Cepaea Surveys: now....and then!

I guess that many members will be aware that the Open University is staging an “Evolution Megalab”, a scheme to increase public awareness of evolution, as a contribution to Darwin Year (2009). They have chosen the famous colour and banding polymorphism of Cepaea hortensis and C. nemoralis (Fig 1) as the object of study, and the scheme involves participants from all the European countries in which the species occur. I did report on it to Council last year. It has received support from the Royal Society and from the British Council.

 

Participants will be invited to submit records online, detailing samples they have made, including the numbers of each colour and banding morph found. They will get feedback on the composition of other nearby samples, and, where possible, an analysis of how their records compare. There is already an interactive website up and running (a test version) with lots of instructions and background material. You can access it at www.evolutionmegalab.org , but you will need to register (free) before you can enter records.

 

The aim of the project is to get as many records as possible from across the range, and to analyse the patterns of variation in morph frequency. We know that there are some broad geographical patterns (with many local exceptions!), and there is often variation with habitat too. Over the course of the last 18 months a few of us have been entering historic records; there are around 8000 site records for Great Britain (some with both species). The earliest are records made by A.E. Boycott in Herefordshire in 1893. We have entered about 1600 records for other countries (including the whole of Ireland), but there will be many more as local organisers enter data for their own countries. I have been told that this is the largest genetic database after that for our own species, Homo sapiens. We shall use this to see if there have been changes over time. We know already that there have been some changes due to climate, and also that there have been many local alterations of ranges.

 

The scheme is managed by the Open University; the team is led by Professor Jonathan Silvertown, with the assistance of people like me who have studied Cepaea for many years. One aspect of it is worth emphasising: the analyses will be at all scales from the very local to rangewide. Where there are local groups who want to analyse and publish their own data, or to use output for local events and recording schemes, all possible help will be given. We have already agreed that all the records will be entered in the Non-marine Recording Scheme of the Society, but I have had time as yet only to give Adrian Norris a tiny part of the total (my own records for Yorkshire!).

 

I will be holding some practical workshop sessions for those interested at the long indoor meeting on January 31, and I will give a more formal talk at the recording meeting on April 18. I think the project will start seeking wide publicity early in 2009, by which time the website should be in its final form. I am happy to give talks or workshops from October this year onwards.

So much for the present! This is not the first time that there has been an attempt to get lots of people to survey Cepaea. The earliest attempt was made in 1901, when the Society set up a Committee for Collective Investigation, who named one project “Do Tachea nemoralis and T. hortensis occur together or separately (1) in the same district, (2) in the same locality, (3) in the same kind of habitat? (Journal of Conchology, 10, page 88). The response was meagre, and the committee disbanded itself the following year, though some reports trickled through between 1903 and 1922. The question, however, is still of interest!

 

Much more significantly, Cyril Diver attempted to organise a survey of the polymorphism in 1938. Diver himself had accumulated many records between 1920 and the Second World War. He had been receiving samples from other people for many years including A.E. Boycott, A.E. Ellis and Greevz Fisher. His own records, and those sent to him are meticulously entered in his notebooks, part of the Diver Archive in the Natural History Museum, London. I have entered more than 300 records into the Megalab database. He evidently wanted to expand this work, and prepared instructions for participants. Here is a transcript. The original (of which there are several copies) is in the Diver Archive in the Mollusca section, Natural History Museum, London (Box 4). It is dated October 1938. It was evidently sent to several people, and there is at least one reply as late as April 1939, from Lionel Adams. In going through the entire archive to extract details of Cepaea samples scored by Diver, I have not found any records later than 1938/9. It would appear that the Second World War put a stop to any further effort, although Diver had clearly returned to his interest in the polymorphisms in the late 1930s. The instructions are interesting both for what he thought it practical to ask of people, and for what ideas were going through his mind at the time. He was clearly considering both selection and drift. They have a surprisingly modern feel for something now 70 years old. Note that today’s instructions are a little different!

 

 

1. The primary object of the enquiry is to determine the frequency with which the different varieties (phenotypes) occur in the colonies of C. hortensis and C. nemoralis; and how these frequencies vary from colony to colony within the same area, or in different parts of the geographical range. A secondary object is to increase our knowledge of the ecology of these two species. The method of the main enquiry is essentially statistical, and to be of value all collections from colonies must be random samples – that is, the collector must not exercise any selection whatever against poor or bad specimens, but all must be included.

 

2. The method of collecting employed is to take every adult shell, alive or dead, that can be found during a single period of collecting. Experience suggests that, apart from exceptional circumstances, such a sample probably represents about 30% of the total adult population, so that the method is not so destructive as it sounds. But obviously continuous collecting from the same colony may well succeed in obliterating it. By “adult” is meant a shell with a completed, firm lip. No young, or shells with a still growing or soft edge, should be taken.

 

3. The least damage will be done to the continued existence of the colony if the collections are made in the autumn (that is after the adults have contributed their full quota of eggs to the next generation), or in the early spring on emergence from hibernation (before the next generation have completed their lips).

 

4. The following notes should be made at the time each colony is sampled:-

(a) Date of collecting: weather conditions.

(b) Location of colony, preferably with reference to the 1 inch Ordnance Survey map of the district. If the geological formation is known, it should be included.

(c) Type of habitat (e.g. hedgerow, beech wood, waste land, down grass, sand dune etc.).

(d) Approximate extent of colony (in paces or yds) and/or of the area actually worked.

(e) Where a series of neighbouring but discontinuous colonies is worked (e.g. along a road or hedge system) distances between colonies should be given or marked on a scaled map.

To which should be added if possible:-

(f) A more detailed description of the habitat (e.g. in the case of hedgerows whether the road bank is regularly ditched and topped by well kept quickset or whether the vegetation is rank and seldom interfered with).

(g) A short list of the most prominent plants particularly those up or in which the snails may be found to be moving or resting (e.g. on sand dunes or dune grass whether concentrated in tufts of marram, Iris etc; or in hedgerows whether dead stems of umbellifers or nettles are frequent. It is not suggested that time should be spent on identifying the exact species of dead umbellifers, unless this can be done with certainty at sight). Notes on other species of snail that may be prominent should be included.

(h) Whether the colony is confined to uniform conditions of habitat, or extends continuously through two or more slightly different habitat types; and whether in the former case the colony extends throughout the whole of the habitat or only occupies a part of it.

(i) Any notes about changes in population density that strike the collector, and facts with which it is suggested they may be correlated.

(j) Any peculiarities or other details which may be observed.

Only the first five headings are essential for the general statistical treatment of the sample. But it should be noted that for any ecological study the fullest descriptive details about the behaviour and distribution of a colony in relation to its habitat is of the greatest value; and in the present state of our ignorance it is impossible to say what detail may safely be neglected.

 

5. The different characters on the shell, e.g. the nature of the ground colour (yellow, pink, brown, etc.), the presence or absence of banding, the band formula, the nature of the band pigment, etc., are known to be inherited on mendelian lines. Therefore, from the distribution of these “phenotypes” it is possible to estimate how the “genotypes” (i.e. the hereditary constitutions) are being distributed by the process of mating, in other words to determine the “breeding structure” of the population. This line of enquiry makes it possible to estimate the parts played by different evolutionary mechanisms. Experience of these species suggests that individual movement for mating purposes is very limited, and consequently there is a large amount of inbreeding, even within different parts of a single colony. It is, therefore, preferable to work any reasonably large colony systematically, keeping the different sub-samples separate.

 

6. I have found that the most convenient method of collecting is to use bags made of canvas or sacking with a pull-string top, each bag bearing a large clear number in indelible pencil. In working dense sand dune populations of nemoralis where within a single quite small sub-sample area anything from 200 – 500 snails can be collected, sacking bags of about 10 x 10 inches are most useful. For ordinary hedgerow and other small discontinuous populations much smaller bags of less coarse material that will hold up to about 80 snails are much more convenient. The bag has great advantages over the cigarette tin (cardboard and paper cannot be used as they are freely eaten). Not only is it easier to carry and to collect into, but the snails can safely be left in it dry for a week or so (perhaps with a little paper to eat) until they are cleaned. Whereas they cannot be left tightly packed in tins without dying. Further if the number of the bag is clearly given in the field notes no other label is necessary.

 

7. Information from really small populations (i.e. where the whole colony can be counted in tens) is particularly wanted even though the sample only contains as few as 10 shells. From more populous colonies a sample of at least 20 should be attempted, and samples of 50 to 100 are preferable. But the size of a sample should never be increased by going outside what are the obvious limits of a sub- sample area (e.g. where the habitat shows signs of a slight change). A series of small adjacent sub- samples, which can always be amalgamated later if the figures and facts justify this course, is far more valuable than a single large sample from a heterogeneous habitat which cannot later be dissected.

 

8. Snails are preferably sent uncleaned (particularly from mixed colonies where the two species live together), and if so are better left in their bags which should be packed in some firm box. The bags will be returned later and, if desired, postage will be refunded. I prefer to retain the shells here where they are available for checking or for biometrical measurements if desired. But individual specimens or whole samples that the collector may desire to keep for his own cabinet will of course be returned if asked for.

 

9. Packages and any requests for further information should be addressed to: Captain C. Diver, 40 Pembroke Square, Kensington, London W. 8. If the systematic working of a large continuous population (e.g. on an area of sand dunes) is contemplated, further information should be applied for before work is begun.

 

10. The data at present in my possession have been drawn from about 450 different populations, but the majority are from the south of England. Further samples from northern England, Wales, Scotland and Ireland are much needed, and particularly from outlying places near or at the limits of the geographical ranges.

 

11. Where a number of broken shells is found round a “bird stone”, all the broken bits (of whatever species) should be carefully collected, the nature of the “stone” recorded, and if possible a control sample of the living population within a radius of about 10 yds or so should be taken. Similarly rat- or mouse-eaten shells should be included in the samples in which they occur.

October, 1938.