A functional analysis of grazing in parrotfishes (family Scaridae): the ecological implications

Seasonal coastal residency and large-scale migration of two grey mullet species in temperate European waters

Grey mullets (family Mugilidae) are widespread across coastal, brackish, and freshwater habitats, and have supported fisheries for millennia. Despite their global distribution and commercial value, little is known about their movement ecology and its role in the co-existence of sympatric mullet species. Gaps in knowledge about migratory behaviour, seasonal occurrence, and movement scales have also impeded effective management, highlighting the need for further research. This study aimed to identify key habitats and timing of grey mullet presence across the Dutch Wadden Sea, North Sea, and freshwater areas, and to explore potential behavioral differences between two grey mullet species: thicklip mullet (Chelon labrosus) and thinlip mullet (Chelon ramada). Using acoustic telemetry, we tracked 86 tagged grey mullet over three years (thicklip mullet, N = 74; thinlip mullet, N = 12), combining data from 100 local acoustic receivers and the European Tracking Network. Both species were detected in the Wadden Sea from April to November, however, thinlip mullet arrived in the Wadden Sea earlier than thicklip mullet (median date = May 16 vs. June 7). Individual residency in the Wadden Sea lasted a median 97 days for thicklip mullet and 94 days for thinlip mullet. Thinlip mullet were also detected by more receivers and over a larger area than thicklip mullet, indicating differences in movement behaviour. Both species showed an affinity for receivers near major harbours, with thinlip mullet more often detected near fresh water outflows. Seasonal migrations between coastal and offshore waters were also observed, with one thinlip mullet returning to freshwater across consecutive years. North Sea detections spanned ten months, with a gap during the presumed spawning period (Jan-Feb). Our data suggest that thinlip mullet show a preference for deeper gullies while thicklip mullet may spend more time in shallow areas and flooded tidal flats. These findings highlight the importance of the Wadden Sea as a seasonal foraging ground and provide insights into the migratory patterns of grey mullets.

Complexity and weak integration promote the diversity of reef fish oral jaws

Major trade-offs often manifest as axes of diversity in organismal functional systems. Overarching trade-offs may result in high trait integration and restrict the trajectory of diversification to be along a single axis. Here, we explore the diversification of the feeding mechanism in coral reef fishes to establish the role of trade-offs and complexity in a spectacular ecological radiation. We show that the primary axis of variation in the measured musculo-skeletal traits is aligned with a trade-off between mobility and force transmission, spanning species that capture prey with suction and those that bite attached prey. We found weak or no covariation between about half the traits, reflecting deviations from the trade-off axis. The dramatic trophic range found among reef fishes occurs along the primary trade-off axis, with numerous departures that use a mosaic of trait combinations to adapt the feeding mechanism to diverse challenges. We suggest that morphological evolution both along and independent of a major axis of variation is a widespread mechanism of diversification in complex systems where a global trade-off shapes major patterns of diversity. Significant additional diversity emerges as systems use weak integration and complexity to assemble functional units with many trait combinations that meet varying ecological demands.

Coral-seeding devices with fish-exclusion features reduce mortality on the Great Barrier Reef

Restoration methods that seed juvenile corals show promise as scalable interventions to promote population persistence through anthropogenic warming. However, challenges including predation by fishes can threaten coral survival. Coral-seeding devices with refugia from fishes offer potential solutions to limit predation-driven mortality. In an 8-month field study, we assessed the efficacy of such devices for increasing the survival of captive-reared Acropora digitifera (spat and microfragments) over control devices (featureless and caged). Devices with fish-exclusion features demonstrated a twofold increase in coral survival, while most corals seeded without protection suffered mortality within 48 h. Overall, spat faced more grazing and higher mortality compared to microfragments, and upward-facing corals were more vulnerable than side-facing corals. Grazing-induced mortality varied by site, with lower activity in locations abundant in mat-forming cyanobacteria or Scleractinian corals. Many scraping parrotfish were found feeding on or near the seeded corals; however, bites by Scarus globiceps explained the most site-related variation in grazing. Cyanobacteria may be preferred over corals as a nutritional resource for scraping parrotfish-advancing our understanding of their foraging ecology. Incorporating side-facing refugia in seeding devices and deploying to sites with nutrient-rich food sources for fish are potential strategies to enhance coral survival in restoration programs.

Two decades of carbonate budget change on shifted coral reef assemblages: are these reefs being locked into low net budget states?

The ecology of coral reefs is rapidly shifting from historical baselines. One key-question is whether under these new, less favourable ecological conditions, coral reefs will be able to sustain key geo-ecological processes such as the capacity to accumulate carbonate structure. Here, we use data from 34 Caribbean reef sites to examine how the carbonate production, net erosion and net carbonate budgets, as well as the organisms underlying these processes, have changed over the past 15 years in the absence of further severe acute disturbances. We find that despite fundamental benthic ecological changes, these ecologically shifted coral assemblages have exhibited a modest but significant increase in their net carbonate budgets over the past 15 years. However, contrary to expectations this trend was driven by a decrease in erosion pressure, largely resulting from changes in the abundance and size-frequency distribution of parrotfishes, and not by an increase in rates of coral carbonate production. Although in the short term, the carbonate budgets seem to have benefitted marginally from reduced parrotfish erosion, the absence of these key substrate grazers, particularly of larger individuals, is unlikely to be conducive to reef recovery and will thus probably lock these reefs into low budget states.

Possible control of acute outbreaks of a marine fungal pathogen by nominally herbivorous tropical reef fish

Primary producers in terrestrial and marine systems can be affected by fungal pathogens threatening the provision of critical ecosystem services. Crustose coralline algae (CCA) are ecologically important members of tropical reef systems and are impacted by coralline fungal disease (CFD) which manifests as overgrowth of the CCA crust by fungal lesions causing partial to complete mortality of the CCA host. No natural controls for CFD have been identified, but nominally herbivorous fish could play a role by consuming pathogenic fungi. We documented preferential grazing on fungal lesions by adults of six common reef-dwelling species of herbivorous Acanthuridae and Labridae, (surgeonfish and parrotfish) which collectively demonstrated an ~ 80-fold higher grazing rate on fungal lesions relative to their proportionate benthic coverage, and a preference for lesions over other palatable substrata (e.g. live scleractinian coral, CCA, or algae). Furthermore, we recorded a ~ 600% increase in live CFD lesion size over an approximately 2-week period when grazing by herbivorous fish was experimentally excluded suggesting that herbivorous reef fish could control CFD progression by directly reducing biomass of the fungal pathogen. Removal rates may be sufficient to allow CCA to recover from infection and explain historically observed natural waning behaviour after an outbreak. Thus, in addition to their well-known role as determinants of macroalgal overgrowth of reefs, herbivorous fish could thus also be important in control of diseases affecting crustose coralline algae that stabilize the foundation of coral reef substrata.

Damselfishes alleviate the impacts of sediments on host corals

Mutualisms play a critical role in ecological communities; however, the importance and prevalence of mutualistic associations can be modified by external stressors. On coral reefs, elevated sediment deposition can be a major stressor reducing the health of corals and reef resilience. Here, we investigated the influence of severe sedimentation on the mutualistic relationship between small damselfishes (Pomacentrus moluccensis and Dascyllus aruanus) and their coral host (Pocillopora damicornis). In an aquarium experiment, corals were exposed to sedimentation rates of approximately 100 mg cm-2 d-1, with and without fishes present, to test whether: (i) fishes influence the accumulation of sediments on coral hosts, and (ii) fishes moderate partial colony mortality and/or coral tissue condition. Colonies with fishes accumulated much less sediment compared with colonies without fishes, and this effect was strongest for colonies with D. aruanus (fivefold less sediment than controls) as opposed to P. moluccensis (twofold less sediment than controls). Colonies with symbiont fishes also had up to 10-fold less sediment-induced partial mortality, as well as higher chlorophyll and protein concentrations. These results demonstrate that fish mutualisms vary in the strength of their benefits, and indicate that some mutualistic or facilitative interactions might become more important for species health and resilience at high-stress levels.

Functional implications of dentition-based morphotypes in piscivorous fishes

Teeth are crucial in elucidating the life history of vertebrates. However, most studies of teeth have focused on mammals. In heterodont mammals, tooth function is based on tooth shape and position along the jaw. However, the vast majority of vertebrates are homodont, and tooth function might not be based on the same principles (in homodonts, tooth shape is broadly similar along the jaw). We provide a quantitative framework and establish dentition-based morphotypes for piscivorous fishes. We then assess how these morphotypes relate to key functional feeding traits. We identified three broad morphotypes: edentulate, villiform and macrodont, with edentulate and villiform species displaying considerable functional overlap; macrodont species are more distinct. When analysing macrodonts exclusively, we found a major axis of variation between 'front-fanged' and 'back-fanged' species. The functional interpretations of this axis suggest that tooth-based functional decoupling could exist, even in homodont vertebrates, where teeth have similar shapes. This diversity is based not only on tooth shape but also solely on the position along the jaw.