Zonation patterns

The zonation of plants and animals is often clearly visible on rocky coastlines of the British Isles. An excellent summary of 'typical' patterns of zonation on sheltered, moderately exposed and exposed shores is given in Hawkins and Jones (1992). For a fuller picture, students should refer to Lewis (1964) where a full account is given of distribution patterns on many different shores throughout the British Isles. Other useful references include Ballantine (1961) and Moyse and Nelson-Smith (1963).

Zonation is a feature of shore populations everywhere. In the bibliography at the end of this chapter are a few references giving information on the zonation of intertidal communities in some other parts of the world (Dakin, 1987; Knox, 1960, 1963; Lawson, 1966; Morton and Miller, 1968; Ricketts and Calvin, 1962; Stephenson and Stephenson, 1972).

In the following pages, Figure 8.11 indicates in a general way the zonation of the more conspicuous fauna and flora on an algal-covered rocky shore in the south-west. Figure 8.12 indicates the distribution of certain species in various intensities of wave action. Some components of rocky shore food webs are shown in Figure 8.13.

Plant zonation

Where waves are not too violent to allow the growth of seaweeds, the rocks across much of the eulittoral zone of the shore have a brownish-green covering of large

Littorina Zone

Figure 8.11 Diagram to illustrate how the distribution of some dominant plants and animals of rocky shores varies with wave intensity. Ü Verrucaria with Littorina saxatilis and Melaraphe neritoides^ U Barnacle zone. A - Ascophyllum nodosum^ Al- Alaria esculenta^ G - Gigartina stallaris^ H - Himanthalia elongata^ L - Fucus vesiculosus f. linearis^ Ld - Laminaria digitata^ Ls - Laminaria saccharina^ M - Mytilus edulis, Pc - Pelvetia caniculata^ Pu - Porphyra umbilicalis^ S - F. serratus^ Sp - F. spiralis^ V - F. vesiculosus.

Figure 8.11 Diagram to illustrate how the distribution of some dominant plants and animals of rocky shores varies with wave intensity. Ü Verrucaria with Littorina saxatilis and Melaraphe neritoides^ U Barnacle zone. A - Ascophyllum nodosum^ Al- Alaria esculenta^ G - Gigartina stallaris^ H - Himanthalia elongata^ L - Fucus vesiculosus f. linearis^ Ld - Laminaria digitata^ Ls - Laminaria saccharina^ M - Mytilus edulis, Pc - Pelvetia caniculata^ Pu - Porphyra umbilicalis^ S - F. serratus^ Sp - F. spiralis^ V - F. vesiculosus.

Fucus Species Zonation
Figure 8.12 Zonation of some common plants and animals of rocky shores exposed to moderate wave intensities along south-west peninsulas of England and Wales.

fucoids, with bands of different colour at different levels indicating the dominance of particular species. Above the level of these large algae there is often a strip of relatively bare rock on which there are few plants except the black tufts of the lichen Lichina pygmaea. Higher still, in the upper part of the littoral fringe, there is a black, tar-like streak of blue-green algae and the encrusting lichen Verrucaria maura. Above this the terrestrial vegetation begins, sometimes as a light-coloured lichen zone tinged with yellow or orange by encrustations of vivid species such as Xanthoria and Caloplaca, and where there are spray-resistant angiosperms, notably sea thrift (Armeria marítima).

Examining the fucoids more closely, we find certain species restricted within fairly close levels. Usually at the highest level there is the channelled wrack, Pelvetia canaliculata, forming a narrow band in the lower part of the littoral fringe where it is wetted by salt spray but only occasionally submerged. Just below

Intertidal Zone Beaches Food Web
Figure 8.13 Simplified food web of a rocky shore.

Pelvetia canaliculata, in the lower limit of the littoral fringe, there is often a narrow zone of Fucus spiralis. The greater part of the eulittoral zone is more or less covered with a mixed growth of Ascophyllum nodosum and Fucus vesiculosus, the former predominating in sheltered areas and the latter in regions more open to waves. Overlapping with A. nodosum and F. vesiculosus, but extending below them into the lower part of the eulittoral zone, is a belt of toothed wrack, Fucus serratus. On many shores the lower limit of the eulittoral zone is indicated by a narrow band of the thong weed, Himanthalia elongata. Conspicuous plants in the sublittoral fringe, usually uncovered only at spring low-tides, are species of Laminaria, or, on the most wave-beaten rocks, Alaria esculenta. Since 1973 another conspicuous weed, Sargassum muticum, has been found along parts of the south coast (Boalch and Potts, 1977; Farnham et al., 1973). Being native to Japan, it is commonly referred to as Japweed and was probably introduced with imported Japanese oysters, Crassostrea gigas. It was first seen at Bembridge, Isle of Wight, and was soon discovered on the Hampshire coast. It now extends along the English Channel coast from Cornwall to Kent.

Animal zonation Barnacles

The zonation of littoral animals is well illustrated by some of the species that live on the surface, for example, the barnacles that cover the rocks where wave action is too strong for plants. The biology of British littoral barnacles is described in full in Rainbow (1984) and their identification in Hawkins and Jones (1992). The most diverse populations of shore barnacles in the British Isles occur on the south-west peninsulas of England and Wales where at least seven species are to be found, namely, Chthamalus montagui, C. stellatus, Semibalanus (Balanus) balanoides, Balanus perforatus, B. crenatus, Verruca stroemia and Elminius modestus. Their distribution can be summarized as follows:

• Chthamalus montagui reaches the highest level, sometimes extending above Pelvetia into the littoral fringe (Southward, 1976). It is a southern species ranging northwards along Atlantic shores from Morocco, the British Isles being the northernmost limit of its distribution. Here it is the dominant barnacle of the upper eulittoral zone in the south-west. It occurs in reduced numbers further north up the west coast to Shetland and eastwards along the English Channel as far as Dorset and the Isle of Wight. C. stellatus is found in the Mediterranean and on Atlantic shores between the tropics and the British Isles. In Britain it is found as far north as the west coast of Shetland (Rainbow, 1984) but is absent from the north Irish Sea. In most British localities it is limited to lower parts of the shore than C. montagui, mainly below mid-tide level. It is usually most abundant on the most heavily wave-beaten coasts whilst C. montagui favours embayed sheltered situations.

• Semibalanus balanoides is a boreal form flourishing in colder water than Chthamalus spp. It is found on all British coasts except the tip of Cornwall and is the dominant eulittoral barnacle on north and east coasts. It does not reach as high a level as C. montagui, and scarcely enters the littoral fringe. In the south and west, it occurs only in the mid- and low-eulittoral where it competes for space with C. montagui. Where conditions of moderate shelter or lower temperature favour Semibalanus balanoides, Chthamalus montagui, if present, tends to be restricted to a narrow fringe zone above the level of S. balanoides. Stronger wave action broadens the Chthamalus zone at the expense of Semibalanus, and favours C. stellatus more than C. montagui. On the Devon, Cornwall and Pembrokeshire coasts these species all compete for space in the lower eulittoral with the larger Balanus perforatus, which in some localities virtually ousts them. This south-western form extends from the lowest levels of the shore up to about the middle of the eulittoral zone.

• Balanus crenatus is essentially a sublittoral species, intolerant of exposure to air. It may be found in rock pools or under algal cover very low on the shore all round the British Isles. Verruca stroemia is a shallow-water species extending to the lower shore, where it is restricted mainly to pools or the underside of stones.

• Elminius modestus is an introduction to this country which appeared in Chichester harbour and the Thames estuary during World War II. It is an Australasian species and is thought to have been brought to Britain on ships' hulls. It has now spread around much of the British coastline (Crisp, 1958), and has crossed the seas to Ireland and the mainland of Europe, extending northwards from the French coast along the shores of Belgium, Holland and the North Sea coasts of Germany and Denmark. The northernmost population yet recorded is in the Shetland Isles. This is over 400 km north of the nearest known population, and it is likely that its spread to the Shetland Isles has again been effected by shipping. In sheltered areas it tends to oust the native population of S. balanoides, being able to survive at shore levels at least as high and as low as Semibalanus and having advantages in its rapid growth rate and extended breeding season through the summer months. It also has a wide tolerance of desiccation, silting, and variations in temperature and salinity.

Periwinkles

Periwinkles, Littorina spp., are very common on almost all rocky shorelines. Several species occur on the coast of the British Isles but there are still taxonomic problems with a number of them. Hawkins and Jones (1992) give basic and advanced identification tables for all species. Graham (1988) gives detailed descriptions and a key. A recent review of the ecology and taxonomy is given by Raffaelli (1982). The following is a summary of their distribution.

• Adults of the small periwinkle, Melaraphe (Littorina) neritoides live mainly in the upper littoral fringe, extending almost to the highest levels of wave splash. They are most numerous on shores exposed to heavy wave action and are often absent from sheltered regions. They are found mainly in crevices on hard rocks, and are consequently absent from most of the eastern part of the English Channel and southern part of the North Sea where suitable substrates are lacking.

• L. obtusata, L. mariae and L. littorea are eulittoral species found on all British coasts but more abundant in sheltered regions. L. obtusata and L. mariae are very similar species frequently referred to together as L. littoralis, the flat periwinkle. Yellow, black, brown and olive morphs are all common. L. obtusata eats the fucoid algae under which it lives. L. mariae feeds on epiphytes on the fucoids. They occur together on algal-dominated shores. On sheltered shores, it is reasonably certain that flat periwinkles in the Ascophyllum zone are L. obtusata and in the F. serratus zone are L. mariae (Hawkins and Jones, 1992). L. littorea, the common periwinkle, feeds by scraping the microflora from the rock surface and on macro-algae. It is often found on bare rocks and is especially numerous in gulleys or on the sheltered faces of boulders.

• The Littorina saxatilis ( = L. rudis) group, often called the rough periwinkle, includes several species with much confusion of nomenclature. Species falling within this group are L. saxatilis, L. rudis, L. arcana, L. nigrolineata and L. neglecta. Identification of the different species in the group is difficult in the field and requires some practice and experience. Students are advised first to learn to recognize the group. They can be distinguished from Melaraphe neritoides and Littorina littorea by looking carefully at the lip of the opening which meets the body whorl at right-angles. In the other two species, it meets tangentially. For more detailed studies, students should refer to the references given above and the papers referred to in them.

The species now called L. saxatilis is the highest littorinid apart from M. neritoides and is mostly found in crevices at and above the Pelvetia zone. L. arcana extends down into the upper eulittoral and prefers more exposed situations. Most workers treat L. rudis as a variant of L. saxatilis though Smith (1981) regards it as a distinct species. L. neglecta is a very small eulittoral form that lives in small crevices, among mussel byssus and in Laminaria holdfasts, but is most reliably found in empty barnacle shells. It is most common on shores exposed to strong wave action. L. nigrolineata is a eulittoral species of rather local distribution on moderately exposed shores to very exposed shores. The shell is distinctively ridged, light coloured and often patterned with dark spiral lines within the grooves.

Reproductive differences between these littorinids are as follows. Melaraphne neritoides and L. littorea are oviparous with planktonic eggs and larvae; L. obtusata and L. mariae are oviparous and lay non-planktonic eggs in jelly-like egg-masses from which the young snails hatch directly; the L. saxatilis group are generally ovoviviparous bearing live young, with the exception of L. arcana and L. nigrolineata which lay egg masses as in L. obtusata.

Limpets

Three species of Patella occur on the British shoreline. The common limpet, Patella vulgata, extends throughout the eulittoral zone on virtually all rocky shores around the British Isles. In the south-west its distribution in the lower eulittoral overlaps with the black-footed limpet, P. depressa ( = P. intermedia) on moderately exposed to exposed shores (Isle of Wight to Anglesey, absent in Ireland). P. aspera (= P. ulyssopensis) occurs on most suitable coasts around the British Isles but not between the Thames and the Humber on the east coast. It lives mainly below MLWN but on exposed shores it can be found in mid-shore rock pools. P. depressa and P. aspersa are rarely found on sheltered shores.

British rocky shore zones

Some of the common species comprising the communities of British rocky shores are listed below.

The littoral fringe

Verrucaria maura, Lichina confinis, L. pygmaea, Myxophyceae, Pelvetia canaliculata, Fucus spiralis, Porphyra umbilicalis, Melaraphe (Littorina) neritoides, Littorina saxatilis, L. rudis, Ligia oceanica, Orchestia gammarella, Petrobius maritimus.

Eulittoral zone

Ascophyllum nodosum, Fucus vesiculosus, F. serratus, Himanthalia elongata, Osmundea pinnatifida, Corallina officinalis, Mastocarpus stellatus, Palmaria palmata, Semibalanus balanoides, Balanus perforatus (SW), Chthamalus mon-tagui and C. stellatus (W and SW), Elminius modestus, Littorina neglecta, L. obtusata, L. littorea, Patella vulgata, Nucella lapillus, Gibbula cineraria, G. umbilicalis (W and SW), Monodonta lineata (W and SW), Actinia equina, Anemonia sulcata (W and SW), Mytilus edulis, Porcellana platycheles, Carcinus maenas, Blennius pholis

Sublittoral fringe

Laminaria spp., Alaria esculenta, Corallina officinalis, Chondrus crispus, Lithothamnia, Balanus crenatus, Verruca stroemia, Gibbula cineraria, Patella aspera, Helcion (Patina) pellucidum, Anemonia sulcata, Asterias rubens, Psammechinus miliaris, Galathea spp., Pisidia longicornis, Halichondria panicea, Alcyonidium spp., Archidoris pseudoargus, Acanthodoris pilosa, Aeolidia papillosa.

Other animals on rocky shores

Inaccessible cliffs, rocky stacks and islands provide breeding grounds for innumerable seabirds (Lloyd et al., 1991). The following are some common British species which nest mainly on rock ledges: the herring gull (Larus argentatus), the greater black-back gull (L. marinus), the kittiwake (Rissa tridactyla), the guillemot (Uria aalge), the razorbill (Alca torda), the shag (Phalocrocorax aristotelis), the cormorant (P. carbo) and the gannet (Sula bassana). The puffin (Fratercula arctica) is a bird of the open sea and forms breeding colonies on certain rocky islands, such as the Farnes and Shetlands, where it makes its nest in burrows in the turf.

At the beginning of the breeding season, the grey seal (Halichoerus grypus) comes ashore onto rock ledges and inaccessible beaches where the pups can be born out of reach of the sea. During the first two weeks of life the young avoid the water and may drown if they fall into the sea. The adult seals usually stay ashore for several weeks at this time.

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Responses

  • yonatan
    What is the advantage of a barnacle having a light coloured shell?
    4 years ago
  • maryam
    Which eulittoral zones are semibalanus balanoides found in?
    4 years ago
  • antje
    Why are there zonation pattern?
    4 years ago
  • stephan
    What are the conditions needed for a melaraphe snail to live in?
    4 years ago
  • zuzanna
    Why do chthamalus genera's are able to live higher the soreline than semibalanus genera's?
    4 years ago
  • gracie
    How is the distribution of rock oysters on rocky shores?
    4 years ago
  • roberta
    What is the order of zonation bands in Wave exposed shores?
    4 years ago
  • pervinca
    What is zonation in ecology?
    4 years ago
  • VERDIANA LUCCHESI
    What tide zone does the Aeolidia papillosa live in?
    4 years ago
  • Principio
    Where is the french littoral fringe?
    4 years ago
  • kidane
    What temperatures can melaraphe survive?
    4 years ago
  • Tekle
    WHAT IS THE FOOD WEB FOR THE INTERTIDAL ZONE?
    3 years ago
  • peter
    How do Melaraphe finds its zone,?
    3 years ago
  • Dan
    What is the food chain in the Intertidal Zone?
    3 years ago
  • adalgisio
    What is the perfect temperature for monodonta lineata to live at?
    20 days ago

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