Processes and mechanisms of predatory interactions on newly settled reef fish

Holmes, Thomas (2009) Processes and mechanisms of predatory interactions on newly settled reef fish. PhD thesis, James Cook University.

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Predation is generally thought to be one of the major processes influencing the size of populations and the structure of ecological communities. As such, the mechanisms of prey survival during predatory interactions will play a large role in determining those characteristics and traits that are passed on to later life stages. These mechanisms will be particularly important during periods of high mortality, such as transitional periods between life history stages for organisms with complex, bi-partite life cycles. One such period is that of settlement from the pelagic larval phase to the more benthic associated juvenile phase in many coral reef fishes.

This project examines the mechanisms influencing survival during interactions with small reef fish predators over this early post-settlement period. The focus is split between two distinct ecological areas that are thought to play a major role in determining survival during transitional life stages: the phenotypic and performance characteristics of predator and prey; and the behavioural responses of prey to potential predation threats. The chapters of this thesis addresses the following questions: 1) how selective is predation with respect to three key prey characteristics: body size, body weight and burst swimming speed; 2) how does predator size and identity influence the nature of size selection; 3) how do behavioural characteristics associated with body size influence sizeselective patterns; 4) what is the role of chemical alarm cues in anti-predator responses and predator identification; and 5) how do anti-predator responses to both visual and chemical predation cues differ with a changing level of threat.

The common Ambon damselfish, Pomacentrus amboinensis, was used as the model prey species throughout all experiments. These were collected during settlement pulses using light traps, so as to keep them naïve to all reef-based processes. All experiments examining the selective nature of predation were conducted in aquaria. Individual predators (Pseudochromis fuscus) were offered a choice of prey, differing in either body size, body weight or burst swimming speed. Predation by this species was found to be highly selective towards larger body size at the time of settlement. In contrast, there was no evidence of selection with regards to either prey body weight or burst swimming speed. These patterns were found to differ from those observed in field based trials, where prey were open to multiple predator communities. These results indicate that body size may be the most important prey characteristic influencing prey survival during predatory encounters over this early period. Further, the discrepancy between single and multiple species trials suggest that the nature of selection towards this trait may differ between predator species and sizes.

Closer examination of this hypothesis using further aquarium trials showed that the intensity and direction of size selectivity differed significantly between four of the key predatory fish species (the dottyback, Pseudochromis fuscus; the moonwrasse Thalassoma lunare; the lizardfish Synodus variegatus; and the rockcod, Cephalopholis microprion). Some species preferentially removed smaller individuals (T. lunare, S. variegatus), while others removed larger individuals (P. fuscus) or were non-selective (C. microprion). However, these patterns of selectivity were not found to differ with predator size. These results suggest that no specific expression of a phenotypic trait holds a definitive survival advantage during all encounters. Instead, prey survival may in part be determined by the behavioural characteristics of different sized prey within a hierarchy, and how this influences vulnerability to predation by different 'modes' of predation.

Size associated differences in prey behaviour within simple hierarchical groups were examined in both aquaria and on small patch reefs constructed immediately adjacent to shallow lagoonal habitat. Small and large individuals were paired and assessed for five behavioural traits. Large individuals were found to make more aggressive strikes on conspecifics and had higher feeding rates than their smaller counterparts. We suggest that the dominant behaviours displayed by larger individuals in a group could result in increased vulnerability of smaller individuals to opportunistic predation, leading to the patterns of predation observed in the previous chapter (with the exception of P. fuscus).

How species react to predation threats and acquire knowledge of them in previously novel habitats will have a large influence on survival during transitional periods. To examine the role that visual and chemical cues play in this process, fishes were assessed for behavioural responses to potential visual predation cues and chemical alarm cues released from injured fishes. Additionally, fish were assessed to determine whether they could use chemical alarm cues to associate novel predator scents with danger. Fish were found to respond to conspecific chemical alarm cues only by reducing their feeding rate. Individuals were able to use these alarm cues to associate a previously novel predator scent with danger, after only a single previous exposure to the paired conspecific alarm/novel scent cue. In contrast, responses to visual cues were more widespread but diffuse, and fish were unable to distinguish between predatory and nonpredatory cues. These results indicate the important role that chemical cues in particular play in both threat detection and learned predator recognition during the early postix settlement period in coral reef fishes. Although visual cues also play a role, their utility appears limited whilst still naïve to reef based processes, due to a lack of innate recognition of predator identity.

When tested across a range of predation threat levels (by manipulating chemical cue concentration and distance from visual cue), behavioural responses were found to be threat dependent in nature. Although significant changes were observed, responses to visual cues were again inconsistent, whilst responses to extremely low chemical cue concentrations were marginal, indicating a possible threshold lower limit. This demonstrates the ability of newly settled fish to assess the level of predation risk using both visual and chemical cues, and respond appropriately.

This project provides us with a detailed insight into the mechanisms and processes of survival during a potentially critical life history period for coral reef fishes. In doing so, it shows how both phenotypic characteristics, predator identity and behavioural changes associated with threat detection and predator learning may influence the outcome of predatory interactions during this early period.

Item ID: 29293
Item Type: Thesis (PhD)
Keywords: predation; coral reef fish; settlement; selectivity; alarm cues; Ambon damselfish; Pomacentrus amboinensis; phenotypic traits
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Holmes, Thomas H., and McCormick, Mark I. (2009) Influence of prey body characteristics and performance on predator selection. Oecologia, 159 (2). pp. 401-413.

Chapter 3: Holmes, Thomas H., and McCormick, Mark I. (2010) Size-selectivity of predatory reef fish on juvenile prey. Marine Ecology Progress Series, 399 . pp. 273-283.

Chapter 5: Holmes, Thomas H., and McCormick, Mark I. (2010) Smell, learn and live: the role of chemical alarm cues in predator learning during early life history in a marine fish. Behavioural Processes, 83 (3). pp. 299-305.

Chapter 6: Holmes, Thomas H., and McCormick, Mark I. (2011) Response across a gradient: behavioural reactions of newly settled fish to predation cues. Animal Behaviour, 81 (3). pp. 543-550.

Date Deposited: 13 Sep 2013 02:24
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 50%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060308 Life Histories @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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