Invertebrate herbivores: Overlooked allies in the recovery of degraded coral reefs?

Evaluating fish foraging behaviour on non-indigenous Asparagopsis taxiformis using a remote video foraging system

The proliferation of pest and invasive marine macroalgae threatens coastal ecosystems, with biotic interactions, including direct effects such as grazing and indirect effects such as the trophic cascades, where one species indirectly affects another through its interactions with a third species, play a critical role in determining the resistance of local communities to these invasions. This study examines the foraging behaviour and preference of native fish communities toward native (Halopteris scoparia, Sargassum vulgare) and non-indigenous (Asparagopsis taxiformis) macroalgae using the Remote Video Foraging System (RVFS). Fifty-four weedpops were deployed across three locations to present these macroalgae, while associated epifaunal assemblages were also collected. Video analysis revealed that four common fish species displayed preference towards native macroalgae, possibly due to by the presence of zoobenthos rather than herbivory. This observation suggests that these fish species identified the macroalgae as a habitat that harboured their preferred food items. In contrast, A. taxiformis was consistently avoided, suggesting limited integration into the local food web. Site-specific variations in fish-macroalgae interactions and epifaunal diversity highlighted the complexity of these dynamics. This study contributes to understanding of the ecological implications of invasive macroalgae and supports the use of RVFS as a tool for assessing local biotic resistance against non-indigenous species in coastal ecosystems globally.

Cryopreservation of the collector urchin embryo, Tripneustes gratilla

The collector urchin, Tripneustes gratilla, is an ecologically important member of the grazing community of Hawai'i's coral reefs. Beyond its ability to maintain balance between native seaweeds and corals, T. gratilla has also been used as a food source and a biocontrol agent against alien invasive algae species. Due to overexploitation, habitat degradation, and other stressors, their populations face local extirpation. However, artificial reproductive techniques, such as cryopreservation, could provide more consistent seedstock throughout the year to supplement aquaculture efforts. Although the sperm and larvae of temperate urchins have been successfully cryopreserved, tropical urchins living on coral reefs have not. Here, we investigated the urchin embryos' tolerance to various cryoprotectants and cooling rates to develop a cryopreservation protocol for T. gratilla. We found that using 1 M Me2SO with a cooling rate of 9.7 °C/min on gastrula stage embryos produced the best results with survival rates of up to 85.5% and up to 50.8% maturation to the 4-arm echinopluteus stage, assessed three days after thawing. Continued research could see cryopreservation added to the repertoire of artificial reproductive techniques for T. gratilla, thereby assisting in the preservation of this ecologically important urchin, all while augmenting aquaculture efforts that contribute to coral reef restoration.

Long-term ecological changes in fishes and macro-invertebrates in the world's warmest coral reefs

The Arabian Gulf is a natural laboratory for examining the consequences of large-scale disturbances due to global warming on coral reef ecosystems because of its extreme temperature regime. Using a coral reef monitoring time series extending from 1985 to 2015, we examined the long-term ecological changes in fish and macro-invertebrate communities as these habitats suffered heat shocks. We used a GLMM modelling framework to obtain clean annual signals in community indicators from noisy data. We also visualized temporal change in the taxonomic composition of fishes and macro-invertebrates. A phase shift from predominantly reef-building corals to barren grounds occurred between 1996 and 2000. Macro-invertebrates responded rapidly, and most of associated indicators recovered to pre-shift levels in 15 years. Fishes generally had lagged responses to the phase shift and had shifted to a new state with lower abundance, as well as different species composition. Increased levels of herbivory first by macro-invertebrates, mostly sea urchins, and then fishes, could have suppressed macro-algae expansion and consequently led to the dominance of barren ground. When the phase shift occurred, most of the 14 fish families declined in abundance while macro-invertebrate groups increased. Fish families able to utilize non-coral habitats appeared more resilient to the disturbances and subsequent coral degradation. Unlike other regions, we observed high resilience of the coral-dependent butterflyfishes to coral loss, possibly due to local migration from other less-impacted coral reefs. We hypothesized a top-down control mechanism mediated by predation by fishes has contributed to shaping the temporal and spatial patterns of the macro-invertebrates. Our results also revealed differences in spatial preferences among fishes and macro-invertebrate groups, which could be used to set priorities and develop effective conservation and management strategies.

Herbivorous Crabs Reverse the Seaweed Dilemma on Coral Reefs

Coral reefs are on a steep trajectory of decline, with natural recovery in many areas unlikely.^1-3 Eutrophication, overfishing, climate change, and disease have fueled the supremacy of seaweeds on reefs,^4,5 particularly in the Caribbean, where many reefs have undergone an ecological phase shift so that seaweeds now dominate previously coral-rich reefs.^6-8 Discovery of the powerful grazing capability of the Caribbean's largest herbivorous crab (Maguimithrax spinosissimus)⁹ led us to test the effectiveness of their grazing on seaweed removal and coral reef recovery in two experiments conducted sequentially at separate locations 15 km apart in the Florida Keys (USA). In those experiments, we transplanted crabs onto several patch reefs, leaving others as controls (n = 24 reefs total; each 10-20 m² in area) and then monitored benthic cover, coral recruitment, and fish community structure on each patch reef for a year. We also compared the effectiveness of crab herbivory to scrubbing reefs by hand to remove algae. Crabs reduced the cover of seaweeds by 50%-80%, resulting in a commensurate 3-5-fold increase in coral recruitment and reef fish community abundance and diversity. Although laborious hand scrubbing of reefs also reduced algal cover, that effect was transitory unless maintained by the addition of herbivorous crabs. With the persistence of Caribbean coral reefs in the balance, our findings demonstrate that large-scale restoration that includes enhancement of invertebrate herbivores can reverse the ecological phase shift on coral reefs away from seaweed dominance.

The timing and causality of ecological shifts on Caribbean reefs

Caribbean reefs have experienced unprecedented changes in the past four decades. Of great concern is the perceived widespread shift from coral to macroalgal dominance and the question of whether it represents a new, stable equilibrium for coral-reef communities. The primary causes of the shift-grazing pressure (top-down), nutrient loading (bottom-up) or direct coral mortality (side-in)-still remain somewhat controversial in the coral-reef literature. We have attempted to tease out the relative importance of each of these causes. Four insights emerge from our analysis of an early regional dataset of information on the benthic composition of Caribbean reefs spanning the years 1977-2001. First, although three-quarters of reef sites have experienced coral declines concomitant with macroalgal increases, fewer than 10% of the more than 200 sites studied were dominated by macroalgae in 2001, by even the most conservative definition of dominance. Using relative dominance as the threshold, a total of 49 coral-to-macroalgae shifts were detected. This total represents ~35% of all sites that were dominated by coral at the start of their monitoring periods. Four shifts (8.2%) occurred because of coral loss with no change in macroalgal cover, 15 (30.6%) occurred because of macroalgal gain without coral loss, and 30 (61.2%) occurred owing to concomitant coral decline and macroalgal increase. Second, the timing of shifts at the regional scale is most consistent with the side-in model of reef degradation, which invokes coral mortality as a precursor to macroalgal takeover, because more shifts occurred after regional coral-mortality events than expected by chance. Third, instantaneous observations taken at the start and end of the time-series for individual sites showed these reefs existed along a continuum of coral and macroalgal cover. The continuous, broadly negative relationship between coral and macroalgal cover suggests that in some cases coral-to-macroalgae phase shifts may be reversed by removing sources of perturbation or restoring critical components such as the herbivorous sea urchin Diadema antillarum to the system. The five instances in which macroalgal dominance was reversed corroborate the conclusion that macroalgal dominance is not a stable, alternative community state as has been commonly assumed. Fourth, the fact that the loss in regional coral cover and concomitant changes to the benthic community are related to punctuated, discrete events with known causes (i.e. coral disease and bleaching), lends credence to the hypothesis that coral reefs of the Caribbean have been under assault from climate-change-related maladies since the 1970s.

Contribution of green turtles Chelonia mydas to total herbivore biomass in shallow tropical reefs of oceanic islands

Green turtles are megaherbivores with a key role in the dynamics of tropical seagrass meadows, but little is known about their relevance as herbivores in tropical reef habitats. We conducted underwater censuses of green turtles, herbivorous fishes and sea urchins in two distinct tropical regions: Fernando de Noronha (Western Atlantic Ocean) and the Hawaiian Archipelago (Central Pacific Ocean), to assess the contribution of green turtles to the total herbivore biomass in shallow reef habitats of tropical oceanic islands. Juvenile green turtles ranging 40–60 cm were observed at most of the surveyed sites, and hence, could be considered typical components of the shallow reef fauna of tropical oceanic islands. Furthermore, they were usually one of the most abundant species of roving herbivores in many of the sites surveyed. However, the biomass of green turtles was usually much lower than the aggregated biomass of fishes or sea urchins, which usually constituted most of the total herbivore biomass. Green turtles made a major contribution to the total herbivore biomass only in sheltered sites with low rugosity, low coral cover and high algal cover. Further investigation on the trophic redundancy between herbivores is required to assess the actual relevance of green turtles in reef ecosystems of oceanic islands, compared to herbivorous fishes and sea urchins, because different herbivores may target different algal resources and complementarity may be needed to maintain ecosystem functioning across large, naturally varied reefscapes.