Few studies have investigated the effects of ocean warming and acidification on marine benthic organisms over ecologically relevant time scales. We used an environmentally controlled coral reef mesocosm system to assess growth and physiological responses of the sea urchin species Echinometra sp. A over 2 generations. Each mesocosm was controlled for temperature and pCO2 over 29 mo under 3 climate change scenarios (present day and predicted states in 2050 and 2100 under RCP 8.5). The system maintained treatment conditions including annual temperature cycles and a daily variation in pCO2. Over 20 mo, adult Echinometra exhibited no significant difference in size and weight among the treatments. Growth rates and respiration rates did not differ significantly among treatments. Urchins from the 2100 treatment had elevated ammonium excretion rates and reduced O2:N ratios, suggesting a change in catabolism. We detected no difference in spawning index scores or oocyte size after 20 mo in the treatments, suggesting that gonad development was not impaired by variations in pCO2 and temperature reflecting anticipated climate change scenarios. Larvae produced from experimentally exposed adults were successfully settled from all treatments and raised for 5 mo inside the mesocosm. The final size of these juveniles exhibited no significant difference among treatments. Overall, we demonstrated that the mesocosm system provided a near natural environment for this urchin species. Climate change and ocean acidification did not affect the benthic life stages investigated here. Importantly, in previous short-term (weeks to months) experiments, this species exhibited reductions in growth and gonad development, highlighting the potential for short-term experiments with non-acclimated animals to yield contrasting, possibly erroneous results.
The corallivorous Crown-of-Thorns Starfish (CoTS, Acanthaster spp.) has been linked with the widespread loss of scleractinian coral cover on Indo-Pacific reefs during periodic population outbreaks. Here, we re-examine CoTS consumption by coral reef fish species by using new DNA technologies to detect Pacific Crown-of-Thorns Starfish (Acanthaster cf. solaris) in fish faecal and gut content samples. CoTS DNA was detected in samples from 18 different coral reef fish species collected on reefs at various stages of CoTS outbreaks in the Great Barrier Reef Marine Park, nine of which had not been previously reported to feed on CoTS. A comprehensive set of negative and positive control samples confirmed that our collection, processing and analysis procedures were robust, although food web transfer of CoTS DNA cannot be ruled out for some fish species. Our results, combined with the (i) presence of CoTS spines in some samples, (ii) reported predation on CoTS gametes, larvae and settled individuals, and (iii) known diet information for fish species examined, strongly indicate that direct fish predation on CoTS may well be more common than is currently appreciated. We provide recommendations for specific management approaches to enhance predation on CoTS by coral reef fishes, and to support the mitigation of CoTS outbreaks and reverse declines in hard coral cover.
Outbreaks of crown-of-thorns seastars (CoTS; Acanthaster spp.) are a major contributor to degradation of Indo-Pacific coral reefs. Understanding the dispersal and fate of planktonic life stages is crucial to understand and manage outbreaks, but visual detection of CoTS larvae is challenging. We apply a quantitative PCR (qPCR) assay to enumerate CoTS larvae in a 3-year time series of plankton samples from two reefs (Agincourt and Moore Reefs) on the Great Barrier Reef. Plankton surveys were complemented with settlement assays, and benthic surveys of juvenile and adult densities over time. Only one out of 109 plankton samples from Agincourt Reef had detectable CoTS mtDNA compared to 41 out of 575 samples from Moore Reef. This may be explained by differences in adult densities, or differences in connectivity and larval retention. Detections of larval CoTS were restricted to summer (November–February), with first detections each year coinciding with water temperatures reaching 28 °C and peak detections late December. A disproportionate number of larval detections occurred in 7 days around full moon. Complementary sampling of settlement and post-settlement life stages confirmed that elevated densities of CoTS larvae at Moore Reef translated to high rates of settlement adding to infestations at this reef. Moreover, there were declines in the detection of larvae, as well densities of juvenile and adult CoTS at Moore Reef, in 2017 and 2018. This study demonstrates that qPCR for genetic identification and quantification of larvae can assist to elucidate life history parameters of nuisance species difficult to obtain with other tools.
Coral reefs are under pressure from a variety of human-induced disturbances, but demonstration of ecosystem changes and identification of stressors are often difficult. We tested whether global change or increased agricultural runoff after European settlement of Northeast Australia (ca. 1860) has affected inshore reefs of the Great Barrier Reef. Eleven sediment cores were retrieved from inner reefs, intermediate reefs, and outer-island reefs, and benthic foraminiferal assemblages were analyzed in dated (14C, 210Pb, 137Cs) core sections (N = 82 samples). Data were grouped into six age bands (< 55, 55-150, 150-500, 500-1000, 1000-1500, and > 1500 yr). Principal component analysis and two-factor (Zone and Age) permutational analysis of variance (PERMANOVA) suggested that assemblages from the three zones were significantly different from each other over several millennia, with symbiont-bearing (mixotrophic) species dominating the outer reefs. A significant interaction term indicated that within-zone patterns varied. Assemblages in outer reefs unaffected from increased land runoff were persistent until present times. In both other zones, assemblages were also persistent until 150 yr ago, suggesting that benthic foraminiferal assemblages are naturally highly persistent over long (> 2000 yr) timescales. Assemblages in core sections < 55 yr old from inner reefs were significantly (post hoc t test) different from those older than 150 yr. Similarly, assemblages < 55 yr old from intermediate reefs were significantly different compared to older assemblages. A multivariate regression tree (environmental variables: Zone and Age) explained 56.8% of the variance in foraminiferal assemblages and confirmed patterns identified by PERMANOVA. With some exceptions, changes on the inner and intermediate reefs were consistent with a model predicting that increased nutrients and higher turbidity enhance relative abundance of heterotrophic species. Given that assemblages did not change in outer-island reefs (not impacted by runoff) we argue that changes in assemblages due to global change can be rejected as an explanation. Thus, the findings are more consistent with the hypothesis that agricultural runoff since European settlement altered foraminiferal assemblages than with the hypothesis that global forcing caused changes.
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