lutjanus argentimaculatus) fed exclusively on bream flesh while housed in tanks with and without refuge. Predator training altered fish behaviour when fish were re-exposed to predators, but the effects were subtle and varied between groups of fish. In contrast, refuge created strong and consistent changes in behaviour, significantly slowing down the amount of time that fish took to consume food. A second experiment focused on the effects of refuge. Bream were trained to use artificial seagrass or house bricks as refuge and then exposed to mangrove jacks in a laboratory predation experiment. When refuge was available, fish significantly slowed down their feeding rate. There was a small, transient increase in survival for fish given seagrass refuges, but this was irrespective of whether the bream were trained to use refuge. The results of this study indicate that the use of refuge may be innate and the benefits of refuge may be available to naive hatchery-reared fish or fish trained to use refuge shortly before release. This suggests that there is potential to improve post-release survival of fingerlings without time-consuming and expensive hatchery training.

The hidden army: corallivorous crown-of-thorns seastars can spend years as herbivorous juveniles.

Crown-of-thorns seastar (COTS) outbreaks are a major threat to coral reefs. Although the herbivorous juveniles and their switch to corallivory are key to seeding outbreaks, they remain a black box in our understanding of COTS. We investigated the impact of a delay in diet transition due to coral scarcity in cohorts reared on crustose coralline algae for 10 months and 6.5 years before being offered coral. Both cohorts achieved an asymptotic size (16-18 mm diameter) on algae and had similar exponential growth on coral. After 6.5 years of herbivory, COTS were competent coral predators. This trophic and growth plasticity results in a marked age-size disconnect adding unappreciated complexity to COTS boom-bust dynamics. The potential that herbivorous juveniles accumulate in the reef infrastructure to seed outbreaks when favourable conditions arise has implications for management of COTS populations.

Caridina malanda, a new species of freshwater shrimp (Crustacea: Decapoda: Atyidae) from the Wet Tropics World Heritage area, north-eastern Queensland, Australia.

Integrated molecular and morphological studies of newly collected and curated specimens of the genus Caridina from the Atherton Tablelands, Wet Tropics World Heritage Area in north-eastern Queensland, Australia indicated the presence of an undescribed species belonging to the Caridina zebra Short 1993 complex. This species is somewhat intermediate, although distinct on the basis of molecular data and morphology, from two known sympatric species, Caridina zebra and C. confusa Choy Marshall 1997, and an allopatric species, C. spinula Choy Marshall 1997, from the Cape York Peninsula, about 500 km north. It is described here as a new species, C. malanda sp. nov., and compared with similar congeners. A key for the identification of the species, as well as notes on its distribution, ecology, and conservation, are provided.

A Crown-of-Thorns Seastar recombinant relaxin-like gonad-stimulating peptide triggers oocyte maturation and ovulation.

The Acanthaster planci species-complex [Crown-of-Thorns Seastar (COTS)] are highly fecund echinoderms that exhibit population outbreaks on coral reef ecosystems worldwide, including the Australian Great Barrier Reef. A better understanding of the COTS molecular biology is critical towards efforts in controlling outbreaks and assisting reef recovery. In seastars, the heterodimeric relaxin-like gonad stimulating peptide (RGP) is responsible for triggering a neuroendocrine cascade that regulates resumption of oocyte meiosis prior to spawning. Our comparative RNA-seq analysis indicates a general increase in RGP gene expression in the female radial nerve cord during the reproductive season. Also, the sensory tentacles demonstrate a significantly higher expression level than radial nerve cord. A recombinant COTS RGP, generated in a yeast expression system, is highly effective in inducing oocyte germinal vesicle breakdown (GVBD), followed by ovulation from ovarian fragments. The findings of this study provide a foundation for more in-depth molecular analysis of the reproductive neuroendocrine physiology of the COTS and the RGP.

Building global change resilience: Concrete has the potential to ameliorate the negative effects of climate-driven ocean change on a newly-settled calcifying invertebrate.

Global climate change is driving sea level rise and increasingly frequent storm events, which are negatively impacting rapidly-growing coastal communities. To mitigate these impacts, coastal infrastructure must be further protected by upgrading hard defences. We propose that incorporating pH-buffering materials into these upgrades could safeguard marine organisms from the adverse effects of ocean acidification and ocean warming during the vulnerable transition from planktonic larvae to benthic juveniles. To test this, we examined the effects of ocean warming (24 or 27 °C), ocean acidification (pH 8.1, 7.9, 7.7), and substratum (concrete, greywacke, granite) in all combinations on the settlement success of an ecologically and commercially important sea urchin, Tripneustes gratilla. Low pH (7.9, 7.7) generally reduced the quantity and size of juveniles four weeks post-settlement, although this was partially ameliorated by increased temperature (24 vs. 27 °C). In the warmed and acidified treatments, settlement rates were lower on concrete than granite or greywacke, but two weeks post-settlement, juveniles on concrete were larger, and had longer spines and higher survival rates than on greywacke or granite, respectively. The benefits provided by concrete to newly-settled juveniles may be related to alkali chemicals leaching from concrete buffering low pH conditions in surrounding seawater and/or increased availability of bicarbonate in the boundary layers around its surface. Our results highlight the potential for pH-buffering materials to assist marine organisms in coping with the effects of changing ocean conditions, but further research is required to understand the generality and mechanism(s) driving the beneficial effects of concrete and to test pH-buffering materials in the field.

Taxonomic revision of the Australian species of Australatya Chace 1983 (Crustacea, Decapoda, Atyidae), and the description of a new species.

A morphological and genetic study of both newly collected and museum specimens of the east Australian freshwater shrimp genus Australatya indicates two genotypes and three morphotypes are present. One genotype and morphotype were of the known species, Australatya striolata (McCulloch McNeill 1923). The second genotype, comprising two morphotypes, is now described as a new species, Australatya hawkei sp. nov.. All Australian Australatya species and morphotypes currently have allopatric distributions. Australatya striolata is widespread along the south-eastern Australian seaboard, from Genoa River in Victoria to the Sunshine Coast in Queensland. The 'southern' morphotype of Australatya hawkei sp. nov. occurs in north Queensland, from near Mackay to about Cooktown, and its 'northern' morphotype is found on Cape York Peninsula, north from about Coen to the Iron Range, just north of Lockhart River. The once monotypic genus now includes two Australian species, Australatya striolata and A. hawkei sp. nov., and a third, A. obscura, recently described from Japan, Taiwan, and the Philippines.

The complex study of complexes: The first well-supported phylogeny of two species complexes within genus Caridina (Decapoda: Caridea: Atyidae) sheds light on evolution, biogeography, and habitat.

Atyid shrimps, a key component of tropical freshwater ecosystems, face multiple anthropogenic threats and thus need special attention. With more than 300 described species, the genus Caridina is the most speciose of all the Caridea infra-order. Caridina spp. occupy diverse habitats in tropical freshwaters of the Indo-West Pacific region. Several species complexes have been recognized, based on common morphological features, but little is known about how well these morphological characteristics align with phylogenetic characteristics. Furthermore, no phylogeny of the genus Caridina published so far has provided well-resolved and supported relationships among different species, thus impeding the possibility of proposing evolutionary hypotheses. In this study we used next generation sequencing (NGS) to provide new insights into the phylogenetic relationships among the genus Caridina, focusing on two complexes: 'Caridina nilotica' and 'Caridina weberi'. We collected 92 specimens belonging to these two groups from most of their known geographical range, representing 50 species, for which we sequenced seven mitochondrial genes and two nuclear markers using ion torrent NGS. We performed a phylogenetic analysis, which yielded the first well-supported tree for the genus Caridina. On this tree were mapped the geographic ranges and the habitats used by the different species, and a time calibration was tested. We found the driving factors that most likely account for separation of clades are differences in habitat and to a lesser extent geography. This work provides new insights into the taxonomy of this group and identifies opportunities for further studies in order to fill knowledge gaps that currently impede the management and conservation of atyid species.

Indirect effects of ocean acidification drive feeding and growth of juvenile crown-of-thorns starfish, Acanthaster planci.

The indirect effects of changing climate in modulating trophic interactions can be as important as the direct effects of climate stressors on consumers. The success of the herbivorous juvenile stage of the crown-of-thorns starfish (COTS), Acanthaster planci, may be affected by the impacts of ocean conditions on its crustose coralline algal (CCA) food. To partition the direct effects of near future ocean acidification on juvenile COTS and indirect effects through changes in their CCA food, COTS were grown in three pHT levels (7.9, 7.8, 7.6) and fed CCA grown at similar pH levels. Consumption of CCA by COTS was bolstered when the COTS were grown in low pH and when they were fed CCA grown in low pH regardless of the pH in which the COTS were reared. COTS fed CCA grown at pH 7.6 grew fastest, but the pH/pCO2 that the COTS were reared in had no direct effect on growth. Ocean acidification conditions decreased the C : N ratio and carbonate levels in the CCA. Bolstered growth in COTS may be driven by enhanced palatability, increased nutritive state and reduced defences of their CCA food. These results indicate that near future acidification will increase the success of early juvenile COTS and boost recruitment into the coral-eating life stage.

Moderate ocean warming mitigates, but more extreme warming exacerbates the impacts of zinc from engineered nanoparticles on a marine larva.

There is growing concern about the combined effects of multiple human-induced stressors on biodiversity. In particular, there are substantial knowledge gaps about the combined effects of existing stressors (e.g. pollution) and predicted environmental stress from climate change (e.g. ocean warming). We investigated the impacts of ocean warming and engineered nanoparticles (nano-zinc oxide, nZnO) on larvae of a cosmopolitan tropical sea urchin, Tripneustes gratilla. Larval T. gratilla were exposed to all combinations of three temperatures, 25, 27 and 29 °C (current SST and near-future predicted warming of +2 and + 4 °C) and six concentrations of nZnO (0, 0.001, 0.01, 0.1, 1 and 10 mg nZnO·L-1). These stressors had strong interactive effects on fertilization, gastrulation and normal development of 5 day old larvae. High concentrations of nZnO had a negative effect, but this impact was less pronounced for sea urchins reared at their preferred temperature of 27 °C compared to 25 or 29 °C. Larval growth was also impacted by combined stress of elevated temperature and nZnO. Subsequent measurement of the dissolution and aggregation of nZnO particles and the direct effect of Zn2+ ions on larvae, suggest the negative effects of nZnO on larval development and growth were most likely due to Zn2+ ions. Our results demonstrate that marine larvae may be more resilient to stressors at optimal temperatures and highlight the potential for ocean warming to exacerbate the effects of pollution on marine larvae.

Future aquafeeds may compromise reproductive fitness in a marine invertebrate.

Aquaculture of higher trophic level species is increasingly dependent on the use of terrestrial oil products. The input of terrestrially derived n-6 polyunsaturated fatty acids (PUFA) into marine environments has subsequently increased, with unknown consequences for recipient species. We exposed a sea urchin, Heliocidaris erythrogramma to three experimental diets for 78 days: a high n-3 PUFA marine imitation treatment, a high n-6 PUFA "future aquafeed" treatment and an intermediate "current aquafeed" treatment. Female urchins fed the high n-6 PUFA diet produced larvae with lower survival rates than all other treatments. Males fed the high n-6 PUFA diet produced no viable sperm. Fatty acid composition in reproductive material revealed comprehensive biosynthetic and dietary sparing capabilities in H. erythrogramma. Despite this, the ratio of n-6 PUFA to n-3 PUFA in reproductive tissue increased significantly with diet. We suggest alterations to this ratio is the likely mechanism of negative impact on larval development.

Biogenic acidification reduces sea urchin gonad growth and increases susceptibility of aquaculture to ocean acidification.

Decreasing oceanic pH (ocean acidification) has emphasised the influence of carbonate chemistry on growth of calcifying marine organisms. However, calcifiers can also change carbonate chemistry of surrounding seawater through respiration and calcification, a potential limitation for aquaculture. This study examined how seawater exchange rate and stocking density of the sea urchin Tripneustes gratilla that were reproductively mature affected carbonate system parameters of their culture water, which in turn influenced growth, gonad production and gonad condition. Growth, relative spine length, gonad production and consumption rates were reduced by up to 67% by increased density (9-43 individuals.m(-2)) and reduced exchange rates (3.0-0.3 exchanges.hr(-1)), but survival and food conversion efficiency were unaffected. Analysis of the influence of seawater parameters indicated that reduced pH and calcite saturation state (ΩCa) were the primary factors limiting gonad production and growth. Uptake of bicarbonate and release of respiratory CO2 by T. gratilla changed the carbonate chemistry of surrounding water. Importantly total alkalinity (AT) was reduced, likely due to calcification by the urchins. Low AT limits the capacity of culture water to buffer against acidification. Direct management to counter biogenic acidification will be required to maintain productivity and reproductive output of marine calcifiers, especially as the ocean carbonate system is altered by climate driven ocean acidification.

Larvae of the coral eating crown-of-thorns starfish, Acanthaster planci in a warmer-high CO2 ocean.

Outbreaks of crown-of-thorns starfish (COTS), Acanthaster planci, contribute to major declines of coral reef ecosystems throughout the Indo-Pacific. As the oceans warm and decrease in pH due to increased anthropogenic CO2 production, coral reefs are also susceptible to bleaching, disease and reduced calcification. The impacts of ocean acidification and warming may be exacerbated by COTS predation, but it is not known how this major predator will fare in a changing ocean. Because larval success is a key driver of population outbreaks, we investigated the sensitivities of larval A. planci to increased temperature (2-4 °C above ambient) and acidification (0.3-0.5 pH units below ambient) in flow-through cross-factorial experiments (3 temperature × 3 pH/pCO2 levels). There was no effect of increased temperature or acidification on fertilization or very early development. Larvae reared in the optimal temperature (28 °C) were the largest across all pH treatments. Development to advanced larva was negatively affected by the high temperature treatment (30 °C) and by both experimental pH levels (pH 7.6, 7.8). Thus, planktonic life stages of A. planci may be negatively impacted by near-future global change. Increased temperature and reduced pH had an additive negative effect on reducing larval size. The 30 °C treatment exceeded larval tolerance regardless of pH. As 30 °C sea surface temperatures may become the norm in low latitude tropical regions, poleward migration of A. planci may be expected as they follow optimal isotherms. In the absence of acclimation or adaptation, declines in low latitude populations may occur. Poleward migration will be facilitated by strong western boundary currents, with possible negative flow-on effects on high latitude coral reefs. The contrasting responses of the larvae of A. planci and those of its coral prey to ocean acidification and warming are considered in context with potential future change in tropical reef ecosystems.

Ingestion of microplastic has limited impact on a marine larva.

There is increasing concern about the impacts of microplastics (<1 mm) on marine biota. Microplastics may be mistaken for food items and ingested by a wide variety of organisms. While the effects of ingesting microplastic have been explored for some adult organisms, there is poor understanding of the effects of microplastic ingestion on marine larvae. Here, we investigated the ingestion of polyethylene microspheres by larvae of the sea urchin, Tripneustes gratilla. Ingestion rates scaled with the concentration of microspheres. Ingestion rates were, however, reduced by biological fouling of microplastic and in the presence of phytoplankton food. T. gratilla larvae were able to egest microspheres from their stomach within hours of ingestion. A microsphere concentration far exceeding those recorded in the marine environment had a small nondose dependent effect on larval growth, but there was no significant effect on survival. In contrast, environmentally realistic concentrations appeared to have little effect. Overall, these results suggest that current levels of microplastic pollution in the oceans only pose a limited threat to T. gratilla and other marine invertebrate larvae, but further research is required on a broad range of species, trophic levels, and polymer types.

Do cues matter? Highly inductive settlement cues don't ensure high post-settlement survival in sea urchin aquaculture.

Increasing settlement and post-settlement survival during the critical transition from planktonic larvae to benthic juveniles will increase efficiency for sea urchin aquaculture. This study investigated the effects of temperature and settlement cues on the settlement and post-settlement survival of the sea urchin Tripneustes gratilla during this phase. The current commercial methodology, which utilises natural biofilm settlement plates, was tested and resulted in low settlement (<2%) and poor post-settlement survival (<1% of settled urchins). In laboratory trials, settlement was high and unaffected by temperatures between 24 and 30°C, but significantly decreased at 33°C. Development of spines, however, was significantly affected by temperatures over 29°C. Mirroring this result, post-settlement survival was optimal between 24-28°C. In laboratory assays, the macroalgae Sargassum linearifolium and Corallina officinalis, and seawater conditioned with these algae, induced significantly higher settlement (>90%) than a natural biofilm (∼25%). The addition of macroalgae-conditioned seawater to natural biofilm significantly increased settlement rates (>85%). Mixed consortia and single strains of bacteria isolated from macroalgae, biofilms and adult conspecifics all induced significant settlement, but at significantly lower rates than macroalgae. No evidence was found that higher rates of settlement to bacteria on macroalgae were generated by a cofactor from the macroalgae. Age of bacterial cultures, culturing bacteria on solid and liquid media and concentration of nutrients in cultures had little effect on settlement rates. Finally, macroalgae-conditioned seawater combined with natural biofilm settlement plates induced significantly higher settlement than to the biofilm plates alone in a commercial scale trial. However, high post-settlement mortality resulted in equivalent survival between treatments after 25 days. This study highlights that settlement studies should extend to post-settlement survival, which remains poor for T. gratilla and is a significant obstacle to increasing efficiency for aquaculture.