Léonore Bonin, Cloé Brochette and Adrián Herrera
Are Sohal surgeonfish (Acanthurus sohal) more willing to chase fish that can compete for food resources in a 1-meter diameter around them?
INTRODUCTION
Coral reefs are among the most highly productive marine ecosystems providing various ecological niches in which thousands of animal species coexist (Loya, 1972; Marshell & Mumby, 2015). One of the most conspicuous and abundant species in shallow coral reefs is the Surgeonfish (Acanthuridae), a diurnal, gregarious and grazing species feeding from small benthic algae (Nursall, 1974; Carpenter, 1986). For our project, we focused on the species named the Sohal surgeonfish (Acanthurus sohal), a vivid blue fish with white stripes along the body; endemic to the Red Sea and living around flat reefs. They are also easily recognizable thanks to their single scalpel-like blade that they use to compete with conspecifics for dominance and mating, but also to attack or to defend against predators. Indeed, as coral reefs are a complex ecosystem, they offer a great habitat and a food resource for many fish species, making it very common that two or several different fish territories overlap (Alwany et al; 2005).
In highly diversified environments, food is a limited resource and fishes have to defend their territory against competitors; a behaviour – territoriality – that might be an adaptation to resource limitation (Alwany et al., 2005). Sohal surgeonfish is known to aggressively defend its feeding territory (Marshell & Mumby, 2015), however, it would be too much demanding for the fish to exclude every intruders in its feeding zone (Craig, 1996). The aim of our project was to establish if the species A. sohal tolerates some families more than others in a 1-meter range around it. As A. sohal are diurnal herbivores that graze on benthic algae or feed on zooplankton, we expect that the intensity of the agonistic reaction will be adjusted regarding the potential threat of the intruder fish. Thus, we expect herbivorous families who share the same diet (like Acanthuridae or Pomacentridae) to be more at risk to be chased than other herbivorous, carnivorous or omnivorous species because of competition.
METHODS
Data were collected in El Quseir, Egypt during three days in June (from the 6th to the 9th).
Surgeon fish were observed during snorkelling sessions. Each individual was observed during 6 to 10 minutes, depending on how easy it was to follow a specific individual.
During this time, we noted down the family of each fish that entered in an estimated 1-meter diameter around our observation subject, so-called visitor fish. Each visitor was counted only once even if he came several times in the perimeter. Additionally, we recorded every time our focal chased the visitor fish. A fish was defined as chased if the surgeon did an acceleration using his caudal fin (chase) or did a sudden change of direction (launch) towards the fish.
The encountered families were the following, grouped into three different feeding habits:
​
Herbivorous = Acanthuridae, Siganidae, Pomacentridae, Scaridae
Omnivorous = Labridae, Chaetodontidae, Monacantidae, Balistidae, Ostracidae
Carnivorous = Priacanthidae, Pomacanthidae, Cirrhitidae, Fistulariidae, Haemulidae, Holocentridae, Scorpaenidae, Serranidae, Tetraodontidae.
​
Sohal surgeonfish (Acanthurus sohal) feeding on benthic algae.
Hypothesis
Sohal surgeonfish (A. sohal) exhibit different toleration rates according to the diet of the intruder fish within a 1-meter range.
Statistical analyses
All analyses are performed with the R software version 3.3.0.
To investigate whether observed surgeon fish were more likely to chase potential food competitors, we use a generalized linear mixed effect model with binomial response variable (chased/unchased). The diet group was taken as categorical explanatory variable, the identity of surgeons was taken as random factor to avoid pseudo-replication error and observations were weighted according to the total number of fish of each diet group meet by each surgeon (to face the fact that all surgeons did not met the same number of fish for each family).
There are no specific assumptions to check for such a binomial model, except overdispersion which is tested with the dispersion_glmer() function from the blmeco package in R, the threshold is fixed to 1. As we find the model to be overdispersed (1.57 > 1), we tried to run the model using the quasibinomial family but this type of model does not accept it. Thus, the results of the model have to be cautiously interpreted.
RESULTS
The diet has been found to be an important factor to explain observed variations in chasing behaviours of A. sohal (p-value < 2.2e-16). Whereas, we found that the duration of observation did not have any impact on our results (p-value > 1).
The following intercepts have been found: carnivorous = 0.01197839, herbivorous = 0.1048088, omnivorous = 0.001781351, meaning that herbivorous have more chances to be chased than others (10% chance against 1% for carnivorous and 1‰ for omnivorous). These results are supported by the figures below, showing the herbivorous species as being more prone to be chased; almost 20% of the total number of chased visitors are herbivorous (Figure 1). The same conclusion can be drown if we look at chasing behaviour of each surgeon fish individually (Figure 2); almost only herbivorous are chased.
 Figure 1: Boxplots of the proportion of chased individual for each diet group of visitors. This boxplot represents the proportion of chased fish (y axis) on the total number of individual met, for each diet group (x axis). We can see a clear difference in the proportion of chased individual between herbivorous and the two other diet groups: herbivorous get chased much more than others, which are almost always tolerated. |  Figure 2a : Chased and unchased visitors of each diet group for each surgeon.This barplot shows, for each surgeon fish observed (1 to 22), the number of chased (pink) and unchased (blue) visitors fishes grouped according to their diet (carnivorous, omnivorous and herbivorous). The y-axis represents the number of visitors which is the total of chased + unchased fishes for each diet group. When there is no bar it means that the surgeon did not met this diet group during the observation time. |  Figure 2b : Chased and unchased visitors of each diet group for each surgeon.This barplot shows, for each surgeon fish observed (23 to 44), the number of chased (pink) and unchased (blue) visitors fishes grouped according to their diet (carnivorous, omnivorous and herbivorous). The y-axis represents the number of visitors which is the total of chased + unchased fishes for each diet group. When there is no bar it means that the surgeon did not met this diet group during the observation time. |
|---|
DISCUSSION
As Surgeonfish live close to coral reefs which are a huge food resource complex in algae, crustaceans or small fishes, many other species are foraging around. Surgeon fishes are known for aggressively defend their territories but they do not chase every species in the same way. Indeed, they are more aggressive against herbivorous species than carnivorous or omnivorous species (Figure 1). So the diet group influences on the probability to be chased away. As showed in Figure 2, they tolerate more carnivorous species because they do not represent a threat at all. However, omnivorous species are chased because their diet is close to the Surgeons’ one and can be considered as weak food competitors. The Surgeonfish defend hardly its territory against other algae-feeding intruders because they are considered as strong food competitors. An interesting study supports our findings by observing the effects of the removal of a territorial herbivorous species on food competitors (Hourigan, 1986). The experiment shows that food competitors - here, herbivorous - replace the vacant place left by the removal of the herbivorous species, thus suggesting a strong territoriality against food competitors in natural sites.
From this point on, we conclude that the tolerance of Sohal surgeonfish toward visitors entering his 1-meter diameter seems to be modulated by the potential food competition of those visitors.
We met the problem of the model overdispersion that cannot be fixed with our data. A bigger sample size might help to reduce the variability of the number of families visitors belong to and thus, maybe the dispersion as well.
Those results are interesting for the study of aggressiveness of surgeonfish, but still quite incomplete. Indeed, to go further in the understanding, the size of the visitors should be taken into account as it influences the strength of the competition with the surgeon. In addition, maybe a more precise classification of diet group should be more relevant as Labridae family for example does not include only omnivorous species. Thus, it might be a bias to our study.
Other factors like the social situation of the observed surgeonfish (in group or solitary), the period of the day coupled with knowledge of the daily cycle of surgeonfish (foraging moment or not) deserved to be tested as well.
References
Alwany, M., Thaler, E., & Stachowitsch, M. (2005). Territorial behaviour of Acanthurus sohal and Plectroglyphidodon leucozona on the fringing Egyptian Red Sea reefs. Environmental Biology of Fishes, 72(3), 321-334.
Carpenter, R. C. (1986). Partitioning herbivory and its effects on coral reef algal communities. Ecological Monographs, 56(4), 345-364.
​
Hourigan, T.F. (1986). An experimental removal of a territorial pomacentrid: effects on the occurrence and behavior of competitors. Environ. Biol. Fishes 15, 161.
​
Loya, Y. (1972). Community structure and species diversity of hermatypic corals at Eilat, Red Sea. Marine Biology, 13(2), 100-123.
Marshell, A., & Mumby, P. J. (2015). The role of surgeonfish (Acanthuridae) in maintaining algal turf biomass on coral reefs. Journal of Experimental Marine Biology and Ecology, 473, 152-160.
Nursall, J. R. (1974). Some territorial behavioral attributes of the surgeonfish Acanthurus lineatus at Heron Island, Queensland. Copeia, 950-959.
Craig, P. (1996). Intertidal territoriality and time-budget of the surgeonfish, Acanthurus lineatus, in American Samoa. Environmental biology of fishes, 46(1), 27-36.