The distribution of small and large Cellana tramoserica on rocky shores

Emily Williams (Supervisors: Associate Professor Ross Coleman, Dr Richard Murphy and Professor Tony Underwood)


Background
Emily with cellana

Distributions of animals are influenced by many factors, such as food, interactions with other species, or aspects of the environment, such as topography. Some places have better resources or shelter, attracting or retaining more individuals of a species. These individuals may be segregated into different size- or age-classes. Adult and juvenile lizards, for example, often seem to be occurring more often in particular microhabitats, thereby segregating size-classes. In some cases, this occurs because of territoriality by larger animals, which exclude smaller individuals. When large animals are removed, smaller individuals move to end up occupying the unused spaces.

An animal’s place within a population may also be determined by its responses to other individuals of the same species. Active avoidance of adult animals by juveniles has been observed in salamanders and chameleons. Juveniles, apparently neutral towards other juveniles, increase the distance between themselves and adults when large individuals are introduced into their habitat. Though the salamanders’ movements were not believed to be a result of cannibalism by adults, some small animals, such as fish, are eaten or attacked by their larger counterparts, creating segregation between age- or size-classes.

These interactions between small and large individuals can have lethal and non-lethal effects. Mortality of one or both size-classes has been observed to occur in a number of marine gastropods when they are confined together at increased densities. Growth is also often affected, typically decreasing for individuals when small and large animals are held together in a confined space. This is usually attributed to resources levels (e.g. food) and competition; one size or age of species often out-competes the other, resulting in decreased growth for the inferior animals.

Aims

The aim of this project is to investigate the distribution and movements of different sizes of the limpet Cellana tramoserica on rocky shores. Specifically, I have examined: (1) sizes of nearest neighbours of small and large focal limpets; (2) whether patterns of distribution for small and large limpets can be explained by movement of animals because of features of the limpets or of the place in which they occur; (3) the attraction or repulsion of small limpets to other small limpets and large limpets.

Importance
Cellana habitat

The limpet Cellana tramoserica is a common gastropod on rocky shores in NSW and occurs abundantly at Cape Banks. By examining the movements of small and large limpets, the distribution and behaviour of these animals may be better understood.

Results

During low tide at Cape Banks, there is a higher proportion of small limpets (< 18 mm) than large limpets (> 22 mm) surrounding small focal limpets on a small scale (10s of cm’s); large focal limpets have a higher proportion of large limpet neighbours. When moved to areas with mostly large or small limpets, small limpets move so that they end up closer to other small limpet neighbours than large, regardless of whether the area contains mostly small or large limpets. Large limpets move so that they end up closer to other large limpets when placed in areas with mostly large limpets, but end up closer to other small limpets when in areas with mostly small animals. This pattern for large limpets is more by random chance and dependent on the number of small limpets within a 30 cm radius.

From these results, small limpets tend to move so that they end up closer to other small limpets because of some feature of small limpets. Currently the attraction or repulsion of small limpets to other small or large limpets is being examined. The distances between small limpets increases over time, regardless of the treatment of neighbouring small and large limpets. This implies that there are experimental artefacts of handling and disturbing limpets, but small limpets may be neutral towards other small limpets and repelled by large limpets.