Counting and measuring limpets in Pembrokeshire

Dale Fort have a couple of places left on their 'Population, ecology and sampling of rocky shores' course which starts on 22nd March 2012. To find out more or to book please contact Dale Fort Field Centre on 01646 636205 or email sally.df@field-studies-council.org

A version of the article below, written by John Archer-Thomson (from Dale Fort), appears in the current / spring edition of Natur Cymru.

Limpets, limpets everywhere but……… How many? Are they changing? Is this normal? How do we know? John Archer-Thomson emphasises the importance of long running data sets to get a true measure of the impact of disasters such as the Sea Empress.

Dale Fort is one of the Field Studies Council’s (FSC) residential Field Centres and has been so for over 60 years. The FSC has three Welsh Centres, one in Snowdonia and two in Pembrokeshire: Dale Fort’s speciality is marine and coastal ecology. I first joined the teaching staff in September 1982: at that time one regular student investigation looked at the population dynamics of common limpets on a moderately sheltered rocky shore called 'Frenchman’s Steps'. We investigated the vertical range of the limpets (how high and low they could live on the shore); their size range and how this might vary with height; and also how their numbers (abundance) varied up and down the shore. Immersion time in sea water decreases significantly with increasing height up a shore, consequently shores exhibit a pronounced environmental gradient from top to bottom. Salty but essentially terrestrial conditions exist at the top of the shore: marine conditions prevail at the base. As a result rocky shores are fascinating places in which to conduct ecological investigations.

The method for data collection is simple. Groups of students are spaced at regular intervals along a tape measure, which runs parallel with the water’s edge, at the known starting height near the base of the shore. Each group has a 50 x 50cm quadrat (sample area) in which they measure the longest diameter of all the limpets they can find, recording the measurements in 5mm size classes. Groups then move up the shore to the next height (75cm above) and repeat the process, continuing upwards at regular height intervals until they run out of limpets to measure. In official sampling terms this is an interrupted belt transect at 75cm vertical height intervals, with up to ten replicates at each height. Results are then standardised so that they are comparable despite originating from different numbers of groups / replicates.

Typical results

Numbers are lower at the top of the shore because of factors such as dehydration and temperature stress: they are also lower at the bottom of the shore because, although conditions are much better for marine organisms, there are issues such as competition for space (with other species better suited to this part of the shore). Indeed, on this particular shore the substrate becomes less suitable as there are more pebbles and less solid rock for limpets to attach to. Optimum conditions, between these two extremes, are to be found in roughly the middle of the shore, so this is where limpet numbers peak. 

Typically most limpets are to be found in the 10-14.99mm size class. To analyse this data we make an assumption that limpet size varies with age, the largest therefore being the oldest. This is a reasonable assumption for any individual shore but definitely not safe if comparing limpets on different shores. Growth rate in limpets is indeterminate, with no fixed maximum, and very sensitive to food supply. There are fewer big (old) limpets because they die (disease, predation etc.). There appear to be fewer small (young) limpets as they are much more difficult to see because of their diminutive size: young limpets tend to live in damp microhabitats such as crevices where they are difficult to spot.  Small limpets grow more quickly and would move into larger size classes relatively quickly.

Limpets also seem to get bigger (on average) with increasing height up the shore. Explanations for this vary and indeed the strength of the trend varies considerably from shore to shore, although it has been a constant in the Frenchman’s Steps data. Most small limpets are to be found on the lower part of the shore, which is immersed for longer periods, because this is where the  limpet larvae settle when they leave their planktonic phase behind them and their thin shells mean they are prone to desiccation. One theory suggests that as limpets grow they need more space so they migrate upshore to where there is less competition for a place on the rocks. This is fine, although some workers disagree that this occurs, but it does seem to conflict with another known aspect of limpet behaviour, that of  'homing'. Limpets have a place on the rocks they return to after foraging for food (they eat green seaweeds, lichens and the biofilm of microscopic algae and cyanobacteria on the rock surface. They scrape their food off the rocks with a tongue-like structure called a radula). In experiments I have done with student groups, 'homing' is over 95% successful. This rather contradicts the idea of limpets migrating up the rocks into space. A suggestion which makes sense is that homing is the norm until the limpet outgrows (or gets ousted from) its home scar, then it migrates up the rocks into space where a new home scar is instigated.

Sea Empress

Student groups vary in their motivation and competence and hence the quality of their results but I decided to keep the sets of data we had collected without being entirely sure why! Then, in 1996, the Sea Empress tanker threw 72, 000 tons of Forties Blend, light crude oil over the coast of the Pembrokeshire Coast National Park and Frenchman’s Steps got its fair share. Suddenly my (warts and all) student data looked very interesting as a record of what was there before the spill. Mortality rates were in the region of 50% for our shores around Dale Fort. Other shores fared rather worse – West Angle Bay suffered close to 95% mortality. The size data showed a shift in the modal class from the 'normal' 10-14.99mm size class to the 15-19.99mm one. Oil kills limpets and young ones are particularly susceptible, hence the shift in the modal class. Within a year, perhaps surprisingly given the extent of the spill, numbers had recovered to within what might be considered a normal range, but the size class data were still skewed to the right. Within two years the modal class had returned to the typical 10-14.99mm slot and the population was back (in gross terms) to what might be considered normal. The rate of apparent recovery was surprisingly rapid.

I wasn’t entirely happy with the quality of the pre-pollution data though, and I wished to know what represented a 'natural' variation in the population of limpets on Frenchman’s Steps. From 1996 onwards, every April, a group of post-graduate students from the University of Leuven, Belgium, and latterly the teaching staff at Dale Fort, has monitored the population and Figure 1 shows the results to date. There are two 1996 data sets (in yellow) as I wished to demonstrate that although this is student data, these two data sets taken within a fortnight of each other, by two different school groups, were remarkably and  reassuringly close to each other. Since then the size class data has resolutely stayed in the 10-14.99mm size class so this seems to be 'normal'. The number of limpets has varied, albeit within what seemed reasonable limits, until 2010 when results exceeded all previous years by so much that I thought my conscientious teaching staff had done it too well! When we all collected data in April 2011 the results showed record breaking numbers of limpets on the shore, confirming that the 2010 data was not a blip. 2011 was a very good year for limpets!

What happens next? It is tempting to speculate that the numbers present on the shore in 2011 are unsustainably high so monitoring will continue to see what happens – roll on next April!

Although counting and measuring limpets in little blue squares may not be considered cutting edge science I think the exercise has tremendous value. Long term data sets showing variation (or lack of it) in 'natural' populations are not that common. It is unwise to speculate on the effects of pollution, climate change etc. if the information about natural fluctuations in populations is not available. I think students benefit educationally from seeing how data they have collected fits into a bigger picture and has relevance to the real world in which, unfortunately, oil spills occur.

John Archer-Thomson is Assistant Head of Centre at the Field Studies Council’s Dale Fort Field Centre. 

Taxonomic note: I have been deliberately vague about the species of limpet we are talking about here. The common limpet (Patella vulgata) is likely to make up the bulk of the experimental population but I can’t rule out the presence of the china limpet (P. ulyssiponensis) more common on the lower shore and in pools and the black-footed limpet (P.intermedia) which seems to favour the lower shore and exposure to wave action. Telling the three species apart is difficult without removing the animal from the rock which stresses and possibly kills it. This would be totally unacceptable for student groups working on a regular basis in practical terms, and ethically best avoided unless scientifically essential. Frenchman’s Steps’ site characteristics favour an almost exclusive population of the common limpet hence the need to identify down to species level is unnecessary, especially considering the ethical cost.