The snails found living on rocky sea shores are among the most rewarding invertebrate animals to study. Species such as dog-whelks, topshells and winkles are easy to find, capture, identify, measure and mark. This book provides a key to common species, background ecology, an overview of rocky shore habitats and the techniques required for anyone to study this fascinating and accessible fauna.
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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.
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 oval operculum and a shell with a dark inner layer (fig. 4).
The shell is formed on a matrix of the protein conchiolin, which is stiffened by the addition of calcium carbonate crystals. In some species, including the common whelk Buccinum undatum, the outermost layer of the shell, the periostracum, remains free from calcium carbonate and the hard shell appears to be covered with brown fur.
The first shell formed by the larval snail (see below) is retained as the apex (the highest point of figs 1–4, see also plate 1.1), although it is often abraded away in old individuals. The outer surface of a snail's body, the mantle, is in contact with the inner surface of the shell. Cells at the edge of the mantle secrete the outer layer of new shell along the lip of the aperture (which extends all the way around the rim in limpets, fig. 5). Just below the arrowhead labelling 'mantle edge' are three black marks showing where the snail is accumulating pigment to form darker shell for the decoration. As the snail increases in size it lays down increasingly thick shell. To ensure that the upper whorls also increase in thickness, the snail secretes shell from the whole outer surface of the mantle. So the outer layers are generally thickest near the lip and the inner layers near the apex, which explains why the outer layer has been abraded from the apex of the top-shell in fig. 3.
Food and feeding
Dog-whelks are carnivores, feeding primarily on sedentary barnacles and mussels but occasionally on snails. All the other species of whelk appear to be carnivores or scavengers as well.
Most other snails are herbivores but it is often far from obvious what they are eating. Indeed rocky shores present a conundrum in that the grazing snails may be most abundant on the shores with the fewest visible algae whilst seaweed-covered shores support comparatively few individuals – except of flat winkles. Lacking a vascular system comparable to that of higher plants, algae are not able to move the products of photosynthesis around the plant body as effectively. Essentially, any excess produced by a cell is released into the water as dissolved organic matter. When the alga dies, more dissolved organic matter is released along with some particulate organic matter (detritus). All this material is available for bacteria to consume and so for animals through eating the bacteria, the particulate organic matter, or by direct absorption of dissolved organic matter. The animals never have to meet their food plants!
Flat winkles are unusual amongst these snails in that they feed on the brown fucoid seaweeds, and/or on any epiphytes growing on the surface of the fronds. Blue-rayed limpets, similarly, feed on kelp and Fucus serratus but the other limpets, topshells and winkles all graze over the rock surface even though, to us, there appears to be nothing there to eat.
No doubt they would prefer to eat larger food, especially green algae, but under normal circumstances these have all been eaten before they have a chance to grow large enough to become visible to us. From time to time, when conditions are particularly favourable for rapid algal growth, the grazers do not manage to consume all the growth and grazing tracks through the vegetation become visible.
In general, prosobranch snails breathe by means of gills. In what are regarded as the most primitive forms alive today, there is a pair of these ctenidia in the mantle cavity. This cavity lies above the head; it is the space into which the head, and often the foot as well, retracts when the snail withdraws under, or into, its shell. In topshells, winkles, and whelks there is but a single gill (ctenidium). Patella limpets have lost their ctenidia and, instead, employ a large number of pallial gills, of a different evolutionary origin, which line the groove between the mantle and the foot (fig. 5).
Gills function well under water but are much less useful out of it as their effective surface area is greatly reduced when the lamellae touch each other. One of the problems facing all marine animals that attempt to live on sea shores is how to breathe at low tide. Those poorly adapted to do so are confined to the lower shore (or to rock pools) and only those that have solved the problem are found higher up. Even then, it is probably their ability to breathe and conserve water that dictates the upper limit of their distribution.
Gaseous exchange may also take place through the skin – but only if it can be kept moist. A snail crawling actively over the rocks at low tide on a warm and windy day would rapidly become dehydrated. That is why rocky shore snails are usually most active at night or whilst the tide is in.
Many species of marine snail have separate sexes, but others are hermaphrodite. Patella limpets, for example, are protandrous hermaphrodites, maturing first as males before undergoing a sex change and developing into females. It is generally assumed that an adult has to accumulate greater food reserves in order to produce yolky eggs than sperm.
In the forms that we regard as more primitive, including Patella limpets and topshells, the males do not have a penis and copulation is impossible. Amongst adults of such species there is no courtship behaviour. Males and females do not even need to meet; they simply release their gametes while they are submerged and fertilisation takes place in the sea water. This appears to be a very wasteful system and it is hard to imagine it being particularly successful at low population densities. Even where the species is abundant, a degree of co-ordination in the timing of gamete release would appear essential for success.
Living, as they do, close together on the rock, members of a breeding population would all be expected to experience much the same blend of environmental conditions and all would 'come into season' at much the same time. It then needs some special stimulus – for example, the shock of cold water flooding the rock as the flood tide engulfs it after a hot day – to trigger the first individual to release his or her gametes. As soon as another individual detects the presence of gametes in the surrounding water, (s)he releases his or her own gametes and a chain reaction is established.
The free-floating fertilised egg hatches into a planktonic larva; in some species this is a trochophore (fig. 6) that later develops into a veliger. Others hatch directly as a veliger (fig. 7). The time spent as a larva varies greatly between species. Scheltema (1971) concluded that some veligers can remain planktonic long enough to cross the Atlantic, although not those of any rocky shore species found in Britain or Ireland (the North Atlantic Drift brings us water from the West Indies). At the other end of the scale, in the common topshell, Osilinus lineatus, the pelagic stage may last as little as four days. In the edible winkle, Littorina littorea, egg and veliger stages may last seven weeks.
Some of the snails that do copulate, including the winkles Littorina littorea and Melarhaphe neritoides, retain planktonic eggs and larvae. Others, however, lay eggs in masses of jelly attached to fucoid seaweeds (L. obtusata and L. fabalis), or rocks (L. arcana, L. compressa) or in capsules attached to the rock (various whelks including Nucella lapillus, fig. 8 and plate 1.6). In all these cases, the larval stages are completed within the jelly mass or capsule and junior emerges as a tiny snail, known as a crawlaway, into the habitat chosen for him (or her) by the mother. Littorina saxatilis females take this a stage further; the mother retains the eggs within her mantle cavity and, apparently, gives birth to live young.
Many features of the biology of rocky shore snails can be related to the form of their life cycle. The wastage incurred by species with planktonic larvae must be enormous. Not only are the larvae vulnerable to predators, but large numbers will find themselves dumped in unsuitable habitats. Yet those that survive may have been distributed over a wide area. It is easy for such species to recolonise sites following an event such as an oil spill, a cold winter or a hot summer, that reduced the population. They may also be well placed to extend their range northwards if temperatures rise.
Those more protective snails, whose young begin their lives in the habitat chosen for them by their mothers, might be expected to suffer from much lower infant mortality but to have more difficulty in extending their range. However, recent events have shown them to be less restricted in this way than was once imagined. Shores depopulated by various 'disasters' have been naturally repopulated.
Excerpted from "Snails on Rocky Sea Shores"
Copyright © 2012 Pelagic Publishing.
Excerpted by permission of Pelagic Publishing.
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Table of Contents
About the author, vi,
1 Introduction, 1,
2 The biology of marine snails, 4,
3 Limpets, 9,
4 Common topshells, 17,
5 Dog-whelks, 23,
6 Winkles, 38,
7 Identification, 45,
8 The rocky shore environment, 58,
9 Techniques and approaches to original work, 66,
10 References and further reading, 83,
11 Index, 87,
12 Pictorial key, 90,