Background

In trying to study the behavioural predator-prey interaction of an organism, prey escape and avoidance response is an important aspect of this complex intra-specific interaction that’s needs to be well understood (Lam 2002). Potential prey will adapt avoidance responses well before it actually encounters the predator when chemical substances emitted by predators (i.e. kairomones) are detected at distances in order to escape being preyed upon by it (Jacobsen & Stabell 1999; Lam 2002). At close proximity, when the prey senses or is touched by a predator, the instantaneous reactions taken by the prey are defined as the escape responses (Lam 2002). Both types of behavioral responses can be chemically triggered and many aquatic animals rely heavily on their chemical sensory system to detect the presence of their predator (Jacobsen & Stabell 1999; McKillup & McKillup 1993).

Characteristic prey escape and avoidance response behaviours have been well documented in many species of marine invertebrates following detection of chemical cues emitted by predators or conspecifics, especially in marine gastropods species (Jacobsen & Stabell 1999; Lam 2002; McKillup & McKillup 1993). Gastropods have developed a suite of sophisticated chemoreception systems which allow them to detect and respond to chemical cues that indicates risk of being predated (Jacobsen & Stabell 1999; Lam 2002).Prey escape and avoidance response in marine gastropod species can ranged from increase movement rates, seeking shelter to avoid detection (i.e. self-burial) to movement upwards on vertical surfaces (negative geotaxis) in order to increase the chances of survival (Lam 2002; McKillup & McKillup 1993).

Many marine invertebrates, including gastropod species, were reported to be able to detect predators indirectly by responding to species-species signaling substances (i.e. pheromones) in the effluent from damaged conspecifics (Jacobsen & Stabell 1999; McKillup & McKillup 1993) These characteristic responses can vary vastly depending on the species involve (McKillup & McKillup 1993). For some, such as a number of hermit crab species, effluent from damaged conspecifics presents an opportunity to acquire a larger shell and hence they are attracted to it despite the risk of being predated (Rittschof et al. 1992). In many others, it has been suggested that the signaling substances in effluent is perceived as an alarm cue suggesting the presence of predators within the vicinity, triggering escape and avoidance responses to the stimulant (Jacobsen & Stabell 1999; McKillup & McKillup 1993). 

This study aims to examine the behavior of Planaxis sulcatus in response to effluent of conspecifics collected from Heron Island on the Southern Great Barrier Reef, Australia under laboratory conditions. It is hypothesis that the effluent of conspecifics will trigger an escape response from P. sulcatus, similar to that observed in Fiji on the Heron Island population (McKillup & McKillup 1993).

Approach

This study was performed on the population of P. sulcatus inhabiting the beachrock on the Southside of Heron Island, Southern Great Barrier Reef, Queensland, Australia (23°26′31.20″S 151°54′50.40″E) in October 2011. Prior to commencement of experiments, P. sulcatus were collected from the field and kept in an aquarium with running seawater and were not fed throughout the duration of captivity. Only active (moving) adult P. sulcatus, with an average shell length of 2.5 cm were used for the experiments. All experiments were performed under controlled laboratory conditions in standard plastic food containers measuring 17 cm (L) X 10 cm (W) X 7 cm (H) containing still, fresh seawater filled to a height of approximately 3 cm (H). Two treatments: addition of crushed P. sulcatus juice in seawater (treatment) or addition of fresh seawater (control), were each replicated six times and assigned at random to individual P. sulcatus specimens. Each specimen was used only once, subjected to only one of either treatment. P. sulcatus juice was prepared by crushing four live P. sulcatus with the shells removed, using a mortar in 8 ml of seawater. Individual P. sulcatus were placed in the middle of the treatment container and allowed to rest (i.e. foots stuck to the bottom of the container and antennae extended) before any treatment were administered. Before each experiment, the containers were washed and filled with fresh seawater to remove any remnant odour from the previous experiment. 1 ml of juice or seawater were introduce into the experimental setup from the bottom right corner of the container, with a single individual being tested each time. The behavior of each test individuals was observed and any movements made were tracked using a marker, timed and recorded.  An individual was deemed to have completed executing a behavior when it remained at a position motionless for > 3 minutes. In control treatments, where individuals were just moving randomly, without any clear direction or motive, the movements of the individuals were tracked over a period of 15 minutes. A picture is then taken of the path traveled by the individual and the distance covered by the individual is calculated using ImageJ image processing program as shown in Figure 1. The speed of the individual is obtained by dividing the distance travel over the time taken to cover that distance. Data analysis was performed using Student’s T-test to determine any statistical significance.