Energy saving through trail following in a marine snail

Mark S Davies, Janine Blackwell

Abstract

Most snails and slugs locomote over a layer of mucus and although the resultant mucus trail is expensive to produce, we show that this expense can be reduced by trail following. When tracking over fresh conspecific trails, the marine intertidal snail Littorina littorea (L.) produced only approximately 27% of the mucus laid by marker snails. When tracking over weathered trails, snails adjusted their mucus production to recreate a convex trail profile of similar shape and thickness to the trail as originally laid. Maximum energy saving occurs when following recently laid trails which are little weathered. Many and diverse ecological roles for trail following have been proposed. Energy saving is the only role that applies across the Gastropoda and so may help to explain why trail following is such a well-established behaviour.

Keywords:

1. Introduction

Most snails and slugs leave behind them a silvery mucus trail as they move. Although this mode of locomotion is almost ubiquitous in the Gastropoda, both its evolution and ancillary functions are yet to be resolved. Davies & Hawkins (1998) suggested that because the mucus acts as glue, a role in locomotion evolved that allowed snails to extend their habitat to verticals and overhangs, with obvious advantages for those terrestrial gastropods that feed on plants. Yet, the production of mucus in locomotion represents a significant energy loss in marine, freshwater and terrestrial species (e.g. Calow 1974; Denny 1980a; Edwards & Welsh 1982; Horn 1986; Peck et al. 1987; Davies et al. 1990, 1992b; Navarro & Torrijos 1995). Owing to this high cost, it seems unlikely that post-depositional functions of mucus have driven the evolution of mucus-based locomotion, since not all species will benefit from these functions whose occurrence shows no relationship to phylogeny. Nevertheless, a range of functions for the mucus trails deposited on the substratum has been proposed. Denny (1989) suggested that trails might be used in navigation where snails with limited vision moving over complex surfaces might find visual tracking difficult. Animals might follow their own trails to home (e.g. Della Santina 1994) and those of conspecifics to aggregate (e.g. Stafford & Davies 2005) or mate (e.g. Cook 1992). Mucus has been implicated in feeding (e.g. Davies et al. 1992a), where it might trap the food of grazers. While trail following might thus deliver benefits that are opportunistic, we examined a more direct benefit: the potential to reduce energy costs by locomoting over existing mucus trails.

We hypothesized that because mucus trails may both stabilize the substratum and produce a smoother surface over which to move, the amount of mucus produced is reduced while animals are trail following. Culley & Sherman (1985) demonstrated that surface topography can influence the amount of mucus required for locomotion in the ormer Haliotis tuberculata. Tankersley (1989) demonstrated that the locomotory force applied is reduced in trail following, as opposed to trail laying, in Littorina irrorata. But if trail following is an energy-saving device then any saving in mucus production is more significant, since the mucus production costs of locomotion outweigh the metabolic costs by 35× (Davies et al. 1992b).

We used the intertidal, rocky shore Littorina littorea (L.) as a model to examine energy saving in trail following because of its widespread distribution in the East and West Atlantic and because of the wealth of information available on its biology. Its pedal mucus will begin to decay shortly after deposition (Herndl & Peduzzi 1989), perhaps as quick as 5–8 h after immersion (Davies & Beckwith 1999). Edwards & Davies (2002) suggested that the mucus of L. littorea is most useful to conspecifics when the trails are 1-day old or less, in terms of nutritional benefit through embedded microalgae. Here we used conventional fluorescence microscopy to visualize mucus trails, giving a profile of mucus thickness. We examined the thickness of mucus trails produced by marker and tracker animals, the relationship between trail thickness and time exposed on the shore, and the response, in terms of trail thickness, of tracker animals to weathered trails.

2. Material and methods

A Leitz Dialux 22EB fluorescence microscope was calibrated using an eyepiece graticule and calibrated slide to measure the diameters of six lengths of stretched wire, each taped separately to a microscope slide. Each wire was then rotated through 90°, re-taped to the slide ensuring that its full length was in contact with the slide, and its diameter re-measured in microscope focusing units. The rotation of the wire was to ensure a proper calibration: we did not assume that the wire had a circular transverse section. For the re-measurement, talcum powder was brushed onto the wire to provide a reference point on which to focus; the microscope was focused on the powder particles on the slide on which the wire was laid and then re-focused on the powder particles on the surface of the wire, noting the number of units moved on the graduated fine focusing knob. Diameters recorded ranged from 28 to 107 μm; the relationship between wire diameter and focusing units was linear (Pearson's r2=0.999, d.f.=4, p<0.001).

Littorina littorea (12–18 mm shell length) were collected from mid-shore at Whitburn, UK (national grid reference NZ 414 616), stored for a maximum of 4 days at approximately 12°C in aerated seawater and used only once. Trails were laid in filtered (0.2 μm) seawater onto an array of touching microscope slides (75×25 mm), etched with approximately 1×1 mm squares. Double trails were laid by introducing a tracker snail to the beginning of a single mucus trail such that it laid a trail directly on top of the single trail, in the same direction as the marker snail.

For measurement of mucus trail thickness, mucus trails were rehydrated, where necessary, in filtered seawater for 10 min. The slide was flooded with a 0.2% (w/v) solution of the fluorescent dye acridine orange in filtered seawater for 30 s. Small volumes of filtered seawater were pipetted at regular intervals onto the mucus being measured to maintain hydration. The microscope was focused on a particle of acridine orange on the surface of the mucus, re-focused on the edge of an etch on the upper surface of the slide beneath the mucus, and the number of focusing units between the two positions noted. Mucus thicknesses were recorded in this way at 10% divisions of the trail widths.

To assess the effect of ‘period of exposure’ on the rate of mucus decay, single and double mucus trails were exposed at mid-shore at a semi-exposed site at Sunderland, UK (national grid reference NZ 412 565). Etched glass slides with single mucus trails were fixed to a frame fixed on near-horizontal rock and covered by cages to prevent grazer ingress. Degree of wave exposure is not a significant factor in the rate at which the pedal mucus of L. littorea decays (Davies et al. 1992a). Slides were recovered and mucus thickness measured after one tidal cycle, two tidal cycles, one week and two and a half weeks. The half-life of L. littorea pedal mucus is approximately 12 days (Davies et al. 1992a): the periods of one week and two and a half weeks straddle this period. Treatments (for which n=10) were carried out in a random order over two months. ‘Laboratory’ controls (n=10) were kept submerged in filtered seawater until required for microscopic measurement (a few minutes). ‘Field’ controls (n=10) were transported to and from the site so they experienced a period of dehydration and hydration prior to being measured (typically 2 h post-laying). To assess the effect of the decay of single mucus trails on the quantity of mucus produced by tracker snails, single trails were exposed onshore for the periods described above. On return to the laboratory, a second trail was laid over the first to create a double trail whose thickness was then measured (n=10 per treatment). To determine the thickness of tracker snail mucus, the difference between the thickness of these double trails and the mean thickness of exposed single trails was calculated.

3. Results

All fresh trails showed a convex cross-section, were thicker in the middle than at the edges, and fresh double trails were significantly thicker than fresh single trails (figure 1a). At the mid (50%) point single trails had a mean thickness of 35.4 μm±1.5 s.e., n=10, though were slightly thicker at the 60% point (mean=37.7 μm±2.3 s.e., n=10). Double trails were significantly thicker than single trails at the 50% point (46.8 μm±1.1 s.e., n=10; Mann–Whitney test U=3.5, p<0.001). At the edges (calculated for the 10 and 90% points combined), double trails were again significantly thicker than single trails (means: single 19.0 μm±1.7 s.e., n=20; double 25.1 μm±0.6 s.e., n=20; U=91.0, p=0.003). By determining the area under the curves in figure 1a, double trails have approximately 27% more mucus than single trails, i.e. a tracker snail produces only approximately 27% of the mucus of a marker snail.

Figure 1

Thickness profiles of mucus trails. (a) Fresh single and double trails laid in the laboratory. (b) The decay of single trails after periods of exposure onshore. (c) The decay of double trails after periods of exposure onshore. (d) Trails produced by tracker animals over decayed single trails (obtained by subtraction). Mucus thickness measures at the midpoint and the edges did not differ significantly between any pair of laboratory and field controls (ANOVAs, ps<0.001). n=10 in every case.

When exposed onshore, the profiles of both double and single trails became thinner and progressively flattened, though a trail was detectable in both cases after one week (figure 1b,c). No trail could be detected after two and a half weeks' exposure. ANOVA showed that both at the edge (repeated measures, n=20) and at the midpoint (n=10), trails showed significant (p<0.05) reductions in thickness at each increasing temporal treatment, and that after one week both single and double trails had decayed to the same profile: their thicknesses were not significantly different. (Means for edges: single=6.1 μm±0.57 s.e., double=6.7 μm±0.55 s.e. Means for midpoint: single=11.0 μm±1.4 s.e., double=10.2 μm±0.45 s.e.)

On laying a trail over a mucus trail that had been onshore for two and a half weeks (figure 1d), animals produced a convex deposit which was similar in profile to that of single trails laid on glass (figure 1a). Statistical comparison showed that these two trail groups were not significantly different in thickness at the midpoint (U=46, p=0.759, n=10), but that single trails laid on glass were significantly thicker at the edges: means, over glass=19.0 μm±1.7 s.e.; over decayed trail=14.0 μm±0.7 s.e., n=20 in both cases (U=121, p=0.032).

On laying a trail over trails that had been onshore for one week or less, animals produced a much flatter deposit, markedly different from that produced on trails exposed for two and a half weeks (figure 1d). An ANOVA on the data in figure 1d revealed that at the midpoint (n=10), there was a significant difference in mean thickness (F5,54=225.96, p<0.001) and a post hoc SNK test showed trails grouped in thickness (from thickest to thinnest) as two and a half weeks>one week=two tidal cycles>one tidal cycle>field control=laboratory control. At the edge (repeated measures, n=20), there were also significant differences (F5,114=21.35, p<0.001), trails grouping as two and a half weeks>one week>two tidal cycles=one tidal cycle=laboratory control>field control. In general then, the longer the marker snail's trail had been onshore, the more mucus the tracker snail laid over it. Thus animals can save mucus and energy by trail following over fresh trails. For example, by determining the area under the curves in figure 1d, an animal moving over a two tidal cycles-old trail produces only approximately 49% of the mucus deposited in moving over a two and a half-week-old trail.

The effect of tracker snails laying a relatively flat trail over aged trails (figure 1d) is to produce a total (marker+tracker) trail profile that is again convex (figure 2), largely restoring the original trail profile and volume.

Figure 2

Resultant double trails laid in the laboratory over single trails after periods of exposure onshore. n=10 in each case.

4. Discussion

In making claims about energy saving through trail following, we make the assumption that the snails we used produced mucus with a constant composition and thus trail thickness is proportional to trail organic content and in turn to energy costs. We regard this assumption as reasonable because while variations in pedal mucus composition in littorinid snails have been described (Smith & Morin 2002), differences have been between the mucus produced while moving and that produced while stationary as an adhesive between the shell and substratum. Thus we attest that snails are able to save energy as mucus by locomoting over previously laid mucus trails. In locomoting over a fresh trail, the saving of approximately 70% of energy costs as mucus is considerable in the context of an animal that expends much energy on mucus production. While values for L. littorea are not available, literature estimates of the proportion of consumed energy expended on mucus in gastropods range from 7% in the nudibranch Navanax inermis (Paine 1965) to 31% in the limpet Patella vulgata (Davies & Hawkins 1998), and for polyplacophorans may be even higher (68% in Chiton pelliserpentis, Horn 1986).

We were unable to detect a mucus trail after two and a half weeks' exposure onshore and tracker animals laid mucus over such a trail which, in terms of its thickness, was indistinguishable from a fresh ‘single’ trail. Indeed, tracker animals appear to lay a mucus trail that recreates the broad profile of the trail as originally laid and this may be a requirement of the locomotory mechanism: a minimum layer of mucus might be required for the coupling of foot to substratum via mucus to be effective in propulsion (Chan et al. 2005). Nevertheless, since snails deposit mucus over fresh, ‘full thickness’ trails it appears that they are unable to switch off mucus production entirely. The mechanism by which the snails perceive the quality of a trail and thus are able to adjust their production of mucus is unknown and warrants further investigation.

Maximum benefit from trail following in energy terms will occur when trails are recently laid. This may explain why recent trails are followed more often (Chapman 1998; Edwards & Davies 2002), rather than as Edwards & Davies (2002) suggested, that snails are responding to the increased food content of these trails. Nevertheless, trail following may have the added benefit of enhancing nutrition for microphagous snails: food particles may be embedded in the mucus (Davies & Beckwith 1999).

Denny (1980b) measured the thickness of the mucus trail of the terrestrial pulmonate slug Ariolimax columbianus as typically 10–20 μm using the method of Lissman (1945). Slugs crawled on aluminium foil and the mucus was recovered by plunging into liquid nitrogen. This slug may indeed leave a thinner mucus trail than those reported here, though the fixing technique may have distorted the structure of the trail. The shape or profile of the trail was not given and it is possible that the convex shape reported here could enhance the capacity of the trail for collecting organic (food) particles from seawater. This is because a domed trail profile, as opposed to a flat profile, will present a greater surface area for organic enrichment.

The significant savings in energy through trail following may help to explain why trail following is such a well-established behaviour. Littorinid snails usually move in order to forage, to shelter or to find a mate. Where compatible with the purpose of the movement, snails may preferentially trail follow for the purpose of energy saving, rather than any other functional reason claimed in the literature (see Davies & Hawkins 1998, for review). Trail following in littorinids tends to occur in an opportunistic manner (see Davies & Beckwith 1999; Edwards & Davies 2002; see Davies & Hawkins 1998, for review) probably because snails are unable to detect mucus trails until they encounter them.

Culley & Sherman (1985) demonstrated that mucus production varies with the microtopography of the substratum, gastropods laying more mucus on rough surfaces to fill pits and crevices, than on smooth surfaces. Hence, crawling on rough surfaces requires more energy in the form of mucus and trail following on a carpet of mucus under such circumstances could be particularly beneficial.

Acknowledgments

We thank two referees for their constructive comments.

Footnotes

    • Received January 11, 2007.
    • Accepted February 6, 2007.

References

Notice of correction

    The first sentence of the Results section is now correct. 1 March 2007

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