The effects of high-risk conditions on coral reef fishes

Abstract

Risk is one of the main drivers in shaping prey phenotypes around the world. Individuals that accurately assess risk have a better chance of mounting a suitable response to a threat cue and hence gain a selective advantage. Phenotypic plasticity in behaviour is advantageous especially for organisms that transition between stages of a complex life history, as it is hard to predict future risk. For coral reef fish, one such stage is known as settlement. At this stage, naive juveniles are exposed to various levels of risk. Individuals that settle on high-risk habitats may develop a risk-adverse phenotype whereby novel cues are initially labelled as risky. There are many effects to having a risk-adverse phenotype that assist prey to increase their probability of survival. However, more studies need to be done to further understand the risk-adverse phenotype. This thesis aims to investigate: (1) How do high-risk conditions affect the fast-start escape response in juvenile coral reef fish; (2) How do high-risk conditions affect the morphology of juvenile coral reef fish. High-risk conditions were created by exposing individuals to conspecific chemical alarm cues three times a day for four days. Low-risk (control) conditions were created by exposing individuals to saltwater three times a day for four days. Chapter 2 investigated the effects of high-risk conditions on the escape responses of a species of juvenile damselfish, the spiny chromis, Acanthochromis polyacanthus (Pomacentridae). Juvenile A. polyacanthus were caught using hand net on SCUBA and transported back to the laboratory. Individuals were then exposed to either a high-/low-risk treatment before being individually tested in a fast-start arena. Prior to the commencement of the test, individuals were exposed to either chemical alarm cues or saltwater to investigate the effect of acute stressors. All fish were filmed at 420 frames per second using a camera pointed at a mirror tilted at a 45 angle underneath the arena. This resulted in a silhouette of the moving fish. A principal component analysis (PCA) was used to analyse how variables measured (response latency, response duration, response distance, mean response speed, maximum response speed, maximum acceleration) differed between treatments. An analysis of variance (ANOVA) was also used to analyse how escape method and turning angle differed between treatments. Risk (background or acute) affected escape responses in two ways. Firstly, the method of escape used by individuals (i.e., C-start or backing away from the threat). Secondly, escape responses were enhanced by individuals exposed to high-risk (with/without acute risk) and low-risk with additional acute risk. Background risk and acute risk acted in a simple additive manner, as seen by the lack of interaction between the two factors. Results showed that escape responses are amplified as the level of perceived risk increases. Chapter 3 investigated the effects of high-risk conditions on the morphology of a species of juvenile damselfish, the ambon damselfish, Pomacentrus amboinesis (Pomacentridae). Naive fish leaving the pelagic phase to settle on reefs were caught by light traps and transported back to the laboratory where they were exposed to high-/low-risk conditions. Following background risk conditioning, individuals were taught to recognise a novel odour from a predator, the Brown dottyback, Pseudochromis fuscus, as risky or safe. Individuals were then housed in plastic containers for 28 days. During this period, individuals were exposed to the predator odour three times a day. Photographs were taken every seven days to document the change in body depth, standard length, ocellus area, ocellus diameter and eye diameter. Results found that individuals exposed to high-risk conditions grew larger ocelli after four days of exposure. At 28 days post risk history treatment, high-risk individuals not only had larger ocelli but significantly smaller eyes and a slightly larger body. Results suggest that exposure to predation risk at a critical time may place these fish on a certain growth trajectory based on experiences during their period of settlement. This study demonstrates the importance of a risk-adverse phenotype for coral reef fishes settling onto high-risk habitats. As naive juveniles are particularly vulnerable to predation at this stage in life, the risk-adverse phenotype increases the probability of survival. This is achieved by not only treating novel cues as a risk, but also altering morphology and escape responses to aid in the pre- and post-attack stages of an interaction with predators.