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Foot anatomy
The foot of gastropods are made up by muscular and connective tissue with a sole used for creeping on its ventral surface. It is split into two parts, the columellar muscle located dorsally and the tarsos muscle located ventrally. The columellar muscle deals with the retraction, extension and twisting of the head and foot, and is made up of large bundles of muscular tissue wrapped in connective tissue. The tarsos muscle, on the other hand deals with the locomotion and prey capture. Egg cases are also moulded by the sole of the foot in females. The sole of the foot consist of three distinct regions, the propodium, mesopodium and metapodium. The propodium in C. parthenopeum consist of several anterior pedal glands that has been suggested to produce mucus that aids in locomotion (Parker, 1911; Graham, 1988). The mesopodium plays the locomotory role while the operculum can be found on the metapodium (Ruppert et al., 2004).
Locomotion
Focusing on ranellidae movement, it can be determined by online literature and the fact that C. parthenopeum has a relatively smaller sized foot as compared to its shell that it utilizes muscular pedal locomotion. Larger footed species would be evidence that they achieve locomotion via ciliary action, which is better suited for sandy substrates. Therefore, muscular movement would be more suited for the hard and muddy substrates that C. parthenopeum inhabits. The ventral foot in C. parthenopeum is responsible for the movement of the species. However, in other gastropods, the use of the shell or operculum for locomotion can also be seen, such as in a number of Strombus species, in which they thrust their foot and operculum on to the substrate to achieve high leaps backwards (Berg, 1972).
Muscular pedal wave movement by gastropods are a result of simultaneous muscle displacement, where a patch or patches of the sole can be seen to move while other parts of the foot remains attached to the substrate, giving it both the ability to move in a desired direction while maintaining tenacity. Patterns of muscular locomotion can be categorized into two categories, rhythmic and arhythmic. This can then be further broken down into monotaxic, ditaxic and composite for rhythmic patterns and distinct and indistinct terminating waves for arhythmic patterns as shown in Figure 8.
Figure 8. Flow chart of different types of muscular
pedal movements found in gastropods. Redrawn
from Miller, 1974.
From the studies of over 300 species of marine prosobranch, Miller, 1974, classified cymatiids as possessing distinct arhythmic terminating wave patterns. However, both the continuous and discontinuous forms can be found in different species of cymatiidae. The following video (video 1) showcases the pedal wave action that was carried out by C. parthenopeum.
Video 1. This video was taken with C. parthenopeum adhered to a
vertical glass surface. This shows the conspicous distinct arhythmic
terminating wave patterns used in muscular pedal locomotion by this
species.
Video credit: Jacob Yeo
Video 2. This video captures the process which C. parthenopeum
adheres to a glass surface and begins moving across the surface at an
enhanced 12x playback speed.
Video credit: Jacob Yeo
Figure 9. Adapted figure from Miller, 1974, depicting
the movements of distinct arhythmic terminating
wave patterns over a period of 10 seconds, with
shaded patches denoting movement, which can be
observed in video 1.
It can also be noted that C. parthenopeum possesses a conspicuous pedal gland located within a transverse groove found right after the anterior edge of the sole (propodium) as shown in figure 10.
Figure 10. Ventral view of C. parthenopeum
with its foot extended and adhered to a
vertical glass surface. Propodium and the
sole of the foot are labelled, along with the
direction of movement.
Photo credit: Jacob Yeo
The propodium has been observed to independently edge over the substratum regardless of the movement patterns carried out by the rest of the sole. This fine rippling of the propodium has been suggested to be the result of localized hemostatic pressures, which could also be aided by ciliary action on the sole (Miller, 1974). This can be seen in the following micrograph (figure 11), that shows the cilia present on the epithelium of the ventral surface.
Figure 11. 400x light micrograph of the ciliated
ventral epithelial cells with a 10µm scale bar.
Photo credit: Jacob Yeo
The well-developed distinct arhythmic terminating wave patterns that can be found in Cymatiidae can be described as irregular patches of the sole moving forward in an unpredictable pattern. The waves do not have any form of division into prominent halves and also are capable of ending anywhere, as compared to ending at the end of the sole uninterrupted (Miller, 1974). Within the patches of sole during its displacement, the movement is retrograde, with pedal movement going opposite of that of the animal. This can be easily seen in the video (video 1) due to the movement of the distinctive dark brown spots of the sole of the foot. The foot is also elongated and narrowed during movement as compared to a more rounded foot when stationary, further emphasizing movement based on muscular locomotion. This pattern of movement are found on animals which live on hard substrata due to their need for higher tenacities (Miller, 1974).
Tenacity
Tenacity of gastropods have been found to be achieved through adhesion of the sole of the foot to the surface it is on. This was achieved by forces holding two closely spaced surfaces with a layer of liquid between them together (Menke, 1911; Davies & Rideal, 1963). The mucus, which are made up of water, protein, carbohydrate and lipids, are released in dehydrated, membrane-bound packages and are most likely made up of a mixture of mucins secreted by different glands (Davis & Hawkins, 1998). These are secreted from the glandular cells of the soles epithelium and subepidermal cells (Miller, 1974). However, some studies suggest that ‘high viscosity’ mucus could also be secreted from the anterior pedal gland in the propodium during locomotion, which remains inconclusive. (Parker, 1911; Graham, 1988). When hydrated, the mucus becomes a viscous elastic ‘solid’ which the animal can use as an adhesive and also as a substrate to move over through ciliary or muscular means (Davis & Hawkins, 1998). Glandular cells at the ventral region of the tarsos shown in Figure 11 and 14 are seen to have mucus stored in its vacuoles such as stated in Davis & Hawkins, 1998.
Foot anatomy
In order to obtain sections of the foot of C. parthenopeum, The animal was placed in a plastic container, which was submerged into a aquarium with flowing seawater. Once the foot of the animal had adhered to the plastic container, it was taken out of the aquarium and water was drained from the container. It was then placed into a -80˚C freezer for 10 minutes to anaesthetise it. A biopsy punch was then used to obtain 2 samples of the foot mesopodium just anterior of the operculum. The animal was then placed back into the freezer to euthanize it. The samples were then fixed in 4% paraformaldehyde fixative and stained using Hematoxylin & Eosin stain.
Although the sections were successfully obtained, the animal did not stay attached to the plastic surface as planned and had retracted its foot while in the freezer. This may have caused the sections to differ from those obtained from different studies such as Thompson et al., 1998, but no major differences were observed in this study.
As seen in figure 12, the difference between the columellar and tarsos muscle of the foot can be seen by the difference in colour of the two regions.
Figure 12. A 40x light micrograph of the
dorsoventral cross section of the foot just
anterior of the operculum, showing the
ventral and dorsal surfaces and hemo-
lymph spaces with a 100µm scale bar.
Photo credit: Jacob Yeo
The numerous ‘holes’ within the foot section in figure can be attributed as small blood spaces where hemolymph flows through. As gastropods lack a large hemocoel in their foot, the muscle tissue and small blood spaces acts as the hydrostatic skeleton (Ruppert et al., 2004).
Figure 13. A composite micrograph of 5 digitally
stitched images of the dorsal section of the C.
parthenopeum foot (columellar muscle) with
labels for observed muscle types and features.
Photo credit: Jacob Yeo
Figure 14. A composite micrograph of 3 digitally
stitched images of the ventral section of the C.
parthenopeum foot (Tarsos muscle) with
labels for observed muscle types and features.
Photo credit: Jacob Yeo
It is not an easy task to describe the various muscles and connective tissue found in the columellar (figure 13) and tarsos (figure 14) muscle due to their spiraling and densely packed nature (Thompson et al., 1998; Kier, 1988). The method employed in this study was similar to the ones used by Thompson et al., 1998, where muscles fibres were described according to how they look in the section. Figure 15 could give an idea of the different muscular organization one could get just by changing the angle of perspective.
Figure 15. Posterior region of columellar
muscle at the metapodium of Bullia digitalis.
Adapted from Trueman, E.R, Kier, 1988.
The use of helical and oblique muscle fibres enables the animal to carry out torsional movements (‘torsion’ in this section refers to the movement and not the developmental feature found in gastropods). The use of left- and right- handed helical and oblique muscles allows twisting in specific directions. Different fibre angles would also allow elongation and contraction of the hydrostatic foot (Kier & Smith, 1985).
The high density of musculature packed into the hydrostatic foot of gastropods gives it the ability to move in ways that cannot be classified into simple terms such as elongation, contraction, bending and torsion. These highly complex movements allows the animal to undulate over uneven terrain with maximum tenacity by keeping as large a contact surface between the sole of the foot and the substratum (Kier, 1988).
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