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Questions and answers

on Rocky Shore topics

If you want help with sediment shores, saltmarsh, sand dunes and succession topics click here

For very general questions and interesting facts try here!

There are a lot of questions here so if you are looking for a specific species use the Control + F key combination to search on the page.

Julie Woodfield asks several questions about vesicles (bladders) in Fucus vesiculosus (Bladderwrack):

How does the air stay in them?Why are there fewer vesicles in exposed places? How does having bladders help in photosynthesis?

Firstly, it could be called "air" but not air as we know it (Jim). It's mostly nitrogen with varying amounts of oxygen, carbon dioxide and water vapour. The gases do not stay in them but diffuse in and out. The amount of oxygen seems to depend upon the partial pressure of oxygen in the surrounding medium. You might expect the gas inside the vesicles to be rich in oxygen (since it's a bi-product of photosynthesis) but this is not the case. The tissues of the vesicle will probably use oxygen in their vicinity for respiration and excrete carbon dioxide. Nitrogen gas is pretty much inert and that presumably is why its concentration builds up inside the vesicle.

The advantage of vesicles is that as soon as the tide rises your fronds are fully supported (because of the buoyancy of the vesicles). This means the individual is presenting maximum area for light absorption (for photosynthesis) and for gas exchange and nutrient absorption (these take place over the whole body of the seaweed). When the tide goes down, like the heroine of a Victorian melodrama you collapse into a moist heap, thus reducing water loss when emersed. Use of vesicles for support also reduces damage due to water movements. If you are flexible rather than rigid and erect (ooer missus...) when waves break over you or when water currents buffet you, there is reduced chance of damage. The drag will be greater on individuals with more knobbly bits (vesicles). More wave action means more buffeting and abrasion and less advantage in possession of vesicles. Fucus vesiculosus can occur at a wide variety of exposure levels because its morphology can vary in repsonse to wave action. On extremely exposed shores, you can find F. vesiculosus without vesicles at all when it is known as F. vesiculosus forma linearis. This can be hard to identify because it it sometimes appears as no more than an abraded bit of stipe clinging to the rocks.

Chloe and Danielle (and many more) have questions about Bladderwrack

How does wave action affect the average size of the bladders on bladderwrack?

Which shores, and why, has the longest fronds? has the widest fronds? most number of bladders? largest bladders? largest holdfast site?

Its all about the amount of surface area.

Bladders create an increased surface area on the fronds of wracks. The greater the surface area the worse it becomes in wave action as this will increase the chance of the weed being ripped off the rocks. In the case of Ascophyllum (Egg or Knotted Wrack) there is a great deal of surface area and it is not adaptable. As wave action increase so we see the alga increasingly damaged until eventually it will be a stump on the rock where the force destroys the fronds. Bladderwrack is different as it is adaptable to the conditions, varying the amount of surface area with wave action. On very sheltered shores you expect to see the density of bladders to be very high and the fronds will be long and broad. As wave action increases the density of bladders lessen, the frond gets shorter and narrower. Eventually, in extreme wave action the plants are small and bladderless. What I have described here is a bit simplistic and other factors may come into play. For example, in estuaries a lower salinity will influence thickness of fronds.


Kylie Thompson asks how changes in pH affect the distribution of Gibbula umbilicalis.

This is not an easy one to give specific information about because (as far as I am aware) there is little specific information available. There has been work on the water relations of some intertidal molluscs, though I've never seen any specifically on G. umbilicalis. Most intertidal molluscs seem to be at about the same osmotic strength and pH as seawater (as you might expect since that's the medium they originated in). Some (like oysters) are euryhaline and can tolerate changes in solute concentration (and hence pH). I have personally done some experiments to investigate the effects of changing salinity on G. umbilicalis (not specifically pH but you would expect pH to change with salinity and I suspect the overall effects would be similar). What happened was: In very dilute seawater the animals walked out of the pot containing them. In anything resembling normal seawater strength they either wandered around the pot or sat there with their foot extended. In stronger solution they walked out and in very strong solution they closed their operculi and sat there (presumably waiting for rain or the tide to come in and refresh their medium). It's only the animals that find themselves in rockpools that will be affected by pH and should they find conditions unpleasant they could walk out.

In conclusion, if you mean how does pH affect animals living out on the rocks or in crevices, I would say probably not much. If you mean animals in rockpools, well yes they would be affected but they could always leave if pH varies beyond what they prefer. (pH can vary quite a lot in rockpools because of respiration and photosynthesis and excretion).

Dan Wilson has requested information on the light absorbing powers and photosynthetic pigments of Fucus vesiculosus (bladderwrack).

This is a very complex subject and any modern textbook on seaweeds will tell you more than we can deal with on this page. Here are some general points:

All seaweeds have chlorophyll a as a photosynthetic pigment. Green seaweeds have chlorophyll a and chlorophyll b so they look green. Red seaweeds have chlorophyll a and extra pigments called phycobiliproteins (mostly phycoerythrin) this is red in colour. Brown seaweeds have chlorophyll a and extra pigments called carotenoids (mostly fucoxanthin) which are brown in colour. (It's more complex than this in reality). The extra pigments are called accessory pigments and their function is to absorb light energy and pass it on to chlorophyll a, which is involved in photosynthesis (in all photosynthetic organisms, including terrestrial plants).

Seaweeds can only photosynthesise using light of appropriate wavelengths. Their pigments can absorb light of wavelengths from about 400 to 700 nm, this is known as photosynthetically active radiation (PAR). Chlorophyll a absorbs strongly at around 675 and 450 nm. Fucoxanthin at around 450nm and phycoerythrin at around 550nm.

In the 1890s Engelman pioneered the spectroscopic investigation of algal pigments and came up with the theory of chromatic adaptation. He thought that algae grew at depths that reflected the quality (wavelength) of light that was available to them. This idea seem to fit very well with observations and was accepted by many for 100 years or so. Modern investigations based on direct observation (not available to Engelmann who relied on dredging up samples) have discredited the theory. It is now thought that total quantity of light is more important than quality. There are also factors other than light that determine seaweed distribution. The general pattern described by Engelmann of green algae near the surface, brown algae a bit deeper and red algae a bit deeper still is not really true either. About 25% of green algal species live deeper than 50m and a green alga holds the depth record for fleshy algae (168m).

Some of the coloured pigments are produced in order to screen the ones involved in photosynthesis from energetic blue and UV light. Accessory pigments of brown and especially red algae are particularly vulnerable to strong sunlight.

The deepest algae ever found (alive and growing) were at an astonishing 268m off the Bahamas. They receive about 0.0005% of the light available at the surface and must have amazingly efficient light gathering powers.

Martin Reynolds asks for reasons why dogwhelks on exposed shores have larger aperture relative to shell length than dogwhelks on sheltered shores.

In your question you mention the effects of wave action. It would seem sensible to have a bigger foot size (as indicated by aperture size) relative to body size (as indicated by shell length) to hold on with on exposed shores.

Some other reasons for any differences might be:

A big opening will render you more vulnerable to desiccation. This might not be a major problem on a very exposed shore where there is continuous spray. On a moderately exposed or sheltered shore it could be a more important factor.

Possession of a large foot and a (relatively) small shell will be an advantage for holding on to the shore but will render you more vulnerable to predators like shore crabs and oyster catchers. They will be able to access your dangley bits by attacking your large operculum (thinner than the rest of the shell). This will not be much of a problem on exposed shores because shore crabs do not thrive in exposed conditions and few will be found there. A shorter, more rounded shape (as you tend to find on exposed shores) will be easier for other predators like gulls to swallow. There have been some experiments which showed that whelks from exposed shores were eaten more quickly than whelks from sheltered shores when confined in cages with hungry crabs. This gives some weight to the idea that predation might be exerting a selection pressure towards smaller openings on sheltered shores.

Taking a longer term view one could argue that sheltered shore dogwhelks (who do not need a big foot) would be wasting valuable resources if they grew one. Thus any animals that do will be selected against (in sheltered conditions) in favour of animals that apportion more of their energies to useful things like reproduction or growing a thicker shell to protect against crabs.

If you can find a copy (try a library or inter-library loans?) you would do well to consult J.H. Crother's DOG-WHELKS An Introduction to the Biology of NUCELLA LAPILLUS (L.). FSC Publication E24 reprinted from Field Studies Vol.6, No.2 (1985) pp291-360

Claire Whittingham asks - I found no significant difference between mean width of Gibbula umbilicalis on sheltered and exposed shores. Why is this? I have found lots of information to suggest that there would have been a difference but am finding it difficult to prove otherwise

Interesting that you found information suggesting there would be a difference. It is a fairly tolerant species and does not tend to show much variation, even between shores of different wave exposure as it feeds when the tide is out. When the tide is in it can become relatively inactive.

If our students (at Dale Fort) do this they don’t often find a significant difference. I believe that this is due to inactivity when the tide is in and active when tide is out. What this means is that as the tide comes in they cease feeding and move to a “sheltered” area, e.g. crevices, rock pools etc, to avoid the wave action. Unlike say, dogwhelks and limpets, they don’t have to stay out on the rocks where the action is, instead avoid it and shelter. When the tide goes out, crawl out and start feeding etc again. So, I would say your results are fine, just develop what I have said here.

Check out the paper on topshells by Crothers (see question below). That has useful stuff in it.

Bethan James asks for the names of books and web sites pertaining to Gibbula umbilicalis.

It is often surprising to people that there are some subjects that no-one has yet written a book on. I do not know of any books specifically about the purple top-shell. What you need to do is conduct a literature search. You can do this in several ways. If you have access to a library with computer search facilities you can ask them to type in Gibbula umbilicalis and you will end up with a list of references to look at. If you do not have this luxury, go to a general book on molluscs and look up Gibbula umbilicalis in the taxonomic (or systematic) index at the back. There you will find a list of references to your creature found in that book . Most general seashore guides will tell you the name of the creature, where to find it and what it looks like. One which goes much further than this is the excellent A Student's Guide to the Seashore by J.D. and S. Fish. It might be difficult to find a copy of this in a general library though. Have you got a local university or college you could ask? There is a recent publication that should help: Common topshells: An introduction to the biology of OSILINUS LINEATUS with notes on other species in the genus. J.H. Crothers, 2001. In: Field Studies Volume 10 pp 115-160. Obviously this paper is primarily about a different species to yours but there will be a lot of useful material in it. The reference list at the back is a veritable treasure trove of relevant material. With regard to web sites. You could try asking a specific question (unlike the present question) at Try any search engine with the name and a variety of related terms (e.g. molluscs, shells, topshell, Gastropoda (the class), Prosobranch (the sub-class), Trochidae (the family) e.t.c.).

In reference to Gibbula umbilicalis would you be able to tell me what is the common food in relation to the zones in which they are found in?

All topshells feed on biofilms. That is they graze on microscopic growth on the rock. Much of this will be very young sporelings of seaweed that have started to grow. As such this means that they will not necessarily be species that originated in that zone but could be any species. Any microscopic "stuff" like detritus and plankton could be caught up in the biofilm.

A similar question: What do limpets eat? Limpets feed on "biofilms". This is a very thin layer on rocks made up of microscopic algae, the majority of which could be just the very young stages of bigger seaweed like bladderwrack. There will also be blue-green algae (cyanobacteria) within it as well as particles of detritus and even tiny plankton that become trapped in the sticky areas. Hence it is a mixture of micro stuff.

Samantha Brewer asks Generally, do seaweed fronds increase in size further down the beach, or do they decrease due to heavy wave action)

Generally, seaweed fronds get bigger to the lower shore – increased biomass. This is due to the longer time under the water as it increases the time that nutrients/minerals can be absorbed. There should be a direct correlation with increased size as you drop vertically down a rocky shore. Wave action is believed to be greatest on any one shore in the middle shore although turbulence could be greater in the lower.

Chris Adams of Barnes asks: Can Pembrokeshire crabs speak Welsh?

Wrth gwrs Chris. Pawb crancod yn gallu siarad Cymraeg. Ydych'in typyn bach twp Chris? Mae beic fodur gorau y Brough Superior.

Matt asks by email: why is it that smaller Carcinus maenas crabs are found higher up the shore than larger C. maenas crabs?

There is no set reason and you have to look at the ecology of the species and expand on all the key adaptations. Carcinus has the ability to breathe air for short time, live in dilute seawater for an hour or so. This means it can survive high on the shore where there is minimal competition. This means that larvae settling here will survive unlike other crabs. However, crabs will be settling all over the shore. Those lower down are bound to do better as they have a greater food supply – more available prey. So growth in the upper shore will be limited. They tend to feed when the tide is in, so less time in the upper shore. So poor growth would account for some size difference. However, most crab species I am aware of tend to be small on the upper part of their range, e.g. edible crabs are small and young on the shore but in sub-littoral are much larger. Second most important thing is that as they age they move into deeper water so that breeding can take place. So all the large crabs in the lower shore are mature individuals and once they have mated the females move into deeper water still to release the larvae to spread into plankton. This is a very quick simplified explanation and could be expanded considerably!

Katie Dunn from Wadebridge Sixth Form asks: How do dogwhelks breathe as they spend time immersed and emersed?

This is a good question that applies to lots of intertidal molluscs as well as dogwhelks. In common with other Prosobranch molluscs (limpets, winkles, top shells) dogwhelks respire via a gill like structure called a ctendium that is found in the mantle cavity. In species that live high up on the shore (e.g. rough periwinkles) more of the ctenidium becomes attached to the mantle surface which begins to fold and become highly vascularised and lung-like. These animals spend a lot of their time in air and so it makes sense for their respiritory kit to work well in air. Dogwhelks spend roughly half their time immersed and half emersed. Their ctenidia work best in water so what happens when the tide goes down? Firstly at low tide our whelky chums will be inactive, so their oxygen requirements will be low. Some gas exchange will be possible across the ctenidia as long as they remain moist. Molluscs can respire through the skin (anything between 20-50% of their requirements apparently). Prosobranchs have haemocyanin, a respiratory pigment which can give up oxygen at low oxygen tensions. (This might be of help although the true significance of it is unclear). Moreover a lot of intertidal molluscs can do facultative anaerobic respiration (we humans can do this with our muscles when exercising heavily) dogwhelks can probably do this as well. Before we depart this interesting excursion into the annals of whelkdome, did you know that Littorina neritoides (small winkle) can survive for several weeks in an atmosphere of pure nitrogen? (I do not know if anyone has tortured dogwhelks in a similar fashion).

Tigerlilly by email asks: How are beadlet anemones able to withstand fluctuations in temperature?

Beadlet anemones are like other primitive inverts in that they almost certainly do not have any specific mechanisms. Organisms either tolerate or regulate; anemones are the former, in other words they will avoid temperature fluctuations first by living in crevices etc but if they have no choice they will have to be tolerant. As temperatures rise so the metabolic rate will increase until eventually it will die. Low temperatures are accompanied by a lower metabolic rate. The only adaptation as such is the ability to cover itself in a slime-mucous. This will give some protection, especially against water loss.

Lots ask for this one: Why do limpets have a taller and pointier conical shell on exposed shores whereas on shelterd shores the conical shell is flatter and wider and less pointy?

If a limpet spends most of its life clinging to a rock (cf. wave action, desiccation) its muscles are tensed most of the time, when shell is secreted it is secreted at a steep angle. If the limpet is relaxed, its shell is secreted at a less steep angle hence limpets are flatter down shore or on sheltered shores. But they are very sensitive to local variation, rock slope, angle to wave action etc. so the patterns will change locally.

No one has a definitive answer and it is up to the individual to look at its biology and see how it might change. However, there are lots of variables.
If a limpet spends most of its life clinging to a rock (cf. wave action, desiccation) its muscles are tensed most of the time, when the shell is secreted it is secreted at a steep angle. If the limpet is relaxed, its shell is secreted at a less steep angle. This is because th eorgan that secretes the shell material  (the mantle) can get outside the shell and lay it down on teh outer edge. Hence limpets are flatter down shore or on sheltered shores. But they are very sensitive to local variation, rock slope, angle to wave action etc. so the patterns will change locally.

Aya Osman asks why is the abundance of lichen greater on a seaward facing rock than a leeward facing rock?

I know of no evidence from scientific papers but thinking how lichens work the following may be helpful: Lichens are completely dependent upon water splashing on rocks as they have no roots to absorb it from soil etc. This splashing is either from rain or sea. Those on the seaward facing side will have options of both whereas the leeward side only when it rains. Hence seaward side has the greater densities and variety. Likewise you will see a greater variety etc on more exposed shores where the splash is greater.

Nuzhat Razavi (and Nilam Khatani) asks please explain why there are more rough periwinkles on the exposed shore rather then the sheltered?

Lots of possible ideas! Rough periwinkles are very common all round the coast living in the upper to splash zone area. In conditions of high wave action they will hide in crevices so being small means more likely to escape. Large ones will be more likely to remain outside of cracks and crevices and get pushed off the rocks also more likely to be eaten. Predators e.g. crabs, are more abundant on sheltered shores. Other issues are that there will be more food on sheltered shores and conditions are better. This means other species (competitors) will be present on sheltered shores. Another reason why rough periwinkle numbers are lower on sheltered shores – more competition. Splash zones will be bigger on exposed shores and so greater area over which more periwinkles get the chance to live. This creates more competition between them and so may not grow so well.

To all organisms there is a limit to the amount of resources available to any population. On exposed shores the conditions favour small individuals as they can escape the wave action and crawl into crevices. Small ones will not consume so much food, so for the amount of food available the density is higher on exposed shores. The more sheltered the shore the fewer the individual and the larger they become. They feed on lichen and microscopic growths on the rocks. This will be fairly constant (amount per square metre) on all shores. However, exposed shores have a much more extensive splash zone; therefore more area, more lichen and more winkles.

All of this is unproven but a strong possibility.

Lance asks What are the reasons for rough periwinkles on an exposed shore being smaller in size, than rough periwinkles on a sheltered shore?

Rough periwinkles are very common all round the coast living in the upper to splash zone area. In conditions of high wave action they will hide in crevices so being small means more likely to escape. Large ones will be more likely to remain outside of cracks and crevices and get pushed off the rocks. This is just an idea and is difficult to prove. Other issues are that there will be more food on sheltered shores and so perhaps they just grow better in those conditions. Splash zones will be bigger on exposed shores and so more periwinkles get the chance to live. This creates more competition between them and so may not grow so well.

Rachel asks: Please could you help me by telling me what the differences are between interrupted belt transects and continuous belt transects are and the advantages/disadvantages to both. Also what is the difference between belt and line transects?

A transect is simply a line along which you sample. They are normally used where some kind of gradient exists e.g. from the lower to the upper part of a seashore, from the bottom to the top of a mountain or from trampled to a non-trampled area of vegetation. It makes sense to use transects in such situations because (hopefully) you get a representative sample for the least amount of sampling effort.

A line transect is the same as a belt transect but thinner. Quite when a line transect becomes a belt and vice-versa is a philosophical point that can provide hours of entertaining debate (the long Winter evenings just fly by). Usually a line transect would be a tape or string or cord which you lay along your gradient. A belt transect is exactly the same but you increase the area sampled by broadening it. You'd commonly do this by using a conventional quadrat frame and simply turning it end over end as you work up or down your line.
What dictates whether you use a line or a belt will usually be time. The only reason for sampling in the first place is because you can't measure/count (or whatever) everything. So if you have little time and a very long transect you might use a line transect. If you've got lots of time and a short transect you might expand your line into a belt. The width of the belt is up to you.

There are of course other considerations. For instance, if you were concerned with recording all species present on your site, a line transect with its small sample area might easily miss some. If it was a long transect and you didn't have time to do a continuous belt, you might decide to use an interrupted belt transect. All this is, is a belt transect with gaps in it. On a sea shore for instance you might decide to sample at a series of heights up the shore. If you were investigating the effects of city-centre pollution on lichens you might decide to sample from the most polluted area to the least, sampling at a series of distances from the centre.

Very often there is no prescribed method for this type of work. It is up to you as the investigator to decide how best to sample given the time, equipment and expertise at your disposal.

How long do limpets live?

Depends on where they are, anything up to 15 years or so on the upper shore (if they're lucky). We (here at Dale Fort) believe that on exposed shores limpets live "hard and fast" with a rapid metabolic rate. Having lived life to the full they die young, 3 - 4 years possibly. However, on very sheltered shores the life is soft with minimal environmental stress. With a slow and steady metabolic rate they live to a ripe old age, 12 years plus.

Julian of Southampton asks: How long do barnacles live?

Probably most of them for no more than about two years although I know of one Elminius modestus that lived for at least 12 years

Tony of Shrewsbury asks: What time of year do dogwhelks breed?

Spring usually


K. Linneaus of Upsaala asks: Why is there a bladderless form of bladderwrack?

Fucus vesiculosus (bladderwrack) has fewer bladders the more exposed the shore is. You can do projects relating a shores exposure to baldder density in bladderwrack. In sheltered conditions bladders offer an advantage lifting the plant up to maximise light and nutrient absorbing capability when immersed. Probably the air bladders are a hindrance in terms of increased drag and abrasion on very exposed shores.

Elizabeth of Windsor asks: What is chart datum?

The height to which lowest astronomical tides rarely fall below. Realistically it's the lowest low tide you can expect in a locality. Each place which has tidal predictions done for it (eg Dover, Milford Haven etc) has its own local chart datum. Depths on sea charts are expressed in metres below chart datum. This means that there will always be at least that depth of water, even if you are brining your ship into harbour at low tide on the lowest tide of the year. (Usually of course it will be deeper).

Jimmy B of Wiltshire asks: What's Ordnance Datum?

If you ever get bored, spare a thought for Arthur Drabble of Newlyn in Cornwall. Between 1st May 1915 and 30th April 1921 he was employed to measure the height of the sea every hour. It sent poor old Arthur over the edge into barkdome but it did produce a magnificent estimate of mean sea level. That's what it is, mean sea level at Newlyn. A bronze bolt can be found under a plate in the floor of a hut on the quay at Newlyn. This bolt is 4.751 metres above Ordnance Datum. All maps of Britain relate their height data to this point.

Tom Hutley asks for information on the anatomy of the holdfast of Pelvetia canaliculata

Large brown algae like Pelvetia have what is known as a pseudo-parenchymatous structure. They have none of the complex structures that you find in flowering plants and thus to ask about the anatomy of one of them is a bit of an inappropriate question. The anatomical part is the holdfast itself. If you cut them up inside you get tissue that resembles the parenchyma of a flowering plant but it's not organised tissue, hence the pseudo bit.

softballgirl179/gypsy (interesting nom de plume) asks What do you know about The reproduction of Anurida maritime (marine springtail)?

Not much is known specifically about Anurida’s reproduction but it seems to comply with the rest of the springtails. This means it is very simple as they are simple as insects go; recently some scientists have questioned whether they are even insects.

They are separate sexes; both the testes and the ovaries are just simple sacs (little bags) with a tube to the outside. These just create lots of cells inside that develop into sperm and eggs. They have very little on the outside (no genitalia as such) and so when they “mate” they the sperm may even be deposited on the ground with the eggs. There is very little courtship that has been seen. The eggs are small white spheres, deposited in small groups on the ground in damp places. The young that hatch out are tiny versions of the adult that moult around 6 to 8 times. As it grows so it adds segments to the body. It is usually sexually mature by the time it has moulted 5 times but this seems to vary a great deal.

How do Pomatoceros worms reproduce?

Pomatoceros is a hermaphrodite and breeds almost continuously during the year, peaking in spring and summer. The larvae live in the plankton for around 3 weeks in summer but up to 3 months in winter. Sexual maturity is reached in 4 months. Once the larvae settle they secrete a small transparent tube which in time becomes calcified.

Phillip Alexander asks Could you tell me please which marine animals filter their food and how they filter food.

I could write a whole book on the subject as so many animals do it and most will filter food differently. On rocky shores several important species filter feed: barnacles and mussels. Barnacles feed by using long hairy legs. The body is stuck to the rock by its head and so the legs easily move through the water in a curve – like a hand – and at the end curl the legs in to trap the particles in the water. These are passed to the mouth and then it repeats the movement of the legs again. Mussels draw water in to the body cavity through a wide hole called a siphon. Inside running the length of the body are many thin sheets called gills. These have thousands of grooves that trap the tiny particles of food which then get moved through the grooves by millions of tiny hairs (called cilia). These hairs make a current of slime that flows across the gills pulling the food to the mouth. Any waste or particles that are too big for the grooves drop into a stream of water that is leaving the body by a narrow siphon. As it is narrower than the one coming in shoots out as a jet of water, getting it clear of the mussel.

Other filter feeders include sedentary marine worms. Look up the Keeled Tube worm, Pomatoceros, on the website. These live in tubes and when the tide covers them the “hairy” tentacles come out and collect small particles. All the above about hairs is that they increase the surface for trapping the food. The particles are mainly very small pieces of organic matter, that is decaying bits of seaweed or animals. Check out this link on the website.

Lynsey Mace asks why is the geology and latitude of a sheltered shore an important variable to keep constant when investigating the abundance of limpets? In other words, does it have an impact on them?

Geology is important due to the ability to attach. Soft, sandy sedimentary rocks are tricky to get a grip; limestone erodes around the limpet. Latitude will only impact over a great distance as it influences temperature. If you keep them constant, like using samples from similar rocks you will have few problems. If you compare samples from soft sandy rock and granite the results will be different.

Emma says Limpets on rock pools are slightly larger in width and height than those living on bare rock, what is the reason for this? Also, how does global warming affect limpets?

Here are a few tips: From what you say you think limpets are generally larger in pools. You would perhaps expect that as there will be more food for them there. This is because it is constantly moist and so young algae can grow. Also limpets will be able to feed when they like – not restricted by tides. So more feeding means they are likely to be bigger than those just outside the pool. The level of wave action can be just the same and so the dimension of the shells are likely to be the same.

The temperature of the water affects things like reproduction and so global warming is likely to allow them to breed earlier in the year

Stephanie Marvier says that her coursework title is How Does the Diameter of Beadlet Anemones Differ in Rock Pools from Bare Rock on a Sheltered Shore. “I have had to accept my null hyothesis that there is no difference but i don't understand why this is. Surely they would be smaller out of water to retain water? Please help as i am very confused!”

There are many factors, age being one. As they age so they get bigger. I don’t know what your controls were and what variables you have considered. They are well adapted for preventing waterloss so can survive out of water for hours. Rockpools sound ideal but you have different and quite severe problems there: temperature and salinity issues that may prevent the anemone opening to feed. When the tide goes out there will be limited food trapped and probably a greater competition for food in the pool – what was the density of anemones in the pool compared to the bare rock? Often there are more anemones in pools. Over crowding will reduce food resources, prevent growth and so will be smaller. If an anemone is one bare rock, say at the base of a boulder, when the tide comes in there will be unlimited flow of water around it bringing constant supply of food – maximum growth.

M Woolley asks - I am trying to find information on Intra  and Inter-specific interactions and what the differences are.  I am really struggling.

This is a huge topic so here is a little information that may help.

Intra-spec comp is that which takes place within a population. E.g. bladderwrack on the seashore is a seaweed that is found in the middle shore. Each bladderwrack plant competes with each other for space on the rock and light. That is intra. However, on the shore are other populations, e.g. Knotted wrack (Ascophyllum). This competes with bladderwrack on sheltered shores for space. The latter lives for 3 years whilst the Knotted lives for 15-20 years. So, when the bladder dies after 3 yrs the knotted around it “muscles” in by spreading out and prevents any new bladder from coming in. Because we now have 2 different populations competing for resources it is termed inter-specific competition. Knotted cannot cope with wave action and so this is limited to sheltered shores. Bladderwrack however adapts and can survive some wave action; on those shores it competes with barnacles for space. Another example of inter-spec comp. Because the knotted has excluded the bladder on some sheltered shores it could be used as an example of “competitive exclusion”. The most well known example of that is between two different barnacle species, Semibalanus and Chthamalus. Here is an extract from the new updated website (due out in a few weeks):

Much research has been done on the principle of Competitive Exclusion with regard to the two species Chthamalus and Semibalanus. The former is a southern species, meaning that it is derived from warmer waters and so is tolerant of higher temperatures than many barnacles not to mention desiccation. The second species is northern in distribution and use to cooler, wetter conditions. In the UK we see the northern limit to the Chthamalus range and the southern limit of Semibalanus. Where they overlap and exist together they compete for space on the rock. Chthamalus is found in the upper shore where it is able to survive the higher temperatures and greater drying effect. Semibalanus is lower down but even so may dominate the shore. Chthamalus should be found lower down but doesn’t exist. This is believe to be due to competitive exclusion where the latter species is being excluded by Semibalanus as it is a stronger species and may crush the weaker one if it tries to grow where Semibalanus is located.



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