Influence of chemically mediated predator perception on the foraging behaviour of Conuber sordidus (Gastropoda: Naticidae)

Introduction

An infaunal predator, the Naticidae are a cosmopolitan family of marine gastropods that take up permanent residence within soft-bottom communities (Ambrose 1991). Commonly referred to as sand snails, members of the Naticid family are recognised for their unique shell-drilling behaviour and is known to have a strong influence on community structure and species diversity (Huelsken 2011; Visaggi et al. 2013)

Conuber sordidus (Leaden sand snail) is indigenous to Eastern and Southern Australia and is an abundant inhabitant of the shallow intertidal zones of the Moreton Bay Marine Park (Davie 2011). C. sordidus are active predators that remain submerged in soft substrate while pursuing their prey and feed primarily on bivalves or other marine snails, although, they have recently been documented feeding on soldier crab species (Davie 2011; Huelsken 2011; Visaggi et al., 2013).

Thomas Huelsken (2011) documented the predation method of C. sordidus at North Stradbroke Island (Queensland, Australia) and Port Welshpool (Victoria, Australia). Travelling at or below the sediment surface, when it is close enough to its prey, it will reach for it with its large propodium (Fig. 1A) then wrap the prey within its foot (Fig. 1B). Once secured, the snail covers its prey in a thick mucus which attaches it to the snail’s mesopodium (Fig. 1C). It then re-buries itself in the sand to feed (Fig. 1D).

The commercial importance of certain epibenthic predators (fish, crabs, and birds) has meant that most research has historically been directed toward them when establishing the role of predation on community structure (Ambrose 1991), but there is increasing interest in the role of predators within infaunal or soft-substrate communities (Ambrose 1991). As well as potentially playing a keystone role in community organisation, infaunal predator influence is reflected in prey population density, abundance, size and age composition and affects their spatial and temporal distribution (Ambrose 1991; Dalesman et al., 2006).

The ability to sense chemical information from your environment (chemoreception) was likely one of the earliest senses to evolve and is thought to play an important role in predator-prey interactions (Breed and Moore 2016). Many aquatic gastropods use chemosensory cells on the surface of their foot and a sensory organ called an osphradium to analyse chemical information from their environment (Hickman et al. 2014; Rochette et al. 1998). These chemical cues mediate many of their essential behaviours such as feeding and mating and evidence has been found to suggest that these behaviours may be affected by the presence of predators (Dalesman et al. 2006; Rochette et al. 1998).

Although it is an active predator, C. sordidus is slow moving when compared to other molluscivorous predators and is found in a marine environment where visual information may be hindered by turbidity or vegetation (Davie 2011). As such, it is assumed that C. sordidus would rely predominantly on water-borne chemical cues for information about its environment, making it an ideal organism to determine the role of chemical cues in assessing predation risk.

The aim of this study was to contribute to current knowledge around the behavioural ecology of predators within infaunal or soft-substrate communities by using C. sordidus samples in a laboratory setting to (1) determine the importance of visual cues in marine predator-prey interactions, (2) assess the ability of C. sordidus to form cue associations and, (3) determine the effect these cue associations have on the behaviour of C. sordidus. It is predicted that (1) C. sordidus will perceive the chemical cue of their predator and (2) demonstrate anti-predator responses such as evasive, escape behaviour.

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Predation-related chemical cues were simulated using mucus from the skin of Arothron hispidus (family Tetraodontidae), a higher order predator common in the Moreton Bay Marine Park.

A. hispidus are found throughout the Indo-Pacific area from depths of three to 35 metres. An active predator, their diet includes calcareous or coralline algae, molluscs, tunicates, sponges, corals, zoanthids, crabs, polychaetes, starfish, urchins, krill, and silversides (Davie 2011). For the purpose of this study, A. hispidus is an assumed predator of the C. sordidus snail.

Thirty-six C. sordidus specimens and mucus from four A. hispidus samples were collected during low-tide from throughout the intertidal zone of Dunwich, North Stradbroke Island (-27° 29′ 59.99″ S, 153° 23′ 59.99″ E).

C. sordidus were kept in holding tanks overnight with ~80mm of salt-water and 20mm of soft- substrate retrieved from the collection site. “Bubble stones” were used to oxygenate the water but the temperature was not manipulated or measured.

To create the mucus extract solution, salt-water samples were placed in a centrifuge at 12,000rpm for 8 mins and suspended sediment and bacteria removed. Four cotton swabs of A. hispidus mucus were soaked in the treated salt-water samples and a vortex machine on high speed used to encourage mucus and salt-water mixing. Swabs were removed leaving 4.5ml of homogenous mucus extract solution.

Laboratory experiments were conducted in 1300 x 200 x 182 mm aquaria with a water depth of 80mm. Natural light was blocked from 20mm of each opposing end of the tank to create a “shaded” environment, commonly preferred by gastropod molluscs (Davie 2011).

Food colouring was used to determine a density gradient of approximately 25 seconds and this measure was used to determine the time-length of observations.

Two large (>30mm) and four small (<30mm) snails were placed in the centre of the tank and two drops of A. hispidus mucus extract added to alternating ends of the tank.

After six minutes, the position of the snails was recorded noting whether they had:

• moved away from the mucus;
• moved toward it;
• moved up the tank wall or;
• not moved at all.

The experiment was replicated six times including one control, each with new, untested C. sordidus specimens. Between each experiment, the salt-water was discarded and aquaria rinsed with fresh water to avoid cross-contamination.

A measure of behavioural responses in this research produced a range of qualitative records which undermined our ability to make any assumptions about the distribution of our data. As a non- parametric test makes no such assumptions, a non-parametric Chi-Squared (X2) analysis was conducted.

Results

When exposed to mucus from A. hispidus, 37% of sample specimens demonstrated anti-predator responses by moving directionally ‘away’ from the treatment, however, 43% displayed no movement response at all.

There was no significant effect of size on treatment when the response of both large (>30mm) and small (<30mm) specimens were analysed, so these results were combined (X2 = 2.81, df = 2, p = 0.246).

Our statistical analysis showed no significant influence of treatment on the directional movement response (Nothing, towards or away) of each snail (X2 =8.31, df = 4, p = 0.081) (Table 1).

Treatment was applied alternately to the left and right-hand sides of the aquaria to account for any effect of the application site. Analysis showed there was no significant effect of treatment (left) on the directional movement response (X2 =3.92, df = 2, p = 0.141), nor was there any significant effect of treatment (right) on the directional movement response (X2 =1.65, df = 2, p = 0.439) (Table 1).

Pooling results from both left and right-hand treatment application sites also produced a non- significant result (X2 =1.03 df = 4, p = 0.598) (Table 1).

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Discussion

In attempting to determine the importance of visual cues in marine predator-prey interactions, our results suggest there is no significance in the behavioural patterns of C. sordidus when exposed to predator chemical cues.

However, when interpreting this data there are several considerations that must be made before any assumptions can be drawn (1) number of control experiments (n = 1) does not compare with treatment experiments (n = 6), (2) small “observed” values and large chi-square results suggest data does not fit this model and may potentially affect the interpretation of results and, (3) experimental design could be improved to account for confounding factors such as; temperature, and absence of sedimentation.

Visaggi et al., 2013 explore the value of controlling for prey health before, during and after experimentation suggesting that research that controls explicitly for prey health seemed to minimise stress in prey specimens. Temperature is an important factor influencing the behaviour of species and is a key determinant in feeding rate (Hickman et al., 2014; Peitso et al., 1994).

As Ansell (1982) describes, Polinices alderi feeding on the bivalve Tellina tenuis is reduced from 4.8% of its body mass per day at 25°C to only 1.7% at 10°C. Edwards and Huebner (1977) found a similar result when measuring the feeding rate of drilling gastropod Polinices duplicatus, showing that feeding rate varied directly with temperature and body mass.

It can be assumed from this that the quality control and monitoring of C. sordidus whilst in laboratory settings may have significantly influenced the result of this research and the ability of C. sordidus to respond to stimuli so a greater consideration of health is recommended for future studies.

Our results may not support the hypothesis that perception of predatory chemical cues affects the behaviour of prey (C. sordidus) in the marine environment but there seems to be little doubt of the importance of chemoreception in modulating behaviour (Breed and Moore 2016). There are several assumptions that could be made from this but little known about the mechanisms behind the interactions between infaunal predators and their prey (Ambrose 1991; Huelsken 2011). It is clear, however, that these predator-prey interactions greatly influence the structure and complexity of soft-substrate communities (Ambrose 1991; Dalesman et al., 2006).

Although our data may indicate no significance in the behaviour of C. sordidus in response to predatory chemical cues, only a single predator and prey species are examined here and a small sample size has influenced results. The following is recommended for further research on chemoreception in marine gastropods in laboratory settings. (1) Substrate depths should be considered with respect to both predator and prey sizes, life habits, and especially when considering any burrowing or escape behaviours. (2) The condition of both predator and prey health should be considered, and where possible, maintained with respect to adequate food availability and monitoring of temperature, light availability and simulation of tide variation.

References

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Ansell, A. (1981). Experimental studies of a benthic predator-prey relationship. I. Feeding, growth, and egg-collar production in long-term cultures of the gastropod drill Polinices alderi (Forbes) feeding on the bivalve Tellina tenuis (da Costa). Journal Of Experimental Marine Biology And Ecology, 56(2-3), 235-255.

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