CHAPTER 1
Introduction
Rocky sea shores are among the best habitats for natural history investigations. Not only is there public access (once you have got there) but also they are different, exciting and, potentially, slightly dangerous places.
The lives of animals on rocky shores seem to be dominated by physical factors that we, too, may experience – including desiccation, inundation, wave action and extremes of temperature. The effects of these physical factors may change significantly over very short distances so that zonation and other distribution patterns may be instantly apparent. As a bonus, most of the animals and plants live out on the open rock surface so that there is often no need to disturb the habitat in order to observe them. Finally, rocky shores are among the most 'natural' of habitats in the British Isles; unless there has been a recent oil spill and away from outfalls, rocky sea shores are unlikely to have been greatly affected by human activity.
Of the many different kinds of invertebrate animals to be found on British and Irish rocky shores, marine snails (Phylum Mollusca, Class Gastropoda, Sub-Class Prosobranchia) are a particularly easy group to investigate, thanks to the strong hard shell that they secrete to protect the delicate body. Shells are easy to measure, and also to mark in various ways without affecting the behaviour of the snail.
This book is concerned with living snails not their empty shells. (Some people refer to these as 'dead shells' but this is a misnomer; they are the shells of dead snails.) Collecting such shells may be a pleasant, and harmless, pastime but their distribution will not provide much biological information. The composition of shell beaches tells us more about the vagaries of water movements or the resistance of certain shells to the erosive effects of wave action than it does about the differential abundance of the living snail fauna.
Some shells are put to a secondary use after the death of their original owner. Hermit crabs use them as protection for the abdomen, regularly up-sizing their homes as they grow. Adults of the largest hermits almost always end up in shells of the common whelk (Buccinum undatum) and the locations of empty whelk shells on the shore may relate more closely to the activities of hermits (or of their avian predators) than to the activities of whelks!
The sea shore is, by definition, the area that is sometimes covered and sometimes uncovered by the sea. Human observations of the invertebrate shore fauna are, not surprisingly, concentrated on the daytime periods of low tide. But the fauna is almost entirely composed of marine species that have colonised the area from the seabed beneath the tidemarks. The animals are usually most active at high tide or at night and the day-time low-tide periods are times to be endured – especially in warm sunny weather. Only the most highly evolved species can survive the conditions high on the shore and it is usual to find that species richness increases with distance down the shore.
Any field work involving sea shores is dependent on the tidal cycle operating at the chosen site; and Britain experiences as great a variation in tidal range as is to be seen anywhere in the world. Our largest tidal range, of more than 17 metres, occurs in the Severn Estuary (under the original Severn Bridge) whilst the smallest, of 0.5 metres, is credited to Machrihanish in southwest Scotland (see p. 62). In those parts, waves can be more significant than tides and atmospheric pressure has more influence than the moon on water levels.
Irrespective of the tidal range at your chosen location, the lowest (and the highest) tides, called spring tides, occur shortly after periods of full and new moon and neap tides, those of smallest amplitude, fall at times of the first and last quarters of the lunar cycle. The most dramatic spring tides are seen at the equinoxes and the least impressive ones at the solstices. Over and above the regular annual pattern, there are longer-term cycles that cause very small variations in the highest and lowest water levels – only really noticeable in the Bristol Channel and other areas of large tidal amplitude.
Tidal predictions, originating from the Proudman Laboratory, Liverpool (known irreverently to some as the Canute Institute), are calculated assuming normal temperature and pressure, but observed water levels are also influenced by variations in atmospheric pressure and in the strength and direction of the wind. High pressure depresses water levels and low pressure allows them to rise higher; strong onshore winds raise levels and strong offshore winds lower them, especially in bays and estuaries.
On most shores, the greatest variety of weird and wonderful sea creatures is to be seen at extreme low water of spring tides. Few marine biologists can resist the call to hunt along the water's edge on such occasions, especially when high pressure and an offshore wind have pushed the tide down further than usual. I expect all readers of this book will be similarly drawn; and why not? But it is unwise to plan any serious investigations on the very low shore. Not only will your time on site be strictly limited but it may be a long time (perhaps years) before the sea goes out that far again.
It is natural for people to be excited by finding an example of a rare species. But, beyond identifying it and making a note of its name and where and when you found it, there are not many questions you can ask of it. The answer to the obvious one, "Why has it turned up here?" is probably "by chance" or "because it made a mistake."
A central theme of this little book will be that, actually, the really common animals are amongst the most interesting. Not only do you not have to spend hours searching for them, when your own ability to find them becomes a major feature of their apparent distribution, but also you can be sure that if they are rare or absent at a particular site there is a very good reason for it.
CHAPTER 2
The biology of marine snails
On almost all rocky shores around the coasts of Britain and Ireland, the snail fauna is dominated by members of four groups – Patella limpets, topshells, winkles and dog-whelks. They are very easily told apart. Limpets have a simple conical shell (fig. 1) and the animal does not have an operculum. Dog-whelk shells have a siphonal groove (fig. 2), so called because the snail extends a siphon or breathing tube along it when moving about. Topshells have a circular operculum and a shell characterised by a nacreous (mother-of-pearl) inner layer (fig. 3). In older individuals, the coloured outer layer is often abraded away, leaving the nacreous layer exposed, especially at the apex. Most species show an open umbilicus, at least when young, although in some it closes in later life and its position is revealed by an umbilical scar. Winkles have an...
