Abstract
Predator-prey interactions are such an important part of ecosystem function and structure.
Predation exerts pressure on prey species who must adapt and respond to predators in order
to survive and reproduce. Many aquatic prey species have developed a mechanism of
detecting prey through the use of chemical alarm cues which alert conspecifics during a predation event. Recently, there has been a growing body of research
that shows the importance of non-consumptive effects of predation via CACs. Studies have
shown that exposure to CACs can induce dramatic behavioural and morphological responses.
Few studies, however, have looked at the physiological responses to predation risk which may
have fitness consequences just as important as behavioural or morphological responses. This
study aims to investigate effects of CACs on the physiology and development of fish during
early life stages. The larval stage generally has the highest mortality rates for most fish species
which is why it is important to understand how non-consumptive effects of predation
influence these life stages. Furthermore, as anthropogenic stressors become more prominent
throughout natural ecosystems, it is important to understand how these affect prey
responses to predation threat.
Chapter 2 looks into the effects of embryonic predation risk by investigating whether there
are changes in the fitness and the anti-predator behaviour of embryonic and larval mottled
triplefin (Forsterygion capito) following chronic, embryonic exposure to CACs. This
experiment contains 2 parts; 1) exposing triplefin embryos to chronic predation risk via pulsed
conspecific CAC’s and measuring changes in yolk sac utilisation and embryonic heart rate as
a proxy for metabolism; 2) measuring post-hatch morphometric attributes in the larvae, such
as size of yolk sac, standard length and weight. Results showed that embryonic CAC exposure
induced significantly higher heart rates at 16 and 37 days post-fertilisation when compared
to a saltwater control. The increased metabolism, associated with elevated heart rates,
resulted in accelerated yolk sac utilization during the embryonic phase and therefore smaller
yolk sacs at 37 dpf. Larval yolk size and standard length and activity rate was measured at <1
days post hatch (dph). Results showed that there was no effect of treatment on larval
standard length or activity rate but yolk size was significantly smaller in larvae exposed to
CACs. Predation is a major influence on prey behaviour and morphology, and understanding
how embryonic and larval development is influenced by predation stress provides insight on
what drives these adaptation.
Chapter 3 examines the morphological and physiological responses of the Ambon damselfish
(Pomacentrus amboinensis) to the combined exposure of CACs and microplastics, highlighting
any interactive effects between the two stressors. Settlement stage juveniles of the Ambon
damselfish (Pomacentrus amboinensis) were exposed to conspecific alarm cues and/or
polystyrene microplastics over a 2.5-week period to observe the effects on morphology and
the oxidative stress response. Antioxidant enzymes vital to the oxidative stress response such
as catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPOX)and
glutathione-S-transferase (GST) were measured as well as markers of oxidative damage,
including protein carbonyls and lipid peroxidation. Morphological parameters such as
standard length and body depth were also be measured to identify any developmental effects
caused by predator threat and microplastic ingestion. Results showed that fish exposed to
CAC and fish exposed to microplastics generated a strong oxidative stress response with an
increase in levels of antioxidant enzymes, protein carbonyls and lipid peroxides. Fish exposed
to combined CACs and microplastics showed an additive response whereby oxidative stress
levels were higher compared to fish exposed to either CACs or microplastics alone. No effect
was found on morphology but fish exposed to CACs did have a smaller mean body depth than
controls. This is some of the first to highlight the physiological stress response to predation
risk and how microplastic pollution can compound this response.
This study demonstrates the physiological and developmental effects of predation risk on
early life stage fish and discusses the potential fitness consequences when combined with
an anthropogenic stressor.