Although the majority of lichens are terrestrial, some grow in areas subject to salt spray and are a conspicuous feature of the upper reaches of many rocky shores. Some withstand daily submersion by the tide and are found in the eulittoral zone, where they compete for space with other organisms, for example, barnacles and seaweeds. Lichens are complex plants composed of fungal and algal components existing in what is believed to be a mutually beneficial association known as symbiosis in which photosynthesis of the algal cells provides the fungus with organic nutrients, while the fungus provides the alga with support and protection. The exact nature of the relationship is still debated. Each species of lichen has its own species of fungus but the algal component is not specific to a particular fungus, and it is believed that the algae can exist independently, unlike the fungus which survives only in the lichen association.

The main part of the lichen, known as the thallus, is typically seen in one of three forms; crustose, foliose and fruticose. The flattened, crust-like thallus of crustose lichens is firmly attached to the substratum and very difficult to remove without damage to the lichen. Foliose lichens have a horizontal, leaf-like thallus usually loosely attached to the substratum and often divided into lobes. In the fruticose lichens the thallus stands upright or hangs downwards from the base.

The phylum Cnidaria, formerly known as the Coelenterata, includes the hydroids or sea-firs, the sea-anemones, jellyfishes and corals. These apparently diverse forms have in common a basic radial symmetry and a relatively simple body structure with a body wall made up of an outer epidermis (ectoderm) and an inner gastrodermis (endoderm), separated by a jelly-like mesogloea. Cnidarians therefore have two layers of cells and are described as diploblastic, compared with the triploblastic grade of organization shown by the majority of animals, in which there is a third cell layer, the mesoderm. The body cavity has a single opening to the exterior called the mouth and this is usually surrounded by tentacles, which are without cilia, but have special cells known as cnidocytes (nematocytes). These contain capsule-like structures, the cnidae, that are unique to the phylum, and from which the phylum gets its name. The cnidae contain a thread which is discharged on stimulation; in some (the nematocysts) the thread penetrates the prey and injects a toxin; in others (the spirocysts) the thread is adhesive. A few cnidarians are suspension feeders but most are carnivorous and when the tentacles make contact with the prey, it is immobilized and passed to the mouth. The majority of cnidarians are dioecious and both asexual reproduction by budding and sexual reproduction are common.

The nemerteans are elongate, often ribbon-like worms with bilateral symmetry. They are non-segmented and have a ciliated epidermis but unlike the flatworms have a separate mouth and anus. Dorsal to the gut, and separate from it, is a muscular, eversible proboscis used in the capture of food and in defence. It is from this structure that the animals get one of their common names, the proboscis worms. They are also known as ribbon worms. Nemerteans are carnivorous and feed on a wide variety of crustaceans, polychaete worms and molluscs, which are captured on the proboscis. In some cases the prey is ingested whole, while in others the body of the prey is torn by the proboscis, which in some species has one or more piercing stylets, and the nemertean feeds suctorially. Many species pursue their prey which is killed by powerful neurotoxins while others scavenge on dead and decaying animals. Despite the presence of toxins in the tissues, nematodes are themselves eaten by a wide range of fishes. Most nemerteans have separate sexes and fertilization is usually external. In some species the gametes are deposited in an egg sac where fertilization and development take place. Several different types of larvae have been described, including a ciliated pelagic larva known as a pilidium. Length of life is believed to be about one year but the larger species possibly live for several years.

The seashore

One of the most striking features of the shore is the rich diversity of plant and animal life to be found there. A wide range of invertebrates, some highly mobile, others fixed or sedentary, and shore fishes, are a characteristic feature. Brightly coloured lichens often form distinct bands on the high shore; seaweeds may be present in abundance, and on mud-flats flowering plants often dominate. Physical factors change rapidly and it is here that the student has the opportunity to observe and study the fascinating adaptations to this environment shown by both plants and animals.

Tides

The dominating force on the shore is the rise and fall of the tide. Tides result from the gravitational forces between the Moon and Sun and the seas and oceans on the Earth's surface. The tides with which we are most familiar in north-west Europe are semidiurnal, that is, there are usually two high tides and two low tides each day. This can be appreciated if the Earth is pictured revolving on its axis during the course of a day and passing through a water envelope which has been distorted by the gravitational forces of the Moon and Sun as inFig. 1.1. Because of the constantly changing positions of the Earth, Sun and Moon, the length of time between successive high tides is not exactly 12 hours but is about 12 hours 25 minutes.

The phylum Sipuncula comprises worm-like, marine animals occurring both intertidally and sublittorally. They are found in burrows in sandy and muddy sediments, in the empty tubes of polychaetes, the empty shells of molluscs, and in rock crevices. Sipunculans have an unsegmented body in which the anterior part, or introvert, is narrow and can be withdrawn, and the posterior part, the trunk, which is wider and more or less cylindrical. In relaxed specimens the mouth is seen at the anterior end of the introvert surrounded completely, or in the dorsal aspect, by lobes or tentacles. The anus is at the anterior end of the trunk in a mid-dorsal position. Sipunculans are mainly deposit feeders, feeding on mud, sand and detritus but filter feeding, using a crown of modified tentacles, has been described in one group. Almost all species have separate sexes with external fertilization, but see Nephasoma minutum (p. 377). There is a free-swimming larva and the length of pelagic life varies from a few days to a month or so.

Identification to species can be difficult and often requires examination of internal anatomy. The species described here can be recognized on external morphology and habitat.

The fusion of identification guide and biological text, the hallmark of A Student's Guide to the Seashore, has been widely acclaimed since the publication of the first edition in 1989 and over the past 20 years we have been greatly encouraged by the support and encouragement received from users of the Guide. It was therefore a pleasure to accept the invitation from Cambridge University Press to prepare a third edition.

The publication of a third edition has given us the opportunity to review the list of species included in the light of our field experience and that of colleagues. Additional species have been added, some of which are introduced species in the sense that they have now become established in Britain and north-west Europe, bringing the total coverage to over 650 species. At the same time, the biological information on individual species has been updated on the basis of the latest research and expanded wherever possible and this, together with the extended bibliography of primary sources of data, will encourage wider lines of enquiry and help those readers who wish to investigate further this fascinating habitat. One of the most significant changes since the publication of the earlier editions has been in taxonomy following the increased use of molecular techniques and there is no doubt that important advances in our understanding of taxonomic relationships will continue to be made through the application of molecular phylogeny.

Molluscs are found in marine, freshwater and terrestrial environments and in terms of numbers of species are second only to the arthropods. Body form varies widely within the phylum but the basic molluscan plan shows a head, often well developed and bearing sensory structures, a muscular foot and a visceral hump. The delicate tissue covering the visceral hump is known as the mantle (or pallium). This projects beyond the edge of the visceral mass to enclose a cavity known as the mantle (pallial) cavity where gills and other structures are found. The mantle secretes a calcareous shell consisting of a number of layers. There is usually an outer proteinaceous layer known as the periostracum, beneath which are layers of calcium carbonate. In some molluscs an inner mother-of-pearl or nacreous layer is often exposed on the surface of the shell by the wearing away of the outer layers. Although described as one of the most characteristic features of the phylum, it is important to note that in some groups the shell is internal, while in others it is absent. The characteristic feeding organ is a ribbon-like, chitinous structure, the radula (Plate 16d), which comprises rows of teeth. In many species these have a rasp-like action and remove adhering microorganisms from the rock surface. In some gastropods, the radula is modified to enable it to bore into prey. The bivalves lack a radula and are deposit or suspension feeders using the gill to filter out fine particles of food.

The arthropods include such well-known representatives as the insects, spiders, shrimps and crabs; they are a large grouping of invertebrate animals which have bilateral symmetry and show metameric segmentation, although the latter is not always clear. They are characterized by paired, jointed appendages and it is from this character that the group gets its name, arthropoda, meaning jointed feet. The appendages show great variation and serve a wide range of functions, including sensory perception, feeding and locomotion. The body is further characterized by the possession of a semi-rigid exoskeleton which in places is thin enough to allow movement. Periodically, the exoskeleton is shed by a process of ecdysis or moulting. A new skeleton develops under the existing one and during moulting the animal frees itself from the old skin which splits along lines of weakness. The new skin is soft and expands rapidly before hardening. Growth increments at moulting can be substantial: for example, the common shore crab increases in carapace breadth by an average of about 30%. The arthropods form the largest group in the animal kingdom and the possession of a hard exoskeleton is a major factor in their colonization of land, sea and air.

The arthropods are a large and diverse group and it is not surprising that the classification has been the subject of much debate. Although some authors consider different groups of arthropods to have different lines of ancestry (polyphyletic) and assign phylum status to these, it is now widely accepted that the arthropods have a single line of ancestry and are monophyletic, the position adopted here.