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Chthamalus stellatus (Poli) and Balanus balanoides (Linneas)

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Invasions of ecosystems by exotic species are increasing, especially in aquatic environments. Barnacles populate the coastlines of countries around the world. Specifically in Britain, we have two main types, which along the southwestern coastline have become distinguished for their different zones. Within this competition for feeding and survival, there is not substantial vertical overlap. This investigation is viable when talking about the competition of species. One of the most famous ecological field studies is that by Joseph Connell on the rocky shores of Scotland. The distribution of two competing species of barnacles, Balanus balanoides and Chthamalus stellatus, has shown to depend on physiology (tolerance of desiccation) and predation as well as aggressive competition by Balanus.

Biology behind my investigation

As Chthamalus stellatus (Poli) has migrated to the British Isles, the competition between the native British barnacle, Balanus balanoides (Linneas) has become evididnt. There is clear zonation between the two species, with little overlay due to the diverse habitats at which the barnacles have become adapted. The main cause for this separation is due to the ability of the different species to survive under different conditions, to be able to function effectively, to feed and to reproduce successfully without hindrance from the environment around them.

Things such a desiccation and the ability to feed are important for the species to productively colonise the rocky seashore and remain dominant. The competition between the two species resulted in Chthamalus migrating further up the shore. As being of southern origin, which is better adapted to heat and desiccation, the reasoning for this is that this species is then able to adapt to being in the exposed to sun and also not being submerged in water for great time periods like Balanus has.

Chthamalus is common or abundant over a few feet at the top of the midlittoral zone, whereas Balanus is abundant and dominant over the remaining midlittoral zone and littoral fringe. The ability to feed as well as reproduce is highly important as barnacles can only do so underwater. Chthamalus has shorter periods whilst in the water over long periods, whereas Balanus is covered with water for longer periods, with shorter periods of being exposed. After hatching from eggs, barnacles are planktonic. They float around the oceans moved by wind and ocean currents. Chthamalus stellatus adults usually found higher in intertidal than Balanus balanoides. As with most barnacles, in fact, there is a sharp line between the zones occupied by these two species.

However, there is much overlap in the vertical distributions of newly settled larvae. The species compete for space. Balanus have heavier shells, and grow more rapidly. As individuals grow, their shells edge underneath those of Chthamalus and pry them off the rock. Chthamalus can grow in the upper parts of the intertidal because, indeed, they tolerate desiccation better. Chthamalus are removed from Balanus so when Chthamalus are removed, Balanus do not invade. Therefore, in this case the species “coexist” in a limited fashion.

The reality is that Balanus competitively excludes Chthamalus wherever it can persist, but Balanus stays out of the intertidal zone because there are places where Chthamalus can live and Balanus cannot. This competition would allow the feeding and fertilisation of Balanus to become more rapid as opposed to Chthamalus, which would have limited time to reproduce and feed. This is the cause of Balanus to have a higher population and percentage cover.

With these facts in mind, clear zonation between the two species, is visible as there would be no means for the northern species to survive in as exposed conditions.

The hypothesis

The two species of barnacle, Chthamalus stellatus (Poli) and Balanus balanoides (Linneas), are self-maintaining, so there will be very little integration; as they have distinguished themselves form each other.

This hypothesis will be suitable and present attainable data due to its grounding in science and its plausibility of completion. It is a known fact to many scientific journals and other scientific literature that there is a definite area of the two species, by trying to prove this hypothesis, the already common knowledge will be deeper, and as primarily, this hypothesis is trying to discover the extent to which the segregation occurs.

Method of the investigation.

The investigation to test the hypothesis, took place on a semi-sheltered coastline in the south west of England. This site was chosen because there was the possibility of reproducing other transects over different locations to allow variations in data to produce a broader range as to which calculate the values. The area which was chosen to place the transect contains similar densities of Chthamalus stellatus and Balanus balanoides, suggesting that this rocky seashore would be ideal for quantifying the invasion of Chthamalus and recording the competition in invasion success.

The percentage density of the two species from high tide level to mean low tide level was measured, along two transects that ran perpendicular to the sea. To this, the hypothesis can be applied and to distinguish a pattern of the distribution of the two species. By taking my quadrat readings at random, and ensuring that they are along the same transect; this would allow there to be an assumption that the test carried out would be a fair test. By doing so, the statistics recorded will not be manipulated and altered, by placing on a spot significant to that of no integration.

By carrying out the experiment in this way, there will be want to strive to disprove the null hypothesis. If the results are concordant to the hypothesis, it will be able to assume that the null hypothesis is not at all significant. Preliminary experiments would have taken place prior to the field study to examine the variation in abundance and zonation, with the exception that this used was to distinguish between the two species and become aware of the differences between the two species.

The apparatus used had to be relevant to the investigation. To aid the difficulties in identification, a hand lens was used to magnify the barnacles. A specific quadrat was used, which was a lot smaller than the standard ones, which would have been too big to grasp any type of percentage. I used a fixed measuring tape to replicate a transect along the beach. This apparatus was specific to my investigation, as they directly enhanced the speed and ease of my investigation.


This is fundamental whilst working in an area with water. I had to ensure that I never had my back t the tide and also that I had enough time to carry out my investigation, as with the changing tides, the rocks became wet and thus slippery.

I had to be aware also that the environment that I was investigating was a living ecosystem, so I had to be careful not to impede this and not to damage it, which could have a knock on effect on the ecosystem and the organisms within it.

My null hypothesis

There is no viable zonation between the two species of barnacles and therefore, a large area of integration without a significant partition of the species.

Problems Faced During Investigation

The main problem faced was the distinction between the two species of barnacles. This was overcame this when carried out in the initial investigation. The barnacles are hardly distinguishable to the naked eye; the differences arise mainly with the shape of the barnacle and the difference of the Scutum and Tergum (Operculum) indicated by 1. This was hard and so, during the initial fieldwork, the time allocated for distinguishing between the two, using flash cards was useful. A hand lens, which used, magnified the operculum enough to distinguish between the kite or diamond shape.

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