Induction of settlement in crab megalopae by ambient underwater reef sound

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Chapter Two: Induction of settlement in crab megalopae by ambient underwater reef sound

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Stanley, J. A., Radford, C. A., Jeffs, A, G. 2010. Induction of settlement in crab megalopae by ambient underwater sound, Behavioral Ecology, 21 (1):113 – 120.

INTRODUCTION

A pelagic larval phase in the lifecycle of many benthic marine organisms typically involves dispersal away from parental habitat with the final stage of larval development selecting a suitable benthic habitat in which to settle (O’Connor & Gregg, 1998). Settlement and metamorphosis often involve a specific cue or a combination of physical and/or chemical settlement cues (Gebauer et al., 2004). These cues include salinity, depth, substrate rugosity, as well as a wide range of chemical cues from sources such as conspecifics, settlement substrates, aquatic vegetation, estuarine water and potential prey. These cues can be both general and apply to many species, such as the specific chemical cues associated with conspecific adults (Uca pugilator, Uca pugnax, Pagurus maclaughlinae, Paguristes tortugae, Chasmagnathus granulata, Panopeus herbstii, and Sesarma curacaoence) (Gebauer et al., 2003) and species specific, such as the presence of certain macroalgae species for the blue crab, Callinectes sapidus (Forward et al., 1996).
The majority of studies on marine invertebrate larval settlement and metamorphosis have concentrated on species that are sedentary as adults, especially commercially important biofouling and aquaculture organisms, such as barnacles and oysters (Forward et al., 2001). By comparison, relatively little is known about settlement cues in mobile marine invertebrates, such as brachyuran crabs which are common and important inhabitants of coastal habitats around the world (Wear & Fielder, 1985).
The late-stage larvae of many marine organisms are known to be capable of extending their larval phase, often for considerable periods, until suitable settlement cues or habitats are encountered. For example, polychaetes (Wilson, 1977), gastropods (Paige, 1988), echinoderms (Strathmann, 1978) and coral reef fish (Victor, 1986; 1991) have all been shown to delay metamorphosis until appropriate settlement cues are encountered. Some larvae will metamorphose spontaneously or even die without metamorphosing in the absence of specific settlement cues (Pechenik, 1990; Zimmerman & Pechenik, 1991; Gebauer et al., 2003).
Brachyuran crabs seem to lack the ability to delay metamorphosis indefinitely as they appear to have a temporal threshold beyond which settlement and metamorphosis occurs even in the absence of settlement cues (Weber & Epifanio, 1996). To determine maximum time to metamorphosis (TTM), megalopae are typically reared in the laboratory and exposed to a control treatment of untainted seawater. The mean TTM in previous studies of brachyuran crabs has varied from 5 to 20 d depending on the species (Forward et al., 2001). In brachyuran crabs the TTM can often be shortened by 15 – 25% upon exposure to chemical cues which serve as indicators of potentially suitable settlement habitat. These chemical cues can be sourced from the presence of adults, aquatic vegetation, biofilms, conspecifics, estuarine water, humic acids, related crab species and potential prey (Forward et al., 2001). The majority of these past studies have been carried out in laboratory aquaria or compartmentalized containers. Consequently the spatial range over which these settlement cues operate in nature is largely unknown and it is assumed that other physical processes, such as tidal currents, serve to initially position the megalopae in the vicinity of these chemical cues. Therefore, the current evidence suggests chemical settlement cues are being used over small distances (m) and do not appear to acting as an orientation cue over larger distances such as on the scale of kilometres (Butman, 1987; Boudreau et al., 1993; Forward et al., 1994).
A number of experimental studies have concluded that ambient underwater sound emanating from coastal habitats may act as a long distance orientation cue for settlement stage crabs and fishes attempting to locate suitable habitats (Stobutzki & Bellwood, 1998; Tolimieri et al., 2000; Jeffs et al., 2003; Jeffs et al., 2005; Leis & Lockett, 2005; Simpson et al., 2005a; Montgomery et al., 2006; Radford et al., 2007). However, previously the role of underwater sound as a settlement cue has not been investigated. Therefore, the aim of this present research was to investigate the potential for underwater sound to trigger settlement behaviour and/or shorten TTM in late-stage larvae (megalopae) of common species of brachyuran crabs from both temperate and tropical waters.

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METHODS

The study was undertaken in temperate waters near the Leigh Marine Laboratory, located in north-eastern New Zealand (36° 15‟ S, 174° 47‟ E), and in tropical waters near the Lizard Island Research Station, north-eastern Australia (14° 39.5‟ S, 145° 26‟ E) during October to December 2008.

Source of megalopae

Light traps were used to capture megalopae for behavioural experiments (Hickford & Schiel, 1999). Up to two light traps were deployed at night within 500 m of the shoreline, 15 m apart and submerged 2 m in water of 5 – 7 m depth. The traps were recovered within 2 h of sunrise the following morning. When large planktivorous fishes were found in a light trap, megalopae were not used in the experiments as they may have altered behaviour due to stress from being in the presence of a predator (Forward & Rittschof, 2000). The megalopae were transported in seawater to a nearby laboratory where they were counted, sorted into settlement stage and identified to lowest taxonomic level possible given the available taxonomic descriptions (Wear & Fielder, 1985; McLay, 1988). Only intermoult pre-settlement (i.e., natant and active swimming) megalopae of a similar size were selected for use in the experiments. The megalopae were held in a flowing filtered (40 µm) seawater system with natural light period and ambient temperature (14 – 31° C, dependant on timing and location) until experiments begun the evening following capture. Five species of brachyuran megalopae were used for this research. The reptant phase in the lifecycle of these three temperate species, Hemigrapsus sexdentatus, Cyclograpsus lavauxi and Macrophthalmus hirtipes are all known to be associated with nearshore subtidal and intertidal habitats. The two tropical species were both identified to be members of the Grapsidae family, however, more detailed taxonomic placement was not possible due to the lack of taxonomic descriptions of megalopae and first instar juveniles of crab species in this region. The researchers observed early juvenile crabs with similar taxonomic characters to both experimental species settled in nearshore subtidal reef habitats. For the purposes of this work the species have been referred to as Grapsidae sp. one and Grapsidae sp. two.

 Behavioural assay

Each treatment and control (i.e., Sound and Silent) consisted of three replicate water baths that were used to maintain a constant water temperature for megalopae throughout the experiment. Each replicate water bath contained one Perspex container housing a group of megalopae (up to five megalopae) and one plastic vial housing a single megalopa. Individually housed megalopa were included in the experiment as a comparison with communally housed megalopae to test for any interactive effects on settlement behaviour that may exist among individuals.
Grouped megalopae were housed in a clear Perspex container (160 160 × 140.5 mm deep) with a square piece of Perspex sheet (100 × 100 × 15 mm) on the bottom imitating a settlement surface. The upper surface of the sheet had been roughened with coarse sandpaper to provide a chemically inert settlement substrate for settling megalopae. Each individually held megalopa was in a plastic vial (250 ml) with a roughened base, within the same water bath as the container holding the group-housed megalopae. Each replicate for both the sound treatment and Silent control had a weighted Sony loudspeaker inside a watertight plastic bag which was submerged in the water bath. For the Sound replicates only, a Sony CD Walkman D – EJ815 was used to continually play a 4 min loop of recorded ambient underwater reef sound into the water bath and through the acoustically transparent plastic containers holding the crabs (Gerber, 1978) (Figure 2.1).
When on a single night sufficient (>30) megalopae of the same species were collected from the light traps to conduct the experiments they were randomly allocated to the experimental treatment or control and replicates within these. Both grouped megalopae and the individuals in each treatment were kept in filtered (1 µm) and UV treated seawater under natural light period and ambient water temperature (14 – 31° C, depending on local temperature) for the duration of the experiment. Sound treatment and Silent control were randomly allocated to water baths for each experiment. Both the treatment and control were located in the same laboratory, but were acoustically isolated using foam rubber mats beneath all water baths to prevent transfer of any external acoustic energy. The absence of any significant acoustic signal in the Silent control tanks was confirmed by recording with a calibrated hydrophone (High Tech, Inc., HTI–96–MIN). In the absence of an anechoic chamber, all laboratory-based experiments were conducted in a quiet concrete floored and walled laboratory.
The megalopae were added to the experiment at 1700 h on the day of their capture and the CD Walkman was switched on to initiate sound in the Sound treatment. Subsequently every 6 h the behaviour of the megalopae were observed, at this time counts were made of the number of individuals that had settled to the substrate and metamorphosed into the first instar benthic juvenile stage. When settlement and metamorphosis occurred in the group containers, first instar juveniles were removed to prevent cannibalism. The period of observation lasted no more than 20 min for both treatment and control. In the current study „settlement‟ is defined as a behavioural process which involves movement out of the water column to a benthic substrate, and „metamorphosis‟ as a physiological process which includes loss of larval characteristics retained in the megalopa and the transformation to the adult reptant body form (Hadfield, 2000; Forward et al., 2001). During each 6 hourly observational period descriptions of behaviour of each megalopa was categorized in the following manner. 1 – „Normal‟ pre-settlement swimming behaviour, i.e., Highly active swimming, low number of downward swimming events , no exploratory crawling behaviour; 2 – Medium swimming activity, medium number of downward swimming events, small amount of exploratory crawling behaviour; 3 – Low swimming activity, high number of downward swimming events, extensive exploratory crawling behaviour; 4 – Complete settlement and metamorphosis, i.e., no swimming activity.
The experiment was terminated when all experimental megalopae in both treatment and control had metamorphosed. The settled juvenile crabs were kept for 10 d following the experiment in flowing seawater and fed, and were monitored for post-experimental mortality.

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Chapter One: General Introduction 
1.1 Introduction
1.2 Objectives and structure of thesis
Chapter Two: Induction of settlement in crab megalopae by ambient underwater reef sound
2.1 Introduction
2.2 Methods
2.3 Results
2.4 Discussion
Chapter Three: Unique habitat acoustic signatures 
3.1 Introduction
3.2 Methods
3.3 Results
3.4 Discussion
Chapter Four: Settlement response to ambient underwater sounds associated with different habitat types 
4.1 Introduction
4.2 Methods
4.3 Results
4.4 Discussion
Chapter Five: Behavioural response thresholds in New Zealand crab megalopae
5.1 Introduction
5.2 Methods
5.3 Results
5.4 Discussion
Chapter Six: General Discussion 
6.1 Overview
6.2 Ambient underwater sound as a novel settlement and metamorphosis cue
6.3 Unique acoustic signatures
6.4 Distingushing among settlement habitats
6.5 Rates of metamorphosis
6.6 Behavioural thresholds
6.7 Anthropogenic noise impacts
6.8 Conclusions and future directions
List of References.
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