Strombid gastropods belong to the Strombidae family, which contains five genera and 75 species (Berg, 1974). Also known as conches, strombid gastropods are important to humans for their shell and as a source of food (Poiner and Catterall, 1988). Strombids have a large variation in their shell morphology between different species, but their internal anatomy remains relatively uniform (Savazzi, 1991).

Strombid gastropods are unique to other gastropods in that they have well developed eyes with an acute sense of vision, and they perform a unique style of locomotion (Savazzi, 1991). This style of locomotion is made possible by their highly flexible and muscularised foot (Catterall and Poiner, 1983). Most gastropods use muscular waves in their foot to ‘glide’ along the substrate, but strombids thrust their foot into the ground and push off, causing them to ‘leap’ forward with each movement (Parker, 1922).

Another interesting behavior that strombids perform with their foot is the righting of the shell when overturned. The leaps performed during locomotion are powerful and can result in the individual being overturned with their body exposed (Savazzi, 1991). This leaves them vulnerable to predators and they must act quickly to right themselves back to their normal orientation (Savazzi, 1991).  Their highly flexible foot is equipped with a serrated operculum which is able to reach around the shell and extend into the substrate and kick away, causing them to right back to their normal position (figure 1).

Some studies have investigated into the righting ability of strombid gastropods (Berg, 1974, Perron, 1978), but none have tested their righting ability over a number of different substrates. The aim of this study was to analyze the righting performance of Strombus luhuanus individuals over four different substrate types. Due to their foot morphology and righting techniques, it was hypothesized that the S. luhuanus individuals would perform better on the deep sand and coral rubble substrates as opposed to the shallow sand and bare substrates, with the worst performance on bare substrates.

Methods                                                                                                                    

Study Site

The study was performed on Heron Island, Australia (-23.441947,151.91268). Heron Island is a coral cay surrounded by a platform coral reef which encloses a lagoon. All study individuals were collected from the lagoon in areas of close proximity to the island.

Study Organisms

Ten adult S. luhuanus individuals were collected from the lagoon and transported to aquaria with flow through sea water. These individuals were kept in the tanks for three days to acclimate before experiments commenced. The aquaria contained coral boulders covered with algae for the individuals to feed on. All individuals were returned to the field after experiments were completed.

Righting Experiments

After the acclimation period was complete, the S. luhuanus individuals were tested for their righting abilities. The individuals were tested over four substrate types, which were: bare substrate (bottom of container), shallow sand, deep sand, and coral rubble (figure 2). To prepare the substrates, sand and coral rubble was collected from the field and placed in separate containers. Each container was filled with water to 25mm above the surface of the substrate.

To control exhaustion effects, individuals were tested over the four substrates in a random order (table 1). The individuals were placed with their ventral side up on each substrate type, and the number of attempts they took to right themselves was recorded. An attempt counted as any time the individual placed their foot onto the substrate and pushed away from it. Each test finished when the individual successfully righted itself 180° over to its normal orientation. A rest period of 30 seconds was allowed between substrate types, to let the individuals recover from righting themselves on the previous substrate type.

Data Analysis

Results

There was a significant difference in the righting abilities of S. luhuanus individuals between certain substrate types (p < 0.001, df = 3). Bare substrate took significantly more attempts on average (7.1) for individuals to right themselves than on any other substrate (figure 3, p <0.001, df = 3). Shallow sand took significantly more attempts on average (4.5) for individuals to right themselves than on deep sand and coral rubble substrates (figure 3, p <0.001, df = 2), but there was an insignificant difference between the average amount of righting attempts for shallow sand and bare substrate (p = 0.07, df = 1). There was no significant difference in the average righting attempts of individuals on deep sand (1.8) and coral rubble (1.7) (figure 3, p = 0.8, df = 1).

Discussion

As hypothesized, S. luhuanus individuals performed significantly better on deep sand and coral rubble than on shallow sand (p<0.001) and bare substrate (p<0.001). This difference in performance can be related to the righting technique of the individuals and the anatomy of the foot. Berg (1974) performed tests and analysis on the righting abilities of strombid gastropods including S. luhuanus. He observed two righting techniques that strombid gastropods used. The first technique was described as a ‘propodium pull’, whereby the individual places the foot and operculum into the substrate and pulls itself back around to its normal orientation (Berg, 1974). The second technique was described as a ‘metapodial kick’, whereby the individual places the foot and operculum into the substrate and quickly kicks away, causing the individual to right back to its normal orientation (Berg, 1974). S. luhuanus only uses the ‘metapodial kick’ technique, and this is what was observed in the experiments (see video 1 & 2).

For the metapodial kick to work, the operculum needs have some level of traction to the substrate. If the kick is attempted and the operculum cannot grip to anything, then the kick simply pushes the operculum away and the shell does not right itself (Berg, 1974, Savazzi, 1991). This is why the deep sand and coral rubble substrates took fewer attempts for the individuals to right themselves. The operculum was able to dig into the deep sand or hook around a piece of coral rubble to grip the substrate and perform an efficient kick. In the shallow sand and bare substrate, the operculum could not properly grip the substrate and slipped with attempts to kick. It was observed that in these two substrate types, the individuals would either right themselves with small rotations per kick or with failed kicks followed by a large kick with the foot extended to a greater reach. The large kick after failure suggests that some individuals would realize that more effort would be needed to right themselves back to their normal orientation.  

In the experiments performed by Berg (1974), he tested individuals on a deep sandy substrate, and S. luhuanus individuals took on average 2.9 attempts to right themselves. In this experiment, individuals on deep sand took on average 1.8 attempts to right themselves, which is similar to that of which was found by Berg (1974).

S. luhuanus is found primarily in sandy habitats associated with coral reefs, and this is likely as to why they perform best in deep sand and coral rubble as opposed to shallow sand and bare substrates (Catterall and Poiner, 1983). Although the individuals performed well on coral rubble, S. luhuanus are not usually found in fields of dense coral rubble (Catterall and Poiner, 1983). This is likely to be due to other reasons, such as the irregular surface of the coral rubble impeding locomotion and increasing the chance of overturning.

In conclusion, strombid gastropods are unique in their eye morphology and by their means of locomotion and righting. The morphology of the foot allows for their ‘leaping’ movements and their ability to right themselves quickly. S. luhuanus individuals performed better on deep sand and coral rubble than on shallow sand and bare substrates, and this reflects their distribution and habitat in which they are found in. The righting abilities of strombid gastropods are an interesting field of study, and further investigation into this field would provide a better understanding of these abilities.