Disturbance-Mediated Changes in Coral Reef Habitat Provoke a Positive Feeding Response in a Major Coral Reef Detritivore, Ctenochaetus striatus

Coral reefs are undergoing global phase shifts from coral-dominated to algae-dominated stages. The negative effects of this substratum shift on the diversity and abundance of fish have been well documented, but the influence on fish feeding is less studied, which may limit a deeper understanding of trophic pathways in such a disturbed system. In this study, we investigated the feeding response of a numerically dominant fish species Ctenochaetus striatus to different substrate types, including hard coral, short algal turfs (SATs, <5 mm), and long algal turfs (LATs, >5 mm), on reefs in the South China Sea. The biomass of C. striatus showed an inverted U-shaped relationship with coral coverage and a significant positive correlation with SAT coverage (p < 0.05), indicating that rising SAT coverage associated with moderate coral loss provoked a feeding response in C. striatus. Stomach contents of C. striatus, analyzed using high-throughput sequencing (HTS), were dominated by algal sequences (relative read abundance, RRA > 80.0%), including macroalgae, filamentous algae, and microalgae (e.g., Symbiodinium and Prorocentrum). The sequence number and diversity of microalgae (mainly dinoflagellates) tended to be abundant (RRA 13.5–36.5%) with increased SAT cover, but brown algae sequences (RRA 17.2–57.8%) or green algae sequences (RRA > 50.7% except one site) dominated the stomach content DNA in reefs with high coral cover and high LAT or macroalgal cover, respectively. Considering the limited ability of C. striatus to remove mature algae, macroalgal DNA might be from algal debris. Our results indicate that C. striatus populations respond positively to conditions of moderate coral loss through increases in body condition identified as increased biomass. These responses are correlated to the expansion of SAT’s as coral cover declined, however, this relationship reverses if coral loss is high due to the succession of LAT’s over SAT’s and a corresponding decrease in the quality of food available. Our use of HTS has nevertheless identified the importance of detritivory in the flow of energy through reefs in the Anthropocene which are increasingly becoming depauperate in hard coral.

Animal Forest Mortality: Following the Consequences of a Gorgonian Coral Loss on a Mediterranean Coralligenous Assemblage

In this work, the consequences of a local gorgonian coral mortality on the whole coralligenous assemblage were studied. A Before/After-Control/Impact sampling design was used: the structure of the coralligenous assemblage was compared before and after the gorgonian mortality event at the mortality site and two control sites. At the mortality site, a relevant decrease in alpha and beta diversity occurred, with a shift from a stratified assemblage characterized by gorgonians and other invertebrates to an assemblage dominated by algal turfs; conversely, neither significant variations of the structure nor decrease in biodiversity were observed at the control sites. The assemblage shift involved the main taxa in different times: in autumn 2018, a large proportion of the plexaurid coral Paramuricea clavata died, but no significant changes were observed in the structure of the remaining assemblage. Then, in autumn 2019, algal turfs increased significantly and, one year later, the abundance of the gorgonian Eunicella cavolini and bryozoans collapsed. Although the mechanisms of the assemblage shift following gorgonian loss will remain uncertain and a cause-effect relationship cannot be derived, results suggest the need for detecting signs of gorgonian forests stress in monitoring programs, which should be considered early indicators of their condition. in the coralligenous monitoring programs for detecting any sign of gorgonian forests stress which should be considered an early indicator of the assemblage condition.

Ecosystem functioning of canopy- and turf- forming algae: contrasting supply of invertebrate prey to pelagic consumers

Macroalgal canopies are declining worldwide and are being replaced by low-lying algal turfs which frequently dominate reefscapes. Their loss may impact reef ecosystems in different ways, including the collapse of small canopy-dwelling invertebrates, and thus the decline of juvenile reef fish that prey on them. To evaluate this potential loss, we assessed (1) the differences between the mobile invertebrate assemblages associated with turf-forming (filamentous and articulated coralline turf) and canopy-forming (Sargassum spp. and Dichotomaria marginata) algae, and (2) the mechanisms underlying those contrasts by examining the invertebrate community assembly of filamentous turf and Sargassum spp. over the main canopy season. Abundance, biomass and diversity almost always differed between canopies and turfs (although not in a consistent way across sampling sites), while differences within canopy and turf algal types were nearly absent. The structure of invertebrate assemblages differed more consistently between canopies and turfs, with certain hard-bodied and soft-bodied invertebrates characterizing canopies and turfs, respectively. This divergence increased as the canopy season advanced. While no temporal changes occurred in turf invertebrate assemblages, clear temporal dynamics occurred in the invertebrate fauna associated with Sargassum. Brittle stars and amphipods were most abundant as early colonizers, followed by hard-shelled gastropods, bivalves and ostracods. By the end of the season, these groups became dominant and decreased diversity in the canopy habitat. As hard-shelled prey are preferred items for the main invertivore fish species in the area, results suggest that canopies may play an important role in the provisioning of trophic resources to pelagic consumers.

A preliminary study on the diversity of invertebrates associated with Corallina officinalis Linnaeus in southern Istrian peninsula

Red coralligenous algae Corallina officinalis provides shelter to many invertebrate species from wave actions, predation and desiccation stress in the intertidal area. Physical structure and complexity of the habitat have a major influence on biodiversity of this community. The aim of this preliminary study was to examine the diversity of invertebrate assemblages inside the red algae C. officinalis turf. Three sampling locations were chosen where algal cover range was above 90%. On each location sampling was done quantitatively by scraping off within 3 replicate quadrats 20 x 20 cm in size. A total of 30,518 specimens were isolated from all sampling locations. The prevalent groups were amphipod crustaceans, polychaetes, bivalves and gastropods that made a total of 86% of all macrofaunal groups associated with algal turfs. The most abundant group were amphipods that made 42% of the total separated individual invertebrates. Our study demonstrated that C. officinalis is a very important habitat with high abundance and diversity of invertebrate assemblages.

Interacciones de corales masivos con céspedes algales y otros organismos en arrecifes del Parque Nacional Natural Tayrona

In coral reefs, competition between sessile benthic organisms for access to space and other resources is a determining factor in community structuring. Recent coral mortality has favored the development of algal turfs, which are now competing with corals, often displacing them,although slowly. To determine the frequency of interactions between coral-turf and coral-other benthic categories, and how their distribution and results (coral wins or coral loses) are modulated by environmental factors, band photo transects were evaluated for 12 stations in the ParqueNacional Natural Tayrona, Santa Marta, Colombian Caribbean. It was found that algal turfs are the most frequent competitors of massive corals, and that the frequency of coral-turf interactions depends on the particular susceptibility of a coral species to the loss of tissue, and past disturbances that have affected the reef in which it lives. Furthermore, the results of these interactions depend on the ways the colonies are organized, meandroidand cerioid colonies being the most successful in competition against turfs, as their vertical growth constitutes an effective avoidance mechanism. By contrast, plocoid colonies are more prone to lose. In general, biological factors (the coral species in question and type of colonial organization) were determinants in the frequency and apparent result of coral-turf interactions, while environmental factors (disturbance gradient, depth, anddegree of wave exposure) appear not to play a predominant role. As a consequence, historical and future changes in coral species cover, and the nature of colony organization in the Santa Marta area, should reflect a combination of their susceptibility to deleterious agents and their capacityto counteract or evade competition, especially with algal turfs.

The effects of an invasive seaweed on native communities vary along a gradient of land-based human impacts

The difficulty in teasing apart the effects of biological invasions from those of other anthropogenic perturbations has hampered our understanding of the mechanisms underpinning the global biodiversity crisis. The recent elaboration of global-scale maps of cumulative human impacts provides a unique opportunity to assess how the impact of invaders varies among areas exposed to different anthropogenic activities. A recent meta-analysis has shown that the effects of invasive seaweeds on native biota tend to be more negative in relatively pristine than in human-impacted environments. Here, we tested this hypothesis through the experimental removal of the invasive green seaweed,Caulerpa cylindracea, from rocky reefs across the Mediterranean Sea. More specifically, we assessed which out of land-based and sea-based cumulative impact scores was a better predictor of the direction and magnitude of the effects of this seaweed on extant and recovering native assemblages. Approximately 15 months after the start of the experiment, the removal ofC. cylindraceafrom extant assemblages enhanced the cover of canopy-forming macroalgae at relatively pristine sites. This did not, however, result in major changes in total cover or species richness of native assemblages. PreventingC. cylindraceare-invasion of cleared plots at pristine sites promoted the recovery of canopy-forming and encrusting macroalgae and hampered that of algal turfs, ultimately resulting in increased species richness. These effects weakened progressively with increasing levels of land-based human impacts and, indeed, shifted in sign at the upper end of the gradient investigated. Thus, at sites exposed to intense disturbance from land-based human activities, the removal ofC. cylindraceafostered the cover of algal turfs and decreased that of encrusting algae, with no net effect on species richness. Our results suggests that competition fromC. cylindraceais an important determinant of benthic assemblage diversity in pristine environments, but less so in species-poor assemblages found at sites exposed to intense disturbance from land-based human activities, where either adverse physical factors or lack of propagules may constrain the number of potential native colonizers. Implementing measures to reduce the establishment and spread ofC. cylindraceain areas little impacted by land-based human activities should be considered a priority for preserving the biodiversity of Mediterranean shallow rocky reefs.