Muscular Foot Tenacity
Figure 1. Scaley Turban (Turbo perspeciosus) specimen, collected on Heron Island.
Gastropods have a large, muscular foot on the ventral side of their body that is used for both locomotion and attachment to substrate (Gade 1988). Using a mucous (made of water, salts and glycoproteins) secreted by the foot, Gastropods are able to attach themselves firmly to rocks during periods of high wave action (Denny 1980), preventing the animal from dislodging, which would render it vulnerable to predation (Gade 1988). Due to the importance of this muscular foot structure and function in Gastropod survival, the effect of foot size and attachment area on the attachment strength of a Gastropod to a surface is of particular interest. The following study was performed in order to investigate this previously understudied field.
The study aimed to investigate the effect of foot attachment area on the attachment strength of a Scaley Turban (Turbo perspeciosus). It was hypothesised that with increasing foot attachment area would be a corresponding increase in attachment strength, as indicated by a greater force requirement to separate an individual from a surface to which is was attached.
  
Figure 2. Assortment of photos from Heron Island, QLD.
The study was conducted at Heron Island Research Station on Heron Island, part of the southern Great Barrier Reef, Queensland, Australia, during the month of September, 2011. At low morning tide, 27 specimens of T. perspeciosus were collected from the reef crest between Shark Bay and the Research Station.
Specimens were placed in a small collection container full of seawater, and carried back to the laboratory. In the lab, the specimens were placed and partially submerged in a large, shallow tray full of seawater (see figure 3).
Figure 3. Specimens partially submerged in a large, shallow tray.
Using Aqua Putty, each specimen has a single washer attached firmly to the dorsal region of its shell (see figure 4). Specimens were left approximately 1.5 hours to allow the Aqua Putty to solidify, at which point a permanent marker was used on the hard Aqua Putty surface to label each specimen from 1 through to 27. One by one, specimens were taken from this tray and placed on a transparent sheet of perspex, that had been prepared with a fine layer of seawater film on the surface (see figure 5). A label with the number corresponding to each specimen was attached to the perspex sheet alongside the animal, as was a transparent ruler. As each individual moved aside its operculum, extended its muscular foot and attached to the perspex sheet, a photograph was taken from the underside of the perspex sheet (see figure 6).
  
Figure 4. Specimens that have had the washer and Aqua Putty apparatus attached to their dorsal surface.
Figure 5. Specimen on a transparent perspex sheet, partially pushing aside its operculum to exposure its muscular foot, just prior to attachment.
Figure 6. Photograph of attached muscular foot, with a specimen label for ID purposes and a ruler for scale.
Specimens were then returned to the initial tray, where they were again partially submerged underwater. The specimens were left for approximately an hour, to allow time for them to relax and attach to the substrate, which in this case was the plastic tray. Using a gentle tugging motion, animals were tested to determine their level of attachment. Each time a specimen was identified as being firmly attached to the surface, an electronic fish scale, which recorded force in newtons (N), was hooked into the washer attached to that individual. The fish scale was then gently but firmly pulled directly upwards and away from the individual, with the force at which the animal was being pulled slowly increasing over time. As the animal detached from the substrate, the force that had been reached was recorded, and the animal was placed back into the initial tray. This process was performed for all individuals, approximately 5 times per individual, allowing approximately 30 minutes between each trial. Upon completion of all trials, the maximum force recording for each individual was identified, to be later used in statistical analyses. All specimens then had their Aqua Putty and washer apparatus removed, by pulling the washer downwards towards the ventral surface of the animal. This left little to no residue or evidence of the previously attached apparatus. Specimens were then released back at the site of collection at high tide.
Figure 7. Specimen collection.
The photographs were analysed using the program ImageJ. Using the ruler for scale, 2 foot measurements were made of each specimen/photograph (see figure 5). To gain an overall measure of
size for each specimens foot muscle, a principal component anlaysis (PCA) was used to combine the two measurements into one. This was then used as the measure of foot size in later analysis. The force measurements collected previously were not normally distributed, which was corrected for by performing a square root transformation. A general linear model (glm) was run on the principal component of foot size and the corrected force measurements.
The force required (√N) to separate a the foot of a T. perspeciosus from the subrate to which it is attached was found to increase with the size of the foot (PCA size) (glm F=28.75 1,25, p <0.001, n=27). A PCA foot size of 0 is the average foot size of those specimens studied, while +/- is how much larger or smaller than average that individuals foot was. The force required to detach the largest PCA foot size of 2.63 was 1.49√N, compared to the smallest PCA foot size of -2.65 which required a force of just 0.49√N to detach the animal completely from the substrate (see figure 8).
Figure 8. Graph demonstrating the effect of foot size (PCA size) on the maximum force (√N) required to detach a T. perspeciosus from the substrate to which it is firmly attached (n=27, R2=0.73).
As can be understood from the results of this study that the area of attachment of the muscular foot, referred to as 'size', has a significant effect on the strength of attachment to a surface of an individual. This is seen by the amount of force that is required to detach the animal from the substrate to which is has attached itself, through the excretion of the sticky mucous produced by the foot.
Observational evidence suggests that foot size increases with overall shell size, however this should be tested in future studies. Such a correlation would suggest, however, that large Scaley Turbans would have a fitness advantage over comparatively smaller individuals, as they would be able to attach more firmly to the substrate within their environment. This would allow them to avoid predation and unwanted detachment as a result of wave action, which renders them vulnerable (Gade 1988). With predicted climate change comes the likelihood of increased storm action and unpredictable weather patterns (UNFCCC 1992). This only stands to increase the pressure and difficulty faced by the Scaley Turban to remain attached to the substrate. As such, if these unprecedented weather predictions and storms do become a reality, and potentially even a common occurrence, there may be a shift in average size of T. perspeciosus to larger individuals with a corresponding large foot. This shift would be a result of natural selection selecting for the trait of a large foot, as these individuals are likely to be best equipped to cope with the predicted trying conditions associated with climate change.
|