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Infaunal hydraulics generate porewater pressure signals.

Introduction

Many activities by infauna, including burrowing and feeding, involve hydraulic mechanisms. Peristaltic irrigation, burrowing, and expansion of soft tissue for use as anchors are common examples (Trueman, 1975). Such activities are known to exert compression forces on sediments; these can be measured using force plates and are a function of the alteration in diameter of the expanded tissue (Trueman, 1967; Trueman and Ansell, 1969; Quillin, 1998, 1999). Additionally, there are nonhydraulic forces produced by organisms such as thalassinid crustaceans compacting their burrow walls, bivalves opening their valves in opposition to the compressive forces of the surrounding sediments, etc. All of these activities, particularly those that move sediments, involve forces exerted on sediments, and would be expected to generate pressure signals in the porewater as well as vibrational signals associated with grain movements. This paper describes pressure signals, generated in the field by the activities of infauna, that are detectable at up to 50 cm from the organism and can be assigned both to a species and to an activity on the basis of amplitude, period, and component waves.

In laboratory studies of burrowing in molluscs and polychaetes, low-frequency (0.1-1 Hz) porewater pressure signals with an amplitude of 100 Pa were observed to propagate over distances of 10 cm through sediment and were attributed to the forces applied to sediments by the organism during burrowing (Trueman, 1966a, b, c, 1967, 1975; Trueman and Ansell, 1969). We hypothesized that such signals could propagate over relatively long distances in natural sediments, because of their very low frequency. The propagation of acoustic signals through sediments is dependent upon frequency, with low-frequency signals having lower attenuation than high-frequency signals. The relationship between attenuation in dB/m and the log of the frequency in the range [10.sup.-2]-[10.sup.7] Hz is roughly linear, with a slope of 1.0 (Kibblewhite, 1989). This low attenuation is the basis for the use of low-frequency "boomers" for subsediment profile acoustic imaging. The behavior of at least one infaunal predator, Glycera alba, suggests that it uses mechanoreceptors for prey localization and vibrational signals as the information source (Ockelmann and Vahl, 1970); other epifaunal predators are also known to be sensitive to such signals (fish: Janssen, 1990; Karlsen, 1992; shorebirds: Piersma et al., 1998). The existence and potential use of interstitial pressure transients by infauna and their predators greatly enlarges the range of available sensory modalities and functions of structural receptors. In addition, the existence of interstitial pressure transients may allow researchers to monitor the activities of infauna remotely and nondestructively as, for example, conditions deteriorate due to anthropogenic effects or increased predator activity.

Materials and Methods

To determine the existence of porewater pressure transients, stainless steel pressure sensors (Sensym ICT Series 19/SPT, 5 psi gauge) were implanted to a depth of 10 cm in intertidal sediments in the vicinity of active burrows of annelids, molluscs, and crustaceans. These sensors have a response time of 0.1 ms to a step change of pressure from 0 to 34.5 kPa, indicating a reliable upper frequency limit of 1 kHz. The pressure reference port was connected to the inside of the sealed watertight housing of a data logger by a polyethylene tube inside the electrical signal cable. The repeatability of pressure readings is 0.01% of full-scale span (3.45 Pa). The sensor openings were filled with seawater and covered with 100-[micro]m Nitex mesh. Pressure sensors were calibrated by placing them in a 1-m-deep water tank, which was filled to different depths above the diaphragm of the sensor (0 to 40 cm in increments of 2 to 5 cm, 1 min at each depth) with ambient seawater. When the sensor is submerged to a depth of 5 to 10 cm, this system can detect both positive and negative deviations from ambient pressure. The analog circuitry was powered by a 3.3-V regulated power supply on the data logger. Excitation of the sensor's Wheatstone bridge was provided with a separate precision voltage reference source on the same chip as the instrumentation amplifier (Burr Brown INA 125), digitized at 40 Hz (CF2/R216 data logger, Persistor Instruments, Bourne, MA), and recorded on compact flash cards. The electronics package, powered by three alkaline D cells, was contained in an aluminum housing, and was able to record continuously on four channels for 4 weeks. To test for the possibility of aliasing of signals, we also sampled at 250 Hz and 1 kHz. The power spectra of pressure signals sampled at all three rates indicated only negligible energy at frequencies higher than 1 Hz. There was no evidence of aliasing in the power spectra or in the time-domain data. Porewater pressure waveforms from animal behaviors were the same, regardless of sampling frequency. Because our data logger would not record reliably at rates above 1 kHz, we could not determine the propagation velocity of the pressure transients, so we do not know whether the transients are infrasound (compressional waves, 1500 [ms.sup.-1] velocity) or shear waves (300 [ms.sup.-1] velocity).

To determine the characteristics of particular species, pressure signals were recorded in the laboratory from isolated individual infauna placed in 20-cm-deep cylindrical plastic tubs of sediment or in thin-walled aquaria, completely immersed in running seawater. The sediment was 15 cm deep in the tubs, which were 20 cm in diameter. There was an air-water boundary at the top of the seawater tank. The sediment surface was photographed simultaneously with the pressure recordings to allow determination of initiation and completion of burrowing, as well as defecation and other modifications of surface topography. Additional measurements were made in aquaria, 15 cm deep, 1 cm thick, and 30 cm wide, made of two sheets of 1.25-cm-thick acrylic plastic and a separator gasket of 1.5-cm-diameter Tygon vinyl tubing. A pressure sensor was screwed into a threaded hole in one side at a depth of 7 cm. Time-lapse images of animal activities taken through the large side of the aquarium were synchronized with the pressure record. In all cases, the dimensions of the laboratory tanks were much smaller than one wavelength of a pressure waveform. Wavelengths of 0.1-Hz waveforms are on the order of 30 to 150 m, depending on whether the waves are surface or compressional, and our tanks were about 0.2 m in size.

A medical ultrasound imager (Teratech Corp, Burlington, MA) was used in the laboratory to observe water flow directions and velocity distributions during defecation by the arenicolid polychaete Abarenicola pacifica. A linear 5- to 10-MHz, 128-element transducer (10L5) was clamped directly above the tail shaft opening of an Abarenicola burrow, and was used in directional power Doppler mode to visualize flow fields in the burrow.

Field recordings were made by implanting the pressure sensors in a muddy sand habitat at False Bay, San Juan Island, Washington (48.4900N, 123.0662W) at a shore level of 2.5 m above chart datum. Maximum water depth above the sensors was 2.5 m. Sensors were deployed in a bed of Abarenicola pacifica (20 to 50 individuals [m.sup.-2]) and in areas with Neanthes brandti and Macoma nasuta. Each data logger had four sensors: three were implanted in the mudflat, and one was attached to the logger housing and used as a surface pressure reference. Plugs of sediment were removed using a 20-ml plastic syringe whose end had been cut off. Sensors were placed in the resulting holes, and the sediment plug was dispersed around the walls of the sensors to seal the gaps. Sensors were separated by 25 to 60 cm and were placed 5 to 10 cm away from traces of infaunal animal activity, such as fecal piles, tube openings, and siphon openings. Placement closer to animal tubes incurred the risk of killing the animal with the syringe corer during implanting of the sensors. Data cards were replaced in the data loggers at regular intervals, and sensors were left in place for 5 to 30 days at a time. Porewater pressure signals in the field were detectable only after sediment had sealed around the sensors, usually within the first tidal immersion cycle.

Pressure signal magnitudes are reported in pressure (pascals, peak to peak), and in sound pressure level, SPL = 20 log (p/[p.sub.ref]), where [p.sub.ref] is the standard underwater pressure reference of [10.sup.-6] Pa, and p is the root mean square (RMS) amplitude of the waveform. RMS amplitudes of waveforms were calculated after smoothing the 10-Hz recordings with a 20-point (2 s) moving average filter to remove high-frequency noise.

Spectral analysis of the defecation signals was carried out using the Speech Filing System (Huckvale, 2004) and the Hidden Markov Toolkit (Young et al., 2002). Spectral characteristics were calculated using a set of second-order Butterworth band-pass filters, centered on 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.17, 0.19, 0.22, 0.24, 0.27, 0.3, 0.33, 0.37, 0.4, 0.45, and 0.5 Hz. The filter bank was derived from a 19-channel vocoder (Huckvale, 2004) with frequencies spaced approximately linearly on a Mel scale. Mean filter bank spectral densities were calculated using the Hidden Markov Toolkit (Young et al, 2002). Traditional spectral densities were calculated using a triangular window with weights 123454321. All spectra were normalized by signal variance, so spectral densities are dimensionless (Box and Jenkins, 1976).

Results

The bivalve mollusc Macoma nasuta and the polychaete annelids Neanthes brandti and Abarenicola pacifica all generated very different interstitial pressure waveforms during burrowing (Fig. 1). The bivalve Macoma created strong positive pressure pulses, at a rate of about 1-2 [min.sup.-1], with a maximum magnitude of 160 Pa. The sound pressure level was 148 dB measured 10-20 cm away (Fig. 1, top). The bivalve also caused rapid increases in porewater pressure with a sound pressure level of 155 dB during siphon relocation and protrusion out of the sediment (Fig. 4). The polychaete Neanthes created pressure waveforms at a rate of about 2 [min.sup.-1] during burrowing, and at a rate of 1 [min.sup.-1] during subsequent burrow irrigation (Fig. 1, trace C: compare signals during first 10 min to those in second 10 min), with maximum pressures of 100 Pa and a sound pressure level of 140 dB during burrowing and 50 Pa (SPL = 136 dB) or less during irrigation. The polychaete Abarenicola created pressure cycles at a rate of 2 [min.sup.-1] during initial probing of the surface and 1 [min.sup.-1] during burrowing (Fig. 1, trace D: compare signal from min 7-12 to min 13-21), with maximum pressures of less than 30 Pa (SPL = 133 dB). The burrowing signals of these three species are all very different from one another, and readily distinguishable.

One of the most common signals in the field recordings was that associated with defecation by Abarenicola. It is a very characteristic signal in that Abarenicola creates cycles of increased interstitial pressure at a rate of 4-6 [min.sup.-1] before and after defecation (Fig. 2, top, before and after min 10), with a rapid decrease in interstitial pressure during expulsion of fecal castings (Fig. 2, min 10). The increases in pressure are associated with flow downward in the tail shaft of the Abarenicola burrow (Fig. 2, bottom left), while the decrease in pressure is associated with upward flow in the tail shaft of the burrow (Fig. 2, bottom right). Large individuals created negative pressure transients of magnitude 400 Pa during defecation and ventilation waveforms, with peak-to-peak magnitudes of 250 Pa. Sound pressure levels were 156-163 dB during ventilation and defecation (Fig. 2), measured at a distance of 5 cm in the laboratory.

Field recordings of pressure signals from undisturbed high intertidal sediments were made at False Bay, San Juan Island, Washington. Porewater pressure signals were detectable during conditions when the sediment was either covered with water or recently drained. As one would expect for porewater pressure transients, signals were not detectable from fully drained sediments, and were not detectable in the overlying water. This habitat was chosen because its predominant large infauna are the polychaetes Abarenicola pacifica and Neanthes brandti and the tellinid bivalve Macoma nasuta, all species that we had recorded signals from in the laboratory. Burrowing, irrigation, and defecation activities were evident in the field recordings (Fig. 3). The top two traces in Figure 3 are most likely defecation activities by a single Abarenicola individual located 5 cm from one sensor (A) and 25 cm from the other (B). These signals are very similar in shape to those we recorded in the laboratory for Abarenicola and those Wells (1949) recorded for Arenicola marina, but they occur at 5-min intervals rather than at the typical 15-min intervals of defecation for A. marina. Krager and Woodin (1993) previously reported a mean defecation interval of 22 min for Abarenicola, but they noted that smaller individuals defecated significantly more often than large ones. Arenicola marina is on average at least 10 times the size of Abarenicola pacifica. These records indicate that pressure signals propagate in excess of 20 cm within sediments. The average difference in RMS sound pressure level between the signals detected by the two sensors was 6.8 dB (standard deviation 1.2, n = 12). This is an attenuation rate of 34 dB [m.sup.-1], assuming that the difference in distance between the source and the two sensors was 20 cm.

Burrowing activities of nereidid polychaetes (Neanthes brandti) are evident in the field records (Fig. 3, trace D), based on comparison with the waveforms seen in the laboratory (Fig. 1, traces B and C). Since the signal in Figure 3 was only picked up by one of the four pressure sensors, the animal was presumably out of range of the other sensors.

Other animals generate different kinds of signals. For example, the siphon movements of surface deposit-feeding bivalves like Macoma nasuta generate large positive pressure transients (Fig. 4, min 2 to 12, sound pressure level 155 dB). These pressure pulses are associated with creation of new holes on the sediment surface (Fig. 4, top). When Macoma moves its siphon, it uses a water jet to penetrate the sediment, and this jet is associated with the positive porewater transients.

To determine whether signal spectra were similar between and within individuals, multiple Abarenicola defecation events for one individual were compared to those from four separate individuals. Frequency spectra calculated from band-pass-filtered defecation signals indicate that most of the signal energy is below 0.2 Hz (Fig. 5). Signal spectra from single individuals in thin-walled aquaria and from groups of four individuals in rectangular aquaria are strikingly similar (Fig. 5).

Discussion

Because the hydraulic signals from different species differ in amplitude, frequency, and waveform (Figs. 1, 2; Trueman, 1966a, b, 1975; Wells, 1949, 1953), they represent a potential means of communication among infaunal organisms. The propagation of these signals over distances of tens of centimeters provides a mechanism for detection of neighbors, competitors, predators, or mates. This mechanism is in a sense analogous to the hydrodynamic wakes left behind by swimming zooplankton, which are used by predators to detect prey (e.g., Strickler, 1977; Yen and Strickler, 1996) at distances of tens of body lengths. We expect that signals characteristic of predator burrowing would generate an escape response in prey, that signals of prey burrowing would attract predators, and that conspecific activity signals would generate an attraction response among mates during the spawning season. Infrasound responses are known from many animals (Hill, 2001), including scorpions (Brownell, 1977; Brownell and van Hemmen, 2001), elephants (Payne et al., 1986; Langbauer et al., 1991; Laron et al., 1997; Garstang, 2004), birds (Montgomerie and Weatherhead, 1997; Piersma et al., 1998; Hagstrum, 2001; Mack and Jones, 2003), fishes (Karlsen, 1992; Sand and Karlsen, 2000; Sand et al., 2000), and clams (Ellers, 1995). Of particular interest is the response of infaunal predators to low-frequency pressure signals. For example, the shorebirds known as red knots, Calidris canutus, have pressure sensors in their beaks, and they appear to use a low-frequency analog of active sonar for detecting clams (Piersma et al., 1998). Since clams also produce their own porewater pressure signals, red knots may also be able to detect them passively. Sea stars, glycerid polychaetes, and freshwater cottid fish may detect prey by vibrational cues (Doering, 1982; Ockelmann and Vahl, 1970; Janssen, 1990).

Additional examples of predators that may detect infaunal prey by either vibrational cues or pressure cues include flatfish. In the Wadden Sea, most of the diet of juvenile flatfish consists of the siphons of clams and the tails of worms (de Vlas, 1979, 1985), indicating that flatfish can detect the protrusion of siphons and tails from the sediment very effectively. The flatfish Pleuronectes platessa detects vibrations in the water column with its otoliths, is sensitive to infrasound in the 0.1-Hz range (Karlsen, 1992), and has a sensitivity threshold to particle acceleration of 3 X [10.sup.-4] m [s.sup.-2], which Sand and Karlsen (1986) equate to a sound pressure level of 180 dB. This is equivalent to the source level at the loud end of whale vocalizations. Other fish have similar sensitivities (Karlsen et al., 2004). Our laboratory measurements of porewater pressure transients from clam siphon movements had peak-to-peak amplitudes of 400 to 500 Pa, or an approximate sound pressure level of 157 dB at a distance of 20 cm, and our field measurements of burrowing signals from worms had amplitudes of 100 Pa, a sound pressure level of about 150 dB at similar distances. Defecation by Abarenicola had a peak-to-peak amplitude of about 400 Pa, or a sound pressure level of 163 dB at a distance of 5 cm. These signals are below the sensitivity threshold of flatfish in the water column; however, when searching for prey, these fish lie on the sediment surface, where they could possibly detect pressure or vibration from subsurface activities for prey location. Donax variabilis, a coquina clam, responds to pressure signals in the range of 20 Pa, or a sound pressure level of 140 dB (Ellers, 1995), an amplitude much lower than that propagating from the burrowing of worms and clams, indicating that infaunal invertebrates can probably detect one another within the sediment. Given our measured attenuation rate of 34 dB [m.sup.-1], we expect that coquina could detect Abarenicola defecation at a distance of 60 cm. An attenuation of 6.8 dB 20 [cm.sup.-1] yields 20 dB attenuation at a distance of 60 cm; an Abarenicola source level of 163 dB minus the 20 dB attenuation yields 143 dB of sound pressure at a distance of 60 cm, slightly in excess of the 140-dB sensitivity of the coquina.

The porewater pressure signals generated by infauna propagate within the sediment, but it is not clear whether they propagate into the overlying water. They are detectable with our porewater pressure sensors only when there is a good seal between the sediment and the sensor. Our sensors in the overlying water do not detect the infaunal signals. The porewater pressure signals are likely to be associated with surface vibrations, however, as a result of movement of sediment grains. Most infaunal activities that compress or fluidize sediments will also move sediments. High population densities of active infauna on the sea floor could therefore be a source of low-frequency vibrational signals, which may contribute to the large background "noise" level in the seismic frequency range in the ocean.

Acknowledgments

Supported by the Office of Naval Research (N00014-0310352). We thank Art Illingworth and Allen Frye for their skilled machining of instrument housings. Roberta Marinelli for interesting discussions, Sarah Berke for assistance in the field, and Clint Cook for assistance with graphics. The data logger system development was partially supported by the National Science Foundation (IBN 01-31308). We thank two anonymous reviewers and Michael LaBarbera for very helpful comments on the manuscript.

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DAVID S. WETHEY* AND SARAH ANN WOODIN

Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 29208

Received 17 February 2004; accepted 12 August 2005.

* To whom correspondence should be addressed. E-mail: wethey@biol.sc.edu
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Author:Wethey, David S.; Woodin, Sarah Ann
Publication:The Biological Bulletin
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Date:Oct 1, 2005
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