SCTLD
Stony Coral Tissue Loss Disease
Stony Coral Tissue Loss Disease (SCTLD) is a coral pathogen that has overtaken Caribbean reefs since 2014 and caused a sharp increase in coral mortality (Aeby et al 2019). While the damage caused by this disease is evident, the mechanism by which it spreads is still unknown (Muller et al 2020). Studies on other coral diseases reveal evidence that corallivores may influence coral disease (Rice et al. 2019), and corallivorous fishes may serve as a vector for disease through oral transmission (Aeby and Santavy 2006), although conflicting evidence has been discovered and more research is necessary to determine if they are linked (Nicolet et al. 2018). Some of the key corallivore players on a Caribbean coral reef are the butterflyfishes (family Chaetodontidae).
Butterflyfish Behaviors
Association with SCTLD
Foureye butterflyfish (C. capistratus) are a species of interest due to their high abundance and previously observed preferential feeding on hard and soft coral substrates, as well as on diseased corals (Liedke et al 2018; Birkeland and Neudecker 1981; Noonan and Childress 2020). However, field observations suggested that a sympatric species, the spotfin butterflyfish (C. ocelatus) exhibited greatly differing foraging, agonistic, and social behaviors than the foureye butterflyfish. The spotfins, while also facultative corallivores, did not show the same feeding preference for hard and soft coral (Birkeland and Neudecker 1981). Considering the similarities these species share on paper, the difference in their behavior indicates a need for investigation into what might be the root cause of these difference so that we can assess the potential for spotfin and foureye butterflyfish to influence SCTLD.
Given that Foureye and Spotfin butterflyfish are both facultative corallivores and are sympatric species, why do they seem to exhibit different behavioral patterns, especially in the presence of coral disease?
QUESTION
Do differences between Foureye Butterflyfish and Spotfin Butterflyfish exist as a result of competetive exclusion or resource partitioning?
Hypotheses and predictions
We hypothesize two explanations for disease association and foraging differences between C. capistratus and C. ocelatus:
Competitive Exclusion
The first is foureye butterflyfish are preventing access to coral as a resource and forcing the spotfin butterflyfish forage on other substrates by competitive exclusion. In this case, we would expect to see foureye butterflyfish acting as the dominant species, defending coral resources – particularly those bearing disease.
Based on previous observations of both species in the field, we expect resource partitioning to be the most likely explanation.
Single foureye butterflyfish had the highest bite rate on hard coral substrate and pairs and groups showed the highest bite rate on soft coral. For spotfin butterflyfish, all social conditions showed the highest bite rate on algae.
METHODS
Site Selection and Substrate Census
This research was conducted on sites in the middle Keys of the Florida Keys National Marine Sanctuary. Reefs were surveyed in the summers (May-July) of 2017, 2018, 2019, 2020, and 2021, with census areas of 50 x 30m, defined by a permanent 50m transect line that ran parallel to the reef and four perpendicular 30m transects at 10, 20, 30, and 40m. Substrate cover was estimated by taking digital photographs (50 x 50 cm) at 10m intervals and calculated using 25 random points per photograph generated by Coral Point Count with Excel extensions (CPCe).
Butterflyfish Population Survey
Butterflyfish abundance was assessed through a population survey completed by a diver counting butterflyfish along the entire reef. The diver swam in a z-shaped pattern through the 10m perpendicular transects along the main 50m transect twice, noting the species of all butterflyfish present and indicating their social status at the time of observation. Species observed were foureye, spotfin, reef, and banded butterflyfish.
Butterflyfish Focal Observations
A diver holding a GoPro mounted to a PVC handle followed butterflyfish and recorded them for 10 minutes. The video observations were uploaded into Behavioral Observation Research Interactive Software (BORIS) and analyzed using an ethogram to highlight behaviors of interest. These behaviors included foraging, aggression, swimming, and hiding. These videos also allowed butterflyfish to be classified as single or paired, with a pair being defined as two or more conspecifics spending at least 50% (5 min) of the video with one another.
Statistical Analyses
Influence of butterflyfish species and social condition on butterflyfish behaviors was analyzed by a mixed model ANOVA with site as a random variable as well as through an ANOVA using site as a fixed variable. Influence of butterflyfish species on butterflyfish behaviors was examined using a multiple regression with three site measures: rugosity, % hard coral cover, and conspecific density.
conclusions
Differences in behavior between Foureye and Spotfin butterflyfish are likely the result of RESOURCE PARTITIONING
DISCUSSION
Ecological and behavioral differences are present in sympatric Chaetodontids C. capistratus and C. ocellatus and appear to be the result of nonoverlapping niches rather than exclusion. By examining data collected both in the presence of Stony Coral Tissue Loss Disease (2017-2019, peaking in 2018) (Florida Keys National Marine Sanctuary 2018), we were able to better understand what influence, if any, coral disease had on the behavior of butterflyfish. This allowed a deeper dive into the behaviors of foureye and spotfin butterflyfish, revealing probable causes of their differences and a narrowed scope of which reef fish might be of interest when studying Stony Coral Tissue Loss Disease.
Our understanding of butterflyfish ecology helps us as we continue to monitor SCTLD in the Florida Keys and unwrap the mystery of its mechanics. With greater understanding of corallivory and reef fish association with coral disease comes greater ability to determine management strategies. By building a better understanding of SCTLD, we are able to know what influences its spread and how it might be stopped, creating hope for a healthier reef system.
