trail following behaviour in relation to pedal mucus production in the intertidal gastropod...

y and Ecology 349 (2007) 313–322www.elsevier.com/locate/jembe

Journal of Experimental Marine Biolog

Trail following behaviour in relation to pedal mucus production inthe intertidal gastropod Monodonta labio (Linnaeus)

Neil Hutchinson a,⁎, Mark S. Davies b, Jasmine S. S. Ng a, Gray A. Williams a

a The Swire Institute of Marine Science, Department of Ecology & Biodiversity, The University of Hong Kong, Hong Kongb School of Health, Natural and Social Sciences, University of Sunderland, Sunderland, SR1 3SD, United Kingdom

Received 14 May 2007; accepted 26 May 2007

Abstract

Trail following behaviour and pedal mucus production were investigated in the mid-shore topshell, Monodonta labio(Linnaeus) in Hong Kong. On the shore, individuals exhibited both conspecific and self trail following while awash on ebb andflood tides, although fidelity to resting sites during emersion on successive days was low. In the laboratory, animals thatencountered trails that had been aged on the shore for different periods showed similar patterns of movement (distance moved,speed and tortuosity) suggesting that degradation of cues in the mucus that animals responded to did not occur until N3 days post-deposition. Animals moved faster, with a lower rate of radular rasping, on freshly laid mucus trails than on a biofilm-coveredsubstratum and did not change their speed when moving over aged (biofilm-covered) mucus compared to fresh mucus. Mucusproduction rates were similar when animals were crawling on vertical or horizontal surfaces, but significantly more mucus wasproduced when animals were emersed than when submerged. Mucus trail profiles were of variable thickness, but ‘double’ mucustrails (marker+ tracker trails) did not contain significantly more mucus than ‘single’ trails (marker mucus only) and wereconsiderably thinner than single trails suggesting tracker snails utilized mucus laid by marker snails, reducing their own depositionof mucus. Thus, while M. labio do not appear to utilize trails for orientation or refuge location, snails that follow trails have thepotential to save energy through reducing mucus production or to gain energy through mucus ingestion. Given the role of pedalmucus production in the overall energy balance of gastropods, such energetic benefits are considerable and may have implicationsfor the life history of the snail.© 2007 Elsevier B.V. All rights reserved.

Keywords: Behaviour; Foraging; Monodonta labio; Mucus; Mucus trail; Trail following; Tropical rocky shore

1. Introduction

The trochid gastropod, Monodonta labio (L.) is acommon grazer found in a variety of intertidal habitats

⁎ Corresponding author. Amakusa Marine Biological Laboratory,Kyushu University, Tomioka 2231, Reihoku-Amakusa, Kumamoto863-2507, Japan. Tel.: +81 969 35 0003; fax: +81 969 35 2413.

E-mail address: [email protected] (N. Hutchinson).

0022-0981/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jembe.2007.05.019

in Hong Kong, including rocky, cobble and bouldershores and mangroves (Chin I Mei, 2003). It has a lifespan of 2–3 years (Takada, 1996) and occurs inabundance on sheltered and semi-exposed rocky shores.Individuals forage when the shore is awash (Hutchinsonand Williams, 2003), feeding on a variety of nutrition-ally rich biofilm species (Takada, 1996; Nagarkar et al.,2004). Inactive periods are generally spent in refugessuch as pools, crevices and under overhangs (Williams1993; Hutchinson and Williams, 2003).

mailto:[email protected]

http://dx.doi.org/10.1016/j.jembe.2007.05.019

314 N. Hutchinson et al. / Journal of Experimental Marine Biology and Ecology 349 (2007) 313–322

In commonwith most gastropods,M. labio glides overthe substratum on a thin layer of secreted mucus, which isdeposited as a mucus trail. In certain species, thisenergetically expensive product (Denny, 1980; Daviesand Williams, 1995; see Davies and Hawkins, 1998 forreview) has been shown to persist on the substratum forweeks (e.g. Davies et al., 1992a) and to provide a varietyof post-deposition functions (reviews by Branch, 1981;Denny, 1989; Davies and Hawkins, 1998). Individuals ofsome species exhibit self trail following as part of homingbehaviour patterns (e.g. Chelazzi, 1990) and trailfollowing by conspecifics has a function in bothaggregatory and mating behaviour (e.g. Erlandsson andKostylev, 1995; Stafford et al., in press), as well as aidingin the discovery of food patches (Hawkins and Hartnoll,1983; Davies and Beckwith, 1999). In addition, somespecies have been shown to ingest mucus trails that havetrapped particles of food (Davies and Beckwith, 1999), oracted as a growth medium for microbes (Peduzzi andHerndl, 1991) and, in some cases, the mucus has beenfound to act as an agent promoting the growth ofmicroalgal species (Connor, 1986; Davies and Hawkins,1998; Williams et al., 2000). If mucus does provide apossible food source, in part through its ability to retain orencourage the growth of food particles, then it would beexpected that grazers will modify their behaviour onencountering such trails (Davies and Beckwith, 1999).Snails make decisions on whether to follow trails or not,based on information present in the trails (Edwards andDavies, 2002) and these decisions can have considerableimpact on spatial distributions of populations (Staffordand Davies, 2005; Stafford et al., in press) and conse-quently resource utilization.

Information on pedal mucus production andfunction in gastropods from tropical shores is availablefor the mangrove littorinids Littoraria melanostomaand L. ardouiniana (Lee and Davies, 2000), and co-existing rocky shore limpets Cellana grata (Daviesand Williams, 1995), Siphonaria laciniosa, and Si-phonaria japonica (Davies and Williams, 1997). Aninvestigation of any potential, post-deposition functionof the mucus of C. grata indicated no provenderingrole. This was suggested to be due to environmentalfactors, including high temperatures and heavy waveaction, causing the mucus to degrade so rapidly thatsuch function was not feasible (Davies and Williams,1995). Nevertheless, grazer density in Hong Kong isso high (Williams, 1993) that much of the intertidalarea will be covered in a layer of deposited mucus,mixed with a cyanobacteria-dominated biofilm (Nagar-kar and Williams, 1999). M. labio is known to exhibitconsiderable trail following activity (authors, pers.

obs.), and this paper formalises those observations andexamines trail following behaviour over progressivelyolder trails in a controlled laboratory setting, in part todetermine the utilization of mucus as a provenderingagent. We hypothesise that the behaviour of M. labiowill be modified on encountering mucus trails thathave been aged in the natural environment and thushave the potential for organic enrichment by collectingmicroalgal particles. In view of the considerable ener-getic cost of mucus production, we additionally aimedto ascertain how mucus production rates vary undera range of conditions, especially in relation to whenanimals are trail following, to determine if there wereany other possible benefits to trail following in thisspecies.

2. Materials and methods

2.1. On-shore observations

The foraging activity of M. labio was observed in situon a 5×5 m semi-exposed rock platform (as defined byKaehler and Williams, 1996) between ∼0.9 and 2 mabove Chart Datum (C.D.; Hong Kong Observatory,1998) at Cape d'Aguilar (22° 13′N, 114° 12′E), HongKong during October and November 2001. The foragingexcursions of individuals were recorded in daylight duringtwo ebbing and two flooding tides when the animals wereactive. A Hi-8 video camera (Sanyo, Japan) fixed to atripod was placed on the shore in a suitable position toobserve the full foraging range of individuals. Recordingbegan prior to foraging excursions when individuals werestationary, and continued until foraging activity ceased, oranimals had moved out of the range of the camera.

Video recordings were viewed on a monitor, with nospatial distortion, and the foraging trails of individualswere traced onto transparent acetate sheets to determinethe proportion of animals that exhibited conspecific andself trail following behaviour and the durations of suchtrail following. As the video camera was not placedperpendicular to the rock surface, it was not possible todetermine the exact distance travelled by individuals orthe proportion of total trail length over which followingbehaviour occurred.

Non-parametric, Mann–Whitney tests were per-formed to compare differences in the proportion oftime that individuals spent trail following during ebband flood tides. Separate analyses were performed forconspecific and self trail following using pooled data for22 individuals from both sampling dates.

To determine fidelity to crevices, three crevicesseparated by N10 m were selected. All the snails in each

315N. Hutchinson et al. / Journal of Experimental Marine Biology and Ecology 349 (2007) 313–322

crevice were marked in situ at low tide using correctionfluid and the number remaining in these crevices at lowwater on three subsequent days was recorded.

2.2. Trail following behaviour

To determine the effect of aged mucus trails on trailfollowing behaviour, three identical square glass arenaswere constructed (400×400×100 mm; l×w×ht). Eacharena was isolated using black polyethylene sheetingwhich also covered the tops of the arenas. To record theactivity of the snails, a Camcorder (Sony, Japan) wasplaced beneath each of the arenas, centrally and normalto the plane of the arena bases, which were lit frombelow with a fluorescent light. Snails were collectedfrom an adjacent platform to that described for fieldobservations and shell height and foot area wererecorded as indices of size. No snail was used morethan once.

In order to produce a mucus trail, a marker snail wasplaced fully submerged in an arena at the centre of aPerspex sheet (300×300 mm) and allowed to crawlacross a central area (250×250 mm). Time taken,distance travelled, speed and tortuosity were recorded.Tortuosity, measured as an index (Davies and Beckwith1999), was calculated as net displacement/distancetravelled which has a value of 1 for a straight path,and smaller values indicate increasingly tortuous paths.Seawater in each arena was changed between each of the40 trials.

Thirty Perspex sheets with trails on were then fixedhorizontally onto plastic pipe frames secured to the rockat ∼mid-tide level on the shore at Cape d'Aguilar inApril 2000. The edges of the frames were covered in 5-mm wire mesh to prevent grazer ingress. After each of 1,2 and 3 days (range chosen given the half-life of C.grata pedal mucus is 1–2 days; Davies and Williams,1995), ten Perspex sheets were randomly selected andreturned to the laboratory (∼100 m away). In thelaboratory they were again placed in an arena and asubmerged tracker snail introduced to the centre of eachplate in the same position as the original marker snail.The tracker snail's behaviour until crawling off thecentral area of the plate was recorded and the seawaterwas changed between trials. As a control, a further tensheets with fresh mucus trails, which had been on-shorebut not fixed to the frame or immersed in the sea, werereturned immediately (b30 min) to the laboratory andtracker behaviour was recorded. Time, distance andspeed while trail following and while not trail following,and a coincidence index of trail following calculated asdistance trail following/marker trail distance (Davies

and Beckwith, 1999) were recorded. If the marker's trailis followed in its entirety the coincidence index has avalue of 1, whereas a value of 0 indicates that themarker's trail was not followed at all.

All measurements were taken by replaying snailbehaviour on a video monitor and marking out the pathsof the snails on an overlaid transparent acetate sheet. Forthe area of the screen displaying the central area of thePerspex sheets there was no linear distortion, although aconstant correcting factor was applied to scale thedistances recorded to life size.

One-factor Analyses of Variances (ANOVA) wereperformed initially to determine differences betweenthe size of marker snails, distance travelled, speed andtortuosity indices with treatment (4 levels; Perspexsheets left on the shore for 1, 2, 3 or 0 (control) days).At the end of the study, one-factor ANOVAs wereperformed to determine significant differences in thecoincidence index between treatments, as well as todetermine significant differences between treatmentsfor time taken, distance travelled and the speed of trailfollowing tracker snails, with separate analyses con-ducted for periods when snails were travelling on oroff marker trails. Data were checked for homogeneityof variances (Cochran's C-test), percentage time anddistance data were arc-sine transformed, and signifi-cant differences were further examined using Student–Newman–Keuls (SNK) multiple comparison tests(Zar, 1996; Underwood, 1997). In addition, χ2 tests(Zar, 1996) were used to determine whether timetaken, distance travelled and the speed of trail fol-lowing by tracker snails varied on and off trails foreach treatment.

A further determination of animal speed and radularactivity was conducted using Perspex sheets that wereplaced in a flow through seawater (∼25 °C) aquariumfor 24 h to allow a biofilm to develop on them. On eachsheet, a marker and tracker snail were allowed totraverse the central area in a straight line (250 mm),guided by stacking glass microscope slides either side ofthe ‘runway’ which ensured both marker and trackersnails (n=19 each) traversed identical paths (pers. obs.).Data were recorded directly during observations frombelow the Perspex and t-tests were performed todetermine whether there were any differences in thespeed and rasping rates between animals moving overthe biofilm and over mucus.

2.3. Mucus production

Snails were again collected from the rock platform,were used once only, and returned to the shore within 1 h

Fig. 1. Proportion of time during a foraging excursion (mean+SD),spent by individuals either conspecific or self trail following. Samplesizes: ebbing tide, 45; flooding tide, 63.


of collection. Individuals were allowed to crawl acrosspre-weighed glass coverslips (50×22 mm) that werethen dried to constant weight at 60 °C to determinemucus production. For each snail the shell height, trail(foot) width and trail length were measured. Becauserates of gastropod pedal mucus production have beenshown to differ when animals are in air or water (e.g.Davies et al., 1990), rates were measured in both media.Snails in air were stimulated to move by spraying themwith a fine mist of seawater at an ambient temperature of∼25 °C.

Initially, in April 2000, treatments included cover-slips that were situated horizontally in air (n=40) andseawater (n=42). A more in-depth examination wasmade of mucus production in air in November 2001,with animals moving over coverslips that were eitherhorizontal (n=40), vertical (n=40), or horizontal andalready coated with a mucus trail by another animal(termed ‘double trails’; n=30).

Correlations were performed to determine whetherthere was a linear relationship between shell height andmucus production (as μg mm−1) or trail (foot) width andmucus production. Significant differences in mucusproduction in air or while submerged in April 2000 wereanalysed using a t-test, and a one-factor ANOVA wasused to examine differences between horizontal single,vertical single and horizontal double trails in November2001 (3 levels). Data were checked for homogeneity ofvariances (Cochran's C-test) as above.

To determine the thickness of mucus trails, 10animals were allowed to crawl across microscope slidesthat had been etched with a fine grid pattern. Each slidewas then immediately flooded for 30 s with a 0.2% (w/v)filtered (0.2 μm) seawater solution of the fluorescentdye, acridine orange. After rinsing by dipping intofiltered seawater, the slide was observed under a laserconfocal microscope (Zeiss 510). Throughout measure-ment, small volumes of filtered seawater were pipettedonto the mucus trail to keep it hydrated. The thickness ofthe mucus was determined at each edge and at the mid-point of each trail by measuring the distance from

Table 1The percentage of individuals of Monodonta labio exhibitingconspecific or self trail following behaviour during each survey period

Sampling period (n=numberof individuals)

% following trails ofconspecifics

% followingown trails

Flooding tide 1 (n=42) 59.5 23.8Flooding tide 2 (n=21) 47.6 0Ebbing tide 1 (n=22) 68.2 31.8Ebbing tide 2 (n=23) 12.1 15.7

particles of acridine orange on the surface of the mucusto the etching below the mucus (Davies and Blackwell,2007), using the microscope's built-in measurementfeature. For three of the trails a more detailed exam-ination was made by measuring thickness at ∼0.5 mmintervals across the trail width. This procedure was alsoemployed for three ‘double’ mucus trails, and t-testswere performed to determine the difference in trailthickness between single and double trails, using meanvalues of trail thickness calculated for each individualtrail.

3. Results

3.1. On-shore observations

At low tide, snails on open rock showed no activity,nor did they when emersed or submerged in creviceswhere they remained in aggregations. Snails in rockpools were also generally quiescent, although a fewmoved near to, and along, the air/water interface viatortuous paths. During both flood and ebb tides thesnails became active when awash or when they receivedregular spray from wave action. In general, snails movedupshore during the flood tide and downshore during theebb tide. The majority of snails moved along cracks inthe rock, at a speed greater than those on flat rock (pers.obs.).

The percentage of observed individuals that exhib-ited either conspecific or self trail following behaviourvaried between each survey with no obvious patterns of

Table 2Fidelity of Monodonta labio to crevices: number of markedindividuals remaining in three areas on the shore

Areanumber

Initial number ofsnails in crevice

Number of marked snails found after

1 day 2 days 3 days

1 23 3 (~13%) 2 (~9%) 02 24 1 (~4%) 0 03 15 0 0 0

Fig. 2. Marker trails: means+SD of foot area, shell height, distancetravelled, speed and tortuosity index. n=10 per bar.


differences between flood or ebb tides (Table 1). Mostincidences of trail following were conspecific, ratherthan self trail following, animals generally followingtrails up shore during the flood tide and down shoreduring ebb tides, often following substratum featuressuch as cracks or shallow depressions in the rocksurface. While the proportion of time during foragingexcursions when animals exhibited trail followingbehaviour varied considerably (Fig. 1), there were nosignificant differences for either conspecific or self trailfollowing between ebb and flood tides (Mann–Whit-ney's: p values N0.05).

The fidelity of snails to crevices on successive dayswas low (Table 2). Snails clearly move out of creviceson foraging excursions and infrequently return to, orremain in, the same crevice on successive days, i.e. theydid not exhibit any collective homing behaviour.

3.2. Trail following behaviour

There were no significant differences between treat-ments (1 day, 2 days, 3 days and control) in the size ofmarker snails (means: foot area=145.2 mm2±33.0 SD;shell height=21.3 mm±1.2 SD; n=40) and their distancetravelled, speed or tortuosity index (ANOVAs: p valuesN0.05; Fig. 2). Trails left by marker snails ranged from125 (the shortest straight line path to the edge) to361 mm, took between 25 and 120 s to lay, with overallspeeds ranging from 1.1 to 5.8 mm s−1. The tortuosityindex ranged from 0.35 to 1.00, but was generally high(Fig. 2) – only one value was b0.5 and 70% were N0.9 –indicating that the snails moved in relatively straightlines.

Trail following varied considerably between individ-ual tracker snails in each of the four treatments, thoughthe tracker snails themselves were of similar size inthe different treatments (ANOVAs: p values N0.05.Means: foot area=127.9 mm2±25.3 SD; shell height=20.4 mm±1.2 SD; n=40). In some cases, tracker snailsalmost exclusively followed trails left by marker snailswhereas in others, no trail following behaviour wasrecorded. All mean coincidence index values were low

(b0.3, Fig. 3A) and there were no significant differencesbetween the treatments, even when those snails that didnot trail follow were excluded from the analysis(ANOVAs: p values N0.05; Fig. 3B).

The majority of tracker animals that did exhibit trailfollowing behaviour travelled significantly greaterdistances and spent more time off pre-laid marker trailsthan on them in all treatments (Fig. 4, Table 3). Inaddition, distance travelled, time taken and the speed oftracker snails did not vary between the treatments, i.e.tracking animals did not adjust their speed whenlocomoting over the biofilm-coated mucus trails presenton treatments after 1, 2 or 3 days, as opposed to

Fig. 3. Degree of trail following in each treatment as indicated bymean+SD coincidence index (see text for explanation). A: All snailsincluded in analysis (n=10 for each treatment). B: Only snails showingtrail following included. Sample sizes given above bars.

Table 3Summary of χ2 tests to compare time spent and distance travelled bytracker snails on and off marker trails

Treatment Number of significanttests (pb0.05)

OnNOff OffNOn

Time 1 day 9 2 72 days 9 1 83 days 9 1 8Control 9 2 7

Distance 1 day 6 0 62 days 9 1 83 days 10 1 9Control 9 2 7

Tests were conducted on each of the 10 replicates for each treatment, asdata could not be pooled.


the control treatment where no biofilm was present(ANOVAs: p values N0.05). Tracker snails followedtrails in the same direction that the trails were laid, andshowed rasping behaviour over all surfaces.

Observations of the speed and radular activity ofanimals established that tracker snails crawling onmucus moved significantly faster (by 50%) than markersnails crawling on a biofilm (t-test: t=34.49, df=18,pb0.001; Fig. 5). The feeding rate of tracker snailswas, however, significantly lower than marker snails (t-test: t=42.98, df=18, pb0.001; Fig. 5).

Fig. 4. Means+SD of distance moved, and speed of tracker snails that

3.3. Mucus production

Mucus trails laid down by individuals (shell heightbetween 14 and 24 mm) varied from 7 to 15 mm inwidth. There were no significant differences betweenthe April 2000 or the November 2001 treatments interms of animal shell height or trail width (ANOVAs: pvalues N0.05). There were no significant correlationsbetween either the shell height of the animals or theirtrail (foot) width and the weight of mucus laid down permm of trail length for any treatment (r values rangedfrom 0.017 to 0.142, all p values N0.1). Analysisshowed that mucus production during April 2000 wassignificantly greater in air than when the animals weresubmerged (t-test: t=2.781, df=39, p=0.008; Fig. 6),and that in November 2001 there were no significant

showed trail following behaviour. Sample sizes given above bars.

Fig. 7. Mean+SD mucus thickness at the edges and mid-points ofsingle trails. n=10.

Fig. 5. Mean+SD of speed and feeding rate of snails when movingover a mucus trail and when moving over a biofilm. n=19 in eachcase.


differences between mucus production in air on hori-zontal, vertical, or on horizontal surfaces where a mucustrail was already present, i.e. for double trails (over-all mean=8.3 μg mm−1 ±7.0 SD, n=110; ANOVA:pN0.05; Fig. 6). This indicates that no additional mucuswas deposited by animals that moved over existingtrails.

Measurement of mucus thickness revealed that forsingle trails there was no significant difference betweenthickness at the edges and at the mid-point of the trail(ANOVA: pN0.05; overall mean=44.8 μm±11.4 SD,

Fig. 6. Mean+SD mucus production rates (μg mm−1) determined inApril 2000 from single mucus trails on horizontal surfaces submergedand in air; and in November 2001 from single mucus trails in air onvertical surfaces and horizontal surfaces, and from double mucus trailsin air on horizontal surfaces. Sample sizes given above bars.

n=30; Fig. 7). Mucus trail profiles showed no consistentpattern in thickness across the trails, but double trails(range 8.1–18.6 μm) were much thinner than singletrails (range 15.7–61.3 μm; t-test: t=15.74, df=4,pb0.001; Fig. 8).

4. Discussion

Observations in situ revealed that, as with manymolluscan grazers (e.g. Acanthopleura gemmata, Che-lazzi et al., 1987; Nodilittorina unifasciata, Chapman,1998; Cellana toreuma, Iwasaki, 1998), trail followingis a common behaviour in M. labio. These snails do notreturn to the same location on successive tides and trailfollowing may facilitate the dispersal of animals oncethey have left resting sites in crevices and pools, andindeed their subsequent aggregation (e.g. Chelazzi,1990).

Environmental exposure and aging of trails forperiods of up to 3 days, during which the trails wereimmersed twice daily, appeared to have no impact on thetrail following behaviour of tracker snails. The cues in

Fig. 8. Individual mucus trail thickness profiles for single and doublemucus trails (n=3 in each case). Note change of scale.


the mucus that M. labio use in order to trail follow are,apparently, still present after 3 day exposure. Whilesignificant degradation of trails is likely to haveoccurred (see Davies et al., 1992a, and Davies andWilliams, 1995 for limpets), it is not to such an extentthat they became unattractive to conspecifics, as hasbeen seen for the temperate species Littorina littorea(Edwards and Davies, 2002). The persistence of M.labio mucus on the shore is unknown, but as theseanimals occur in relatively high abundances, it isunlikely that long persistence times are necessary ifthe animals are utilizing trails to follow conspecifics inorder to find food resources or refuges.

Mucus trails laid by the co-occurring limpet C. gratahave been found to persist on shores for a maximum of6 days, with a half-life of 1–2 days (Davies andWilliams, 1995). It has been suggested that this shortpersistence time may explain why this species does nottrail follow, unlike other species such as Lottia giganteathat lay mucus trails that persist for longer (Connor1986). It is tempting to suggest that M. labio mucusdecays as quickly as C. grata mucus, making theresponse of M. labio to old trails remarkable, but inDavies and Williams's (1995) study the deposited C.grata mucus was exposed to much more stressfulconditions as it was measured in the summer on waveexposed shores.

Initial tests suggest that, unlike the mucus producedby L. littorea (Davies and Beckwith, 1999), the pedalmucus of M. labio does not provide an additional foodsource for the snails by trapping a significant quantity ofmicroalgal particles (Ng, unpublished data). Despitethis, the rasping rate of M. labio was higher whenmoving over a biofilm as opposed to moving overmucus, a pattern also noted by Davies and Beckwith(1999) for L. littorea which they ascribed to theincreased food content of the biofilm. M. labio,however, rasps at a rate 2–3× lower than that of L.littorea and this perhaps reflects the high energycontent of readily available biofilm species on HongKong shores (Nagarkar et al., 2004).

M. labio moved at similar speeds to the co-occurringlimpet, C. grata (Davies et al., 2006) and at least 3×faster than the temperate L. littorea (Davies andBeckwith, 1999; Edwards and Davies, 2002), presum-ably owing to environmental temperature differences.Tracker snails moved faster over a layer of mucus thanmarker snails did over biofilm and this may be related tothe amount of mucus that markers and trackers deposit(see below), trackers being able to travel faster becausethe production of a smaller quantity of mucus may takeless time.

No relationship was found between animal size andthe rate of mucus production within the size classes ofindividuals used in the study. This lack of relationshiphas been observed before in the co-occurring limpetS. japonica (Davies and Williams, 1997), althoughpositive relationships have been recorded in C. grata(Davies and Williams, 1995), S. laciniosa (Davies andWilliams, 1997), and the mangrove snails L. melanos-toma and L. ardouiniana (Lee and Davies, 2000). As allindividuals used here were of a similar size, it is possiblethat analysis of a wider size range containing multiplecohorts, found at different tidal levels (Takada, 1995),would reveal a relationship, especially since the allocationof energy to differentmetabolic processes has been shownto vary with size and age (e.g., Branch, 1981; Parry,1982).

M. labio secreted pedal mucus at a much higher ratein air than while submerged, a pattern seen in both L.littorea (Davies et al., 1992b) and Patella vulgata(Davies et al., 1990). It has been suggested that this isowing to the increased weight of emersed animals and tothe lack of a diluent for mucus in air (Davies et al.,1992a,b). Movement in air is therefore energeticallymore demanding than movement while submerged andwould mean that animals are less able to fulfil otherenergetic demands. Thus increased movement underwater (or awash), as observed in this study, may be afunction of conservation of energy in addition to otherproposed reasons such as predator avoidance andvulnerability to physical stresses (Garrity, 1984).

Mucus production rates while mobile are similar tothose reported for C. grata (Davies and Williams,1995), L. melanostoma and L. ardouiniana (Lee andDavies, 2000), S. japonica and S. laciniosa (Davies andWilliams, 1997), and the temperate L. littorea (Davieset al., 1992a), despite the differences in the size of theseanimals. This suggests differing thicknesses of mucus inthese different species, inconsistent with a theory ofpropulsion using a uniformly thick layer of mucus (seeChan et al., 2005). We did not attempt to standardise foranimal size in cross-species comparisons owing tospecies-specific patterns of allometry and size-scaling ofphysiological characters.

Our results, though with small sample sizes,demonstrate that mucus trails vary in thickness acrossthe profile of the trail, again suggesting that a uniformlythick layer is not necessary for locomotion. Mucus trailprofiles were unlike the convex shape reported for L.littorea by Davies and Blackwell (2007) and may be aresult of the shape and movement of the trailing edge ofthe foot as the trail is laid. In particular, trail thicknessmay vary dependent on whether the trailing edge of the


foot was in contact with the substratum or lifted from itas the snail moved over the point we measured. Thusvariations in thickness across the trail may be accountedfor by the fact that M. labio shows an alternate ditaxicdirect locomotion (pers. obs.): while the right side of thetrailing edge will be in contact with the substratum, theleft side might not, and vice versa.

Measuring both by weight of mucus produced and byits thickness, we recorded no additional mucus produc-tion by tracker animals. Indeed it seems that doubletrails may be thinner than single trails. This we cannotexplain, save a suggestion that tracker animals may eat aproportion of the trails they travel over. It is unlikely thatthe mucus is compressed and thus rendered thinner bytracker animals since mucus is∼90% water (Davies andHawkins, 1998) and thus barely compressible.

Trail following M. labio snails appear not to depositmucus, instead utilizing that which is already present intrails for locomotion; or producing only small quantitiesequivalent to that which they may consume. A similarphenomenon has been observed in L. littorea, wheretracking snails produce ∼70% less mucus than markersnails, and as suggested for L. littorea (Davies andBlackwell, 2007), M. labio may preferentially trailfollow to save energy rather than for any post-depositionfunction of mucus as has previously been speculated(see Davies and Hawkins, 1998, for review). Theresultant saving in energy is likely to be considerable.For the co-occurring species S. laciniosa (Davies andWilliams, 1997) and C. grata (Davies and Williams,1995) mucus has calorific values of 9.080 and 10.81 kJg−1, respectively and pedal mucus production in marinegastropods can amount to as much as 31% of the overallproportion of consumed energy (for P. vulgata, seeDavies and Hawkins, 1998, for review). As M. labio iscommonly seen moving across wet rock surfaces in air,conditions under which they produce significantlyhigher quantities of mucus than while submerged, trailfollowing will reduce the increased amounts of energyrequired for locomotion under these conditions. As anindication of the increased energy requirement duringlocomotion, in L. littorea oxygen consumption is 15times higher when crawling than when at rest (Newell,1970). If savings in energy are to be made by trailfollowing then the energetic requirements of movementmay not be as great as previously thought, snails may beable to increase their fecundity through energy ‘saved’,and their spatial distribution may in part be determinedby trail following in addition to exogenous biotic andabiotic factors. As a result, trail following is likely tohave important implications for their life historystrategies.

Acknowledgements

We are grateful to the Agriculture, Fisheries andConservation Department of the HongKongGovernmentfor permission to work in the Cape d'Aguilar MarineReserve. Mr Jason Tam and Dr Sanjay Nagarkar kindlyassisted with the Laser Confocal Microscopy.

References

Branch, G.M., 1981. The biology of limpets: physical factors, energyflow, and ecological interactions. Oceanogr. Mar. Biol. Annu. Rev.,Lond. 19, 235–380.

Chan, B., Balmforth, N.J., Hosoi, A.E., 2005. Building a better snail:lubrication and adhesive locomotion. Phys. Fluids 17, 113101.

Chapman, M.G., 1998. Variability in trail-following and aggregationin Nodilittorina unifasciata Gray. J. Exp. Mar. Biol. Ecol. 224,49–71.

Chelazzi, G., 1990. Eco-ethological aspects of homing behaviour inmolluscs. Ethol. Ecol. Evol. 2, 11–26.

Chelazzi, G., Della Santina, P., Parpagnoli, D., 1987. Trail following inthe chiton Acanthopleura gemmata: operational and ecologicalproblems. Mar. Biol. 95, 539–545.

M. Chin I Mei, 2003. Variation in Monodonta labio among differentintertidal habitats in Hong Kong. PhD thesis, The University ofHong Kong, Hong Kong.

Connor, V.M., 1986. The use of mucous trails by intertidal limpets toenhance food resources. Biol. Bull. 171, 548–564.

Davies, M.S., Williams, G.A., 1995. Pedal mucus of a tropicallimpet, Cellana grata (Gould) — energetics, production and fate.J. Exp. Mar. Biol. Ecol. 186, 77–87.

Davies, M.S., Williams, G.A., 1997. Mucus production by species ofSiphonaria (Mollusca: Gastropoda: Pulmonata) in Hong Kong.In: Morton, B. (Ed.), The marine flora and fauna of Hong Kongand southern China IV. Hong Kong University Press, Hong Kong,pp. 303–314.

Davies, M.S., Hawkins, S.J., 1998. Mucus from marine molluscs.Adv. Mar. Biol. 34, 1–71.

Davies, M.S., Beckwith, P., 1999. Role of mucus trails and trail-following in the behaviour and nutrition of the periwinkleLittorina littorea (L.). Mar. Ecol. Prog. Ser. 179, 247–257.

Davies, M.S., Blackwell, J., 2007. Energy saving through trailfollowing in a marine snail. Proc. R. Soc. B. 274, 1233–1236.

Davies, M.S., Hawkins, S.J., Jones, H.D., 1990. Mucus production andphysiological energetics in Patella vulgata L. J. Molluscan. Stud.56, 499–503.

Davies, M.S., Hawkins, S.J., Jones, H.D., 1992a. Pedal mucus and itsinfluence on the microbial food-supply of two intertidal gastropods,Patella vulgata L. and Littorina littorea (L.). J. Exp. Mar. Biol. Ecol.161, 57–77.

Davies, M.S., Jones, H.D., Hawkins, S.J., 1992b. Pedal mucusproduction in Littorina littorea (L.). In: Grahame, J., Mill, P.J.,Reid, D.G. (Eds.), Proceedings of the Third InternationalSymposium on Littorinid Biology. The Malacological Society ofLondon, London, pp. 227–233.

Davies, M.S., Edwards, M., Williams, G.A., 2006. Movement patternsof the limpet Cellana grata (Gould) observed over a continuousperiod through a changing tidal regime. Mar. Biol. 149, 775–787.

Denny, M.W., 1980. The role of gastropod pedal mucus in locomotion.Nature 285, 160–161.


Denny, M.W., 1989. Invertebrate mucous secretions: functionalalternatives to vertebrate paradigms. In: Chantler, E., Ratcliffe,N.A. (Eds.), Symposia of the Society for Experimental Biologynumber XLIII. Mucus and related topics. The Company ofBiologists Limited, Cambridge, pp. 337–366.

Edwards, M., Davies, M.S., 2002. Functional and ecologicalaspects of the mucus trails of the intertidal prosobranchgastropod Littorina littorea (L.). Mar. Ecol. Prog. Ser. 239,129–137.

Erlandsson, J., Kostylev, V., 1995. Trail-following, speed and fractaldimension of movement in amarine prosobranch, Littorina littorea,during a mating and a non-mating season. Mar. Biol. 122, 87–94.

Garrity, S.D., 1984. Some adaptations of gastropods to physical stresson a tropical rocky shore. Ecology 65, 559–574.

Hawkins, S.J., Hartnoll, R.G., 1983. Grazing of intertidal algae bymarine invertebrates. Oceanogr. Mar. Biol. Annu. Rev., Lond. 21,195–282.

Hong Kong Observatory, 1998. Monthly Weather Summary. TheObservatory, Hong Kong.

Hutchinson, N., Williams, G.A., 2003. An assessment of variation inmolluscan grazing pressure on Hong Kong rocky shores. Mar.Biol. 142, 495–507.

Iwasaki, K., 1998. Interindividual trail following by the intertidalpatellid limpet Cellana toreuma. J. Mar. Biol. Assoc. U.K. 78,1019–1022.

Kaehler, S., Williams, G.A., 1996. Distribution of algae on tropicalrocky shores: spatial and temporal patterns of non-corallineencrusting algae in Hong Kong. Mar. Biol. 125, 177–187.

Lee, O.H.K., Davies, M.S., 2000. Mucus production and morpho-metrics in the mangrove littorinids, Littoraria melanostoma andL. ardouiniana. In: Morton, B. (Ed.), The Tenth InternationalMarine Biological Workshop: The Marine Flora and Fauna ofHong Kong and Southern China, vol. V. Hong Kong UniversityPress, Hong Kong, pp. 241–253.

Nagarkar, S., Williams, G.A., 1999. Cyanobacteria dominated epilithicbiofilms: spatial and temporal variation on semi-exposed tropicalrocky shores. Phycologia 38, 385–393.

Nagarkar, S., Williams, G.A., Subramanian, G., Saha, S.K., 2004.Cyanobacteria-dominated biofilms: a high quality food resourcefor intertidal grazers. Hydrobiologia 512, 89–95.

Newell, R.C., 1970. Biology of Intertidal Animals. Lagoon Press,London.

Parry, G.D., 1982. Reproductive effort in four species of intertidallimpets. Mar. Biol. 67, 267–282.

Peduzzi, P., Herndl, G.J., 1991. Mucus trails in the rocky intertidal: ahighly active microenvironment. Mar. Ecol. Prog. Ser. 75,267–274.

Stafford, R., Davies, M.S., 2005. Examining refuge location mechan-isms in intertidal snails using artificial life simulation techniques.Lect. Notes Artif. Intell. 3630, 520–529.

Stafford, R., Davies,M.S.,Williams,G.A., in press. Computer simulationsof high shore littorinids predict small-scale spatial and temporaldistribution patterns on real rocky shores. Mar. Ecol. Prog. Ser.

Takada, Y., 1995. Variation of growth rate with tidal level in thegastropod Monodonta labio on a boulder shore. Mar. Ecol. Prog.Ser. 117, 103–110.

Takada, Y., 1996. Seasonal and vertical variations in size structure andrecruitment of the intertidal gastropod, Monodonta labio. Venus55, 105–113.

Underwood, A.J., 1997. Experiments in Ecology: their Logical Designand Interpretation Using Analysis of Variance. CambridgeUniversity Press, Cambridge.

Williams, G.A., 1993. The relationship between herbivorous molluscsand algae on moderately exposed Hong Kong Shores. In: Morton,B. (Ed.), Proceedings of the 1st International Conference on theMarine Biology of the South China Sea. Hong Kong UniversityPress, Hong Kong, pp. 459–470.

Williams, G.A., Davies, M.S., Nagarkar, S., 2000. Primary successionon a seasonal tropical rocky shore: the relative roles of spatialheterogeneity and herbivory. Mar. Ecol. Prog. Ser. 203, 81–94.

Zar, J.H., 1996. Biostatistical Analysis. Prentice-Hall, London.

trail following behaviour in relation to pedal mucus production in the intertidal gastropod...

Documents