Investigation on the size of Limpets.

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Investigation on the size of Limpets

Abstract:

During my preliminary investigations, I established that sheltered and exposed rocky shores, differed immensely in their size of Limpets.  Thus I was inspired to compare the size of Limpets on the sheltered rocky shore, Frenchman’s Steps.  The background knowledge and preliminary work enabled me conclude the following hypothesis; there will be a statistically significant difference between the size of limpets found on the sheltered rocky shore at two different vertical heights.  I decided to look at 3M and 6M above chart datum for my investigation, as looking at the profile graph (appendix) the number of limpets was high at that chart datum.

Aim: 

To determine if there is a difference in the size of limpets at two heights in there

Vertical range.

Scientific Background

 Limpets are animals that belong to the family Patellidae, subclass Prosobranchia of the phylum Molluscs. They have a strong, conical shell composed mainly of calcium carbonate, protecting the internal tissue, underneath which is a sucker, also known as the limpet foot, which allows limpets to adhere to rock surfaces, as shown in the photographs. Limpets are usually immobile unless they are foraging and so zonation is less apparent.

As can be seen from the pictures above, limpets have a hydrodynamic shape so that resistance to water flow is reduced. This ensures that it doesn’t overturn as it is less of an ‘obstacle’.  

Limpets feed mainly on microalgae, using a shovel-shaped radula. As the radula, also known as the ‘docoglossan’, is swept back and forth across the rocks, the rows of teeth hardened with goethite (iron oxide) rasp into the rock thereby removing the microalgae and subsequently ingesting them. Once a limpet has finished feeding, it returns to its home ‘scar’, a shallow depression in the rock formed by the abrasion between limpet shell and rock so that its profile matches shell shape, thereby reducing rate of water loss from internal tissue during emersion. The mechanism by which this occurs is still unknown.

An increased amount of algae results in more energy being available in the food chain as algae are photosynthetic organisms that convert sunlight energy into chemical energy. Algae, as well as other plants, store this energy as carbohydrate through the process of photosynthesis. Photosynthesis occurs in two stages – the light dependant stage followed by the light independent stage. In the first stage, reduced NADP (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate) are synthesised by conversion of photon energy to electrical energy in the light harvesting clusters found on the thylakoids of chloroplasts. These chemicals are then utilised in the next stage, known also as the Calvin Cycle, to form carbohydrates. Energy is thus stored within the bonds of the resulting molecules such as glucose. On absorption of these molecules, primary consumers that feed on these algae, such as limpets, harness this stored energy through respiration. First the glucose is converted to pyruvate through a series of biochemical pathways collectively known as glycolysis, which occurs in the cytoplasm of the cell. The pyruvate is then decarboxylated and dehydrogenated to acetyl coenzyme A, which subsequently binds to oxaloacetate to form citrate. After another series of reactions it reforms oxaloacetate through a biochemical pathway known as the Krebs cycle or the tricarboxylic acid cycle. Coupled to these reactions is the reduction of NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). These hydrogen acceptors pass on their electrons to the electron transport chain. As the electrons progress down the chain of electron carriers, electrical energy is utilised to synthesise ATP, also known as the energy currency of life. The hydrolysis of ATP yields 30.5 kJ of energy that can be used in active transport, locomotion, and anabolism - the synthesis of complex molecules from simpler ones under enzyme action. Therefore, primary consumers found on sheltered shores can synthesise more ATP than their counterparts on exposed shores where there is less algae. As a result the growth rate of limpets on sheltered shores will be greater than those on exposed shores.  

   As mentioned before, one of the most influential factors affecting species distribution is desiccation or aridity. The loss of water from tissue during emersion by any relatively immobile organism, including limpets, living on the shore poses a serious threat to its survival. Patella vulgata from lower shore lose water rapidly and on average half of the population dies when they have lost 50 – 55% of their body water. On the other hand, Patella vulgata from higher up the shore lose water very slowly, but half of the population dies only when they have lost 60 – 65% of their body water. In other words, the limpets found higher up the shore have adapted to the increased risk of desiccation, or drying out, experienced at this part of the shore. In fact, limpets found on the higher parts of the shore tend to grow larger than those belonging to the same species, found at the lower shore that is closer to the water. Larger limpets have a lower surface area : body volume ratio thereby reducing the rate of water loss. Species living at the bottom of the shore (sublittoral zone) such as the Patella aspera haven’t adapted in this manner, as the risk of desiccation is far less, consequently having the greatest rates of evaporation and usually die when 30 – 35% of their body water has been lost.  

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   The average lifespan of a limpet varies considerably depending on environmental conditions. Where growth is fast, such as on a sheltered shore, the lifespan can be 4 to 5 years while in places of slow growth this can be up to 15 to 17 years. The limpet species Patella vulgata is said to be a ‘protandrous hermaphrodite’ as younger individuals are predominantly male until the age of about 1 year when they undergo a sex change and progressively become more female. Therefore the male limpets of this species are usually smaller than the females, and their size usually ...

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***** A thorough account of the investigation which includes good attention to detail and use of A level biological terminology throughout.