Effectsofenvironmentonelectricﬁelddetectionbysmall...

Effects of environment onelectricfielddetectionby smallspotted catshark Scyliorhinus canicula (L.)

J. L. FILER*, C. G. BOOKER* AND D. W. SIMS*†‡

*Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill,Plymouth PL1 2PB, U.K. and †School of Biological Sciences, University of Plymouth,

Drake Circus, Plymouth PL4 8AA, U.K.

(Received 22 January 2007, Accepted 9 January 2008)

This study evaluated the influence of environment (substratum type and depth) on the

electroreception capabilities of small spotted catsharks Scyliorhinus canicula in response to

prey-simulating electric fields. In experiments where electric fields (applied current 15 mA) were

presented beneath different substrata (sand, pebbles, rocks and control) it was found that search

effort was not different between substrata or S. canicula sexes, however, both rates of turning

and biting towards active electrodes were decreased over pebbles and rocks compared with sand

and the control (no substratum). There was no significant effect of sex on turn and bite rates

over any substrata. Electric fields were then presented beneath different depths of sand to

examine the depth-limits of fish electroreception. Turn and bite rates were significantly lower at

depths below 10 mm, with no bites towards electrodes made when they were >30 mm depth.

Search effort was not found to be different between different burial depth treatments or between

sexes. These results indicate substratum type and depth influences the ability of S. canicula to

detect prey-simulating electric fields. This variation in electroreceptive performance may

influence space use of sharks. # 2008 The Authors

Journal compilation # 2008 The Fisheries Society of the British Isles

Key words: elasmobranch; electroreception; foraging behaviour; habitat selection; movement;

space use.

INTRODUCTION

Some species of predatory fishes are capable of detecting the minute electricalfields emanating from living organisms over distances between 100–150 mmand 1–2 m, depending upon the organism and surrounding medium. Electrore-ception is used by a diverse range of teleost and non-teleost species for huntingand, or spatial orientation (Watt et al., 1999). The ability of elasmobranchs touse this electroreceptive sense in order to detect, locate and capture prey hasbeen well documented in the literature from experimental studies (Kalmijn,1971, 1974, 1982; Dawson et al., 1980; Blonder & Alevizon, 1988; Montgomery& Bodznick, 1999). In a foraging context electroreception can be considered

‡Author to whom correspondence should be addressed. Tel.: þ44 (0) 1752 633227; fax: þ44 (0) 1752

633102; email: [email protected]

Journal of Fish Biology (2008) 72, 1450–1462

doi:10.1111/j.1095-8649.2008.01807.x, available online at http://www.blackwell-synergy.com

1450# 2008 The Authors

Journal compilation # 2008 The Fisheries Society of the British Isles

a near-distance sense that enables close target location of prey without the needfor visual or other sensory clues, for example, when prey is hidden beneath thesubstratum of the seabed (Kalmijn, 1971). The effects of physical environmenton the electroreceptive ability of elasmobranchs, however, has received rela-tively little attention in studies of fish electroreception, even though such influ-ences may affect which habitats are selected during foraging by free-rangingsharks, skates and rays.The ability to detect bioelectric fields makes even hidden prey available, but

the environment is likely to affect detection ability by altering the distance overwhich stimuli can be detected. This may occur through high ‘backgroundnoise’, or the damping or distortion effects of the physical environment (e.g.substratum type) on electric fields. For example, in the marine environmentthere are electrical fields of biotic origin (bioelectric fields), but also of abioticorigin, including those caused by water mass movements through the Earth’smagnetic field (Kalmijn, 1982; von der Emde, 1998). In addition, the predatoritself will generate its own bioelectric fields through body movements (Bullock,1973; Montgomery, 1984; Kalmijn, 1987; Bodznick et al., 1992; Montgomery &Bodznick, 1999). Thus, if the level of background noise in an environment ishigh, this may hinder the ability of elasmobranchs to locate accurately hiddenprey that they may not be able to locate using vision, chemoreception or mech-anoreception. Sharks are known to filter ventilatory noise and still respond tosmall localized dipole electrical fields (Bodznick et al., 1999; Montgomery &Bodznick, 1999). Therefore, elasmobranchs have mechanisms for tuning in tothe signal of prey items against other background noise. Nevertheless, levelsand types of background noise may influence this filtering ability.Aspects of natural habitats such as substratum type may modify, distort or

mask the electrical signal of prey in different ways, thereby affecting the effi-ciency of the filtering and detection capacity of elasmobranchs in habitats withdifferent characteristics. Examples of substrata that may have a high potentialto modify electric fields include pebbles or rocks as these substrata are denseand may contain metals with magnetic properties, which could cause distor-tions in the electrical fields emanating from prey. Therefore, habitats contain-ing these substrata may have a high potential for modifying electric fields.Habitats containing substrata like sand, however, may have a low potentialto modify electric fields since sand is highly porous. Consequently, the effi-ciency of filtering and detection of electrical fields may be enhanced in thesehabitats and thus could be more favourable foraging habitats. Since habitatselection is a key process influencing population distribution and abundanceof fishes including elasmobranchs (Sims et al., 2001), understanding the factorsthat influence habitat selection, such as the influence of the environment onelectroreception, may aid the interpretation of observed free-ranging foragingmovements and spatial distribution patterns (Sims, 2003).To investigate the potential role of electroreception in the decision-making

processes of foraging habitat selection this experimental study, under controlledlaboratory conditions, examined the influence of the environment on electricfield detection capabilities of the small spotted catshark Scyliorhinus canicula(L., 1758). The aim was to determine the effects of different substratum typesand depths on the ability of small spotted catsharks to detect and locate

ELECTRIC FIELD DETECTION BY CATSHARKS 1451

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Journal compilation # 2008 The Fisheries Society of the British Isles, Journal of Fish Biology 2008, 72, 1450–1462

prey-simulating electric fields. Since the substrata used reflect those found intheir natural habitats the responses to electric fields within the substrata mayprovide clues as to why certain habitats are chosen for foraging over others.A secondary aim was to examine whether responses differed between the sexes.

MATERIALS AND METHODS

FISH COLLECTION AND MAINTENANCE

Adult male and female S. canicula (500–750 mm total length, LT) were collected inotter trawls in the western English Channel off Plymouth (50–51° N; 04–05° W), as partof the standard trawling programme undertaken by the Marine Biological Association.The fish were transported to the laboratory where they were maintained in large rect-angular aquaria (length, 4�8 m; width, 1�7 m; depth, 0�5 m) with filtered flow-throughsea water. Water temperature was maintained at 14° C throughout the investigation.They were fed a diet of chopped whiting Merlangius merlangus (L., 1758) and squidAlloteuthis subulata (Lamarck, 1798) five times per week from Monday to Friday for2 weeks. In the third week food was withheld prior to a trial, whereafter each fish wasfed normal rations.

EXPERIMENTAL EQUIPMENT

The experimental arena was separated into two halves (one for holding and one asthe test arena) by a PVC partition with an upward sliding door to control access, withplastic mesh screens to maintain water flow between sections. During the 3 week exper-imental period the door was left open to allow free movement of fish between bothsides, however, during experiments the door remained closed.

A stimulus generator similar to that used by Kajiura & Holland (2002) was con-structed to produce prey-simulating electric fields within the test arena. The stimulusgenerator consisted of a 12 V direct current (DC) circuit that passed current throughthe sea water that acted as a series resistor. In the circuit there were three variable re-sistors and a number of resistors, which allowed the experimenter to vary the strengthof the electric stimulus. There were also a number of switches that allowed any one ormore of four electrode pairs (dipoles) to be activated. An ammeter in series enabled theamount of current supplied through the circuit to be monitored.

The current was supplied to each of four dipoles through salt bridges. Each saltbridge comprised two 600 mm lengths of seawater-filled polyethylene tubing (Tygon,3�2 mm internal diameter). These tubes were tightly sealed to underwater cables(20 AWG SO) via underwater connectors with gold-plated stainless steel pins (PDMNeptec, MCBH). The ends of the tubes were pulled snugly over the rubber sleeves thatpartially encased the pins of the underwater connectors to avoid current leakage. Theother end of the cable was connected to the stimulus generator. The ‘open’ ends of theseawater-filled tubing were covered by fine plastic mesh to prevent blockage by sedi-ment and inserted into pre-drilled holes within an acrylic housing for the electrodearray (Fig. 1). These holes were arranged in four pairs equally spaced from one another(300 mm) with each of the holes within a pair separated by 10 mm, mimicking the sizeof naturally occurring prey of S. canicula. An odour delivery tube was also inserted intoa pre-drilled hole in the centre of the electrode array. This allowed an olfactory cue tobe released via a 100 ml syringe at the edge of the tank into the water surrounding thearray at the start of each trial. The olfactory cue consisted of a seawater solution con-taining the ‘juice’ from a mixture of chopped whiting and squid.

The array housing was 0�6 � 0�6 m in area and 80 mm deep, with the top-side 40 mmdeep to hold the substrata and the bottom side 30 mm deep to allow the tubes to bendwithout being compressed by the mass of the array and substratum. This electrode

1452 J . L . FILER ET AL .

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array housing was fixed in the centre of the test arena. Rocks were placed around theedges of the array to facilitate movement of individuals over the housing.

A CCTV camera (Panasonic WV-BP122E with Computar H2Z4515CS wide-anglelens) was fixed directly above the array in the test arena and video recordings of alltrials were made. Dark, waterproof fabric screens were placed along the side of thearena to eliminate visual stimuli or disturbance from observers during experimentaltreatments.

EXPERIMENTAL PROCEDURE

Substratum type experimentsOverall, 32 male and 30 female adult S. canicula were tested with different sub-

stratum types (habitats). Each individual was allocated to one of four experimentaltreatments whereby prey-simulating electric fields were presented during normalodour-induced food searching behaviour. The first treatment involved the presentationof an electric field (active dipole) combined with no substratum, the second was presen-tation of an active dipole buried under a layer of sand, and the third and fourth treat-ments comprised a dipole under a layer of pebbles and rocks, respectively (Fig. 2). Thedepth of the layers of sand and pebbles were the same but the layer of rocks wasslightly deeper due to their physical size. Electrode dipoles, however, were not posi-tioned directly under the centre of a pebble or rock but were located towards the edgesnear the interstitial spaces. Fish were tested in single sex groups of three because duringa pilot study it was not possible to motivate an individual to search for food whenalone in the test arena. No more than three individuals formed a test group to eliminatedifficulties of identifying individuals repeatedly from video recordings. Each group ofthree fish was tested separately and was introduced into the test arena and allowedto acclimate for c. 1 h. After this time, an olfactory cue was introduced into the testarena via the odour delivery tube (Fig. 1). At the same time, one of the four dipoles

Video camera

Monitor

Video recorder Syringe

Whiting ‘juice’

Odour delivery tube

Ammeter

Circuit control box

Plastic tray

Electrodepair

Seawater-filledpolyethylene tubes

Underwater cable withsealed underwater connections

FIG. 1. Schematic of the experimental apparatus used to test the behavioural responses of Scyliorhinus

canicula to prey-simulating electric fields. Adapted from Kajiura (2003).


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was activated with an applied current of 15 mA. This current was calculated to produceelectrical field gradients in the absence of substrata of between 0�119 and 0�004 mV cm�1

at distances of between 100 and 300 mm, respectively. These values are similar to thoseproduced by a variety of prey species commonly found in the diet of S. canicula andhave previously been reported to elicit predatory responses by them (Kalmijn, 1966,1971, 1982). Video recordings were made for 15 min following olfactory cue introduc-tion, during which time the current applied to one of the four dipoles remained active.After each trial fish were fed and then removed from the test arena. The array housingwas then cleared of substratum and washed thoroughly ready for the next experimentaltreatment. If an individual moved and searched for food (dipoles) successfully or not, itwas not used in any further trials to ensure data independence. Any individual thatshowed no movement when tested was classified as not having been exposed to thetreatment and was re-tested at a later time with the same treatment until movementwas observed. Overall, data from eight individuals of each sex with each experimentaltreatment were collected except with rock substrata where data from eight males andsix females were collected.

Substratum depth experimentsSince the detection of electrical fields buried deeper within the substratum would be

expected to be more difficult a further 30 males and 30 females were used in behaviou-ral experiments to test this hypothesis. This involved the presentation of simulatedbioelectric fields (a single dipole activated with a current of 15 mA, as before) with

FIG. 2. Photographs of the electrode array used for testing responses of Scyliorhinus canicula to buried

prey-simulating electric fields: (a) control, no substratum, (b) sand, (c) pebbles and (d) rocks. The

square acrylic array housing measured 0�6 � 0�6 m.


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no substratum (control 0�0 mm) and at four different burial depths within sand (5, 10,30 and 50 mm) Besides this, the procedure used in these trials was the same as for thesubstratum-only experiments, however, only six males and six females were assignedto each treatment.

DATA ANALYSES

Several behavioural variables were determined from video records of trials. Fromvideo recordings, the time that each individual first began searching for food (thelatency to response) was determined. The total amount of time an individual spentsearching was also calculated, together with the number of passes, the number of turnstowards and the number of bites at an active dipole by each individual throughout each15 min trial. Passes were defined as any occasion where the head of an individualpassed over the array, and before another pass was counted the individual’s whole bodymust have left the array. A turn was defined as any sudden change in trajectory or devi-ation from a swimming path by an individual towards an active dipole. A bite wasclearly identifiable by the positioning of the head over the active dipole along with asso-ciated jaw movements and flaring of the gill slits as water, or in some cases substratum,passed through the mouth and over the gills.

Once these behavioural variables were determined for individuals of each sex for eachexperimental treatment, the counts were converted into rates per minute. Each ofthe recorded behavioural variables was tested for normality and equal variances usingKolmogorov–Smirnov and Levene’s equality of variance tests, respectively. If necessary,data were transformed using square-root transformations. Data for each variable wereanalysed using two-way analysis of variance (ANOVA). Any data that were not nor-mally distributed and did not have equal variances, and that could not be transformed,could not be analysed using ANOVA (Quinn & Keough, 2002). Therefore, a generalizedlinear model (GLM) was fitted to the data using sex and substratum as categorical pre-dictors. Since the count data often contained zeros the data were transformed by add-ing one to each value. The distribution of the data resembled a Poisson distribution soa factorial, log-linear model with a Poisson error structure and a log-link function wasused (Quinn & Keough, 2002). The explanatory variables in the systematic componentof the model were sex, substratum and the interaction term sex � substratum. Sinceoverdispersion can inflate the risk of a type I error when using a Poisson GLM (Seavyet al., 2005), overdispersion was evaluated using the deviance (G2) statistic (Quinn &Keough, 2002). Overdispersion was evident upon examination of the deviance (G2) sta-tistic and therefore the negative binomial distribution was used in the GLM to accountfor the extra variance since the negative binomial distribution incorporates a dispersionparameter (Crawley, 1997). Basic statistical analyses including normality and equalityof variance tests and two-way ANOVA were conducted in Minitab release 14 (MinitabInc., State College, PA, U.S.A.). Generalized linear modelling techniques were con-ducted in GenStat for Windows 9th edition (VSN International Ltd, Hemel Hemp-stead, Hertfordshire, U.K.). Since no differences between the sexes were foundduring experiments with different substrata the data for males and females duringexperiments with different depths of sand were pooled. One-way ANOVA and Kruskal–Wallis analysis of variances were used as appropriate.

RESULTS

SUBSTRATUM TYPE EXPERIMENTS

Initiation of searching or foraging behaviour by individuals did not occur inall individuals. In first trials 74�4% of individuals commenced searching inresponse to the odour stimulus; however, the remaining individuals did notrespond (Fig. 3). A small number of individuals (14�1%) did not respond at


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all on subsequent attempts and so were not included in the analysis since theyremained immobile and were at least a metre away from the electrodes andwere assumed not to have been exposed to the experimental treatment. Ofthe individuals that did not respond during experimental trials there wasroughly an equal percentage of males and females (45�5 and 54�5%, respec-tively) (Fig. 3).The onset of foraging behaviour was characterized by a sudden increase in

the swimming speed and number of turns in addition to increased circling ofthe arena. This increased the pass rate over the electrode array housing. Thelatency period between the introduction of an odour stimulus and the onsetof foraging behaviour was variable but was not significantly influenced byeither the substratum presented with the electric field or the sex of the individ-ual (two-way ANOVA: substratum, F3,54 ¼ 0�14, P > 0�05; sex, F1,54 ¼ 0�29,P > 0�05) [Fig. 4(a)]. Similarly, search effort was not influenced by either sexor substratum since both sex and substratum did not significantly influencethe time spent searching or the rates of passes over the electrode array housingby individuals during experiments [Fig. 4(b), (c)].The results for predatory responses of individuals towards simulated prey,

however, showed a different pattern. There was a significant effect of substra-tum type on the rate of turns towards active electrodes (simulated prey) byindividuals, with turn rates towards dipoles with no substratum and buriedwithin sand higher than towards dipoles buried beneath pebbles or rocks(two-way ANOVA: F3,54 ¼ 7�83, P < 0�001) [Fig. 4(d)]. There was no signifi-cant effect of sex on turn rates and there was no significant interaction betweensubstratum and sex (two-way ANOVA: sex, F1,54 ¼ 0�52, P > 0�05; sex � sub-stratum, F3,54 ¼ 0�51, P > 0�05). Generalized linear model results of the bite-frequency data showed the same pattern as data for turn rates. The resultsshowed a significant effect of substratum with a greater frequency of bitestowards electrodes with no substratum and buried beneath sand than when

FIG. 3. The percentage of Scyliorhinus canicula individuals that either responded to the odour source by

initiating searching behaviour on the first attempt ( ), did not respond on the first attempt but did

respond on subsequent attempts ( ), and did not respond at all ( ).


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buried beneath pebbles or rocks [Fig. 4(e)]. Both models showed that therewere no differences between the sexes and there were no significant interactions(Table I). Using a negative binomial distribution, the resulting deviance statis-tic showed the model fitted the data well (Table I).

SUBSTRATUM DEPTH EXPERIMENTS

As with the substratum-type experiments the latency period between theintroduction of an odour stimulus and the onset of foraging behaviour wasvariable but was not significantly influenced by the burial depth of electrodes

FIG. 4. Foraging behaviour of Scyliorhinus canicula when exposed to prey-simulating electric fields

presented within different substrata: (a) latency period from the introduction of an odour stimulus to

the onset of foraging behaviour, (b) rate of passes over the electrode array, (c) time spent searching,

(d) turn rates and (e) bite frequencies towards active electrodes. Values are means � S.E. ( , males;

, females). SQRT, square-root transformed values are displayed.


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TABLEI.

Resultsofthegeneralizedlinearmodel

analysesoftransform

ed(x

þ1)bite-frequency

data

Reference

level

Poisson

Negativebinomial

Variable

Estim

ate

S.E.

Pvalue

Deviance

(G2)

d.f.

Estim

ate

S.E.

Pvalue

Deviance

(G2)

d.f.

Sex

¼female

Male

0� 192

0� 235

0� 413

96� 31

54

0� 192

0� 352

0� 585

45� 21

54

Substratum

¼nosubstratum

Sand

0� 167

0� 236

0� 480

0� 167

0� 353

0� 636

Pebbles

�1� 417

0� 394

<0� 001

�1� 417

0� 473

0� 003

Rocks

�1� 417

0� 444

0� 001

�1� 417

0� 526

0� 007

Male

�sand

�0� 359

0� 333

0� 281

�0� 359

0� 499

0� 471

Male

�pebbles

�0� 192

0� 553

0� 728

�0� 192

0� 665

0� 772

Male

�rocks

�0� 192

0� 589

0� 744

�0� 192

0� 704

0� 785

Dispersion(k)

na

na

na

3� 637

Variablesforfactors

are

differencescomparedwiththereference

level.

na,Notapplicable.


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(ANOVA: F4,55 ¼ 0�33, P > 0�05) [Fig. 5(a)]. Similarly, search effort was notinfluenced by burial depth since the time spent searching and the rates of passesover the electrode array housing by individuals during experiments were notsignificantly different between burial depths [Fig. 5(b), (c)]. As with the substra-tum type experiments, however, predatory responses of fish towards simulatedprey at different burial depths were significantly different. Both the trans-formed turn rates and untransformed bite rates showed significant effects ofburial depth (ANOVA with square-root transformed turn rates: F4,55 ¼6�44, P < 0�001; bite rates, Kruskal–Wallis, H ¼ 25�82, d.f. ¼ 4, P < 0�001).There were greater turn rates and bite rates in the control and shallower sand

FIG. 5. Foraging behaviour of Scyliorhinus canicula when exposed to prey-simulating electric fields

presented at different sand burial depths: (a) latency period from the introduction of an odour

stimulus to the onset of foraging behaviour, (b) rate of passes over the electrode array, (c) time spent

searching, (d) turn rates and (e) bite frequencies towards active electrodes. Values are means � S.E.

SQRT, square-root transformed values are displayed. Treatments with the same lower case letters

indicate no significant difference at P < 0�05 level.


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depths (0, 5 and 10 mm) than in the deeper sand depths (30 and 50 mm)(examined with post hoc tests) [Fig. 5(d), (e)].

DISCUSSION

As far as is known, this study is the first to examine the role of habitat type(substratum) on the electro-detection ability of a marine elasmobranch. It washypothesized that the rate of detection of prey-simulating electric fields wouldbe lower in certain substrata compared with others due to distortion or highbackground noise effects on the electric field against which the signal of preycould not be detected. In experiments to test this idea it was found that despitethe high individual variability of both turn rates and bite frequencies towardsprey-simulating electric fields, there was a significant difference between sub-stratum types. Both males and females exhibited greater turn rates and bite fre-quencies towards prey-simulating electric fields presented with no substratumand within sand compared to those beneath pebble and rock arrays. These re-sults support the hypothesis that some habitat (substratum) types affect theelectroreception performance of sharks.The results show clearly that individuals were not as successful in locating

electric fields within matrices of pebbles or rocks as they were in sand or whenelectrodes were presented without substratum. It is possible that this was due,at least to some degree, to electric field modification or distortion by pebblesand rocks that made the electric fields unrecognizable as prey during searchingbehaviour. Published physical data shows that the three test substrata usedhave different resistivities. Resistivity values are related to the porosity of thesubstratum as the current is conducted predominantly through the electrolyte(for example, sea water) in the pores (Telford et al., 1976; van Blaricom,1980). The resistivity of wet sand has been reported to be up to 200 Om,but the resistivity of pebbles is generally higher and reported to be up to500 Om, with rocks higher again, being up to 1 000 000 Om. One possibleexplanation for the present findings is that the electric fields presented to S.canicula were conducted better (i.e. there was less alteration to the strengthand geometry of the field) through a highly porous substratum such as wetsand, compared with the pebble and rock arrays. This may have been due todifferences in the length and tortuosity of the pathways that the electrical cur-rent had to pass through to reach the surface of the substrata, even thoughdipoles were positioned close to the interstitial spaces and the distance betweenthe dipole and the surface of each matrix was similar. Thus, substratum typemay affect electro-detection of prey by fish by influencing the form of the elec-tric field, perhaps making it more difficult to target precisely signal location.Tracking studies of free-ranging adult S. canicula show individuals have rel-

atively small home ranges and actively forage in specific locations (Sims et al.,2001, 2006). Male fish were found to forage nocturnally in warm, shallow areaswith high abundances of prey that either burrowed in soft, fine sediment or re-mained hidden under dense layers of filamentous algae (Sims et al., 2006). It isprobable that these habitats had minimal distorting effects and may representoptimal habitats for detecting hidden prey using electroreception. Interestingly,S. canicula tracked in these studies did not forage in areas with hard substrata


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(Sims et al., 2006). It can be speculated that the effects of physical environmenton electric field detection of S. canicula demonstrated in this study supports thehypothesis that S. canicula may select habitats with optimal properties for preydetection.The results of this study showed no sex differences in electroreceptive ability

when fish foraged across different substrata. Both male and female fish showedsimilar behaviour when detecting electric fields and consequently males and fe-males probably have the ability to forage in the same habitats for the samefood. Although sexual segregation with respect to foraging is well documentedfor many species including sharks (McCord & Campana, 2003; Mathot &Elner, 2004; Gomes-Ferreira et al., 2005), for small spotted catsharks at least,sex differences in foraging habitat selection is unlikely to be attributable to dif-ferential electroreceptive ability when foraging across different substratum types.In experiments with different depths of sand covering the paired electrodes,

the latency between odour introduction and onset of foraging, search timeand pass rates over the electrode array were not different between treatments,but responses showed high individual variability as before (unpubl. data). Turnand bite rates of S. canicula did differ significantly between treatments withgreater turn and bite rates exhibited in trials where the prey-simulating fieldswere either not buried or buried in shallow sand (0, 5 and 10 mm) as opposedto buried in deep sand (30 and 50 mm). Despite the differences in bite ratesbetween sand depths, there was high variability between individuals. The differ-ence between bite rates of fish towards prey-simulating electric fields in differentsand depths may be explained by the ‘damping’ effect of a greater volume ofsand above the electrodes (which may also increase the relative resistivity).The greater the amount of substratum above the electrodes the more disrupteda particular field will be because it has to penetrate more material to be detect-able by the fish. This suggests S. canicula may only detect prey which producea sufficiently large bioelectric field (e.g. larger prey) when buried in deep sand,consequently very small prey at such depths may go undetected by this predator.

This research was supported by the Natural Environment Research Council (NERC)and the Marine Biological Association of the United Kingdom. We thank J. Rundle foranimal husbandry, P. Rendle and J. Mavin for technical assistance, J. Murua, G. Budd,S. Rogers, J. Kimber, A. Smith, V. Wearmouth and the crews of R.V. Plymouth Questand R.V. MBA Sepia for assistance with specimen collection. D.W.S. was supported bya NERC-funded MBA Research Fellowship.

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