Novel field observations of coral reef fishes feeding on epiphytic and epizoic organisms associated with the allelopathic seaweed Galaxaura divaricata

Abstract

In degraded coral reef ecosystems, allelopathic macroalgae have received increasing attention from marine ecologists because their secondary metabolites (also known as allelochemicals) kill corals that grow adjacent to them and weaken the recovery of degraded reefs. One well‐known coral‐killing macroalga is the calcareous red seaweed Galaxaura . However, our knowledge of how coral reef fishes interact with allelopathic algae like Galaxaura is very limited. Here, we documented novel observations of feeding interactions of 17 species of coral reef fishes (herbivorous and carnivorous) with the filamentous Galaxaura divaricata on degraded lagoon patch reefs in Dongsha Atoll (South China Sea). Video analyses showed that territorial farming damselfishes (i.e., Dischistodus perspicillatus , D. prosopotaenia , Hemiglyphidodon plagiometopon , Pomacentrus grammorhynchus , P. adelus , and Neoglyphidodon nigroris ) and juvenile parrotfishes ( Scarus schlegeli , S. ghobban , S. rivulatus , and Chlorurus spilurus ) likely used G. divaricata as a feeding substratum. Further, microscopic analyses revealed that the filamentous surface of G. divaricata harbored a wealth of epiphytic microalgae, such as filamentous cyanobacteria (i.e., Leptolyngbya , Lyngbya , Rivularia , Oscillatoria , and Stigonema ), diatoms (i.e., Synedra , Nitzschia , Mastogloia , and Pleurosigma ), and filamentous red algae (i.e., Heterosiphonia ), suggesting that these fishes targeted the nutrient‐rich microscopic epiphytes rather than the nutrient‐poor host. Juvenile benthic carnivores (i.e., Labridae, Parupeneus multifasciatus , and Meiacanthus grammistes ) form feeding assemblages with roving parrotfishes to feed on small invertebrates (i.e., amphipods, copepods, isopods, gastropods, and polychaetes) associated with G. divaricata . Given that coral reef fishes appear to target the epiphytes associated with Galaxaura rather than the alga itself, these observations thus substantiate the threat posed by the overgrowth of G. divaricata to coral recovery in degraded reef systems due to the lack of natural grazers.

Testing sea urchin and green sea turtle consumption of the allelopathic macroalga Galaxaura divaricata

“…divaricata , but not on G . divaricata itself (Nieder et al., 2022). According to our preliminary observations made in Dongsha Atoll, only two species of rabbitfishes, Siganus guttatus and S .…”

“…Herbivorous fishes are also major consumers of macroalgae; however, few studies that have investigated herbivorous fish feeding interactions with Galaxaura suggest most species avoid feeding on Galaxaura (Loffler et al, 2015;Mantyka & Bellwood, 2007a, 2007bNieder et al, 2022). Damselfishes and young parrotfishes are known to feed on macro-and micro-epiphytes from the surface of G. divaricata, but not on G. divaricata itself (Nieder et al, 2022).…”

“…Mantyka and Bellwood (2007a) found the lowest grazing frequencies and minimal biomass removal of G. filamentosa by common herbivorous fishes compared to other macroalgae. Recent field observations further suggest that, while several species of coral reef fish feed on epiphytes and epifauna from the surface of G. divaricata, they do not readily remove G. divaricata itself (Nieder et al, 2022). Consequently, there is a need to understand whether G. divaricata is consumed by macroherbivores other than fish that could potentially control the abundance of this algae on coral reefs.…”

“…divaricata , they do not readily remove G . divaricata itself (Nieder et al., 2022). Consequently, there is a need to understand whether G .…”

Epiphytic Algae on Dominant Macrophytes in Lotic Ecosystems in the Eastern Flanks of Mount Cameroon

“…Epiphytic algae are a group of algae found attached on plants, usually submerged aquatic plants [5]. They are very important primary producers and constitute a good food source for fishes, insects and other aquatic invertebrates inhabiting the waters [6,7]. These microscopic organisms play vital roles in carbon fixation, allelopathy amongst others [8,9].…”