Physiological and behavioural adaptations are necessary to withstand the fluctuating nature of the shore environment (Gibson, 1969). The wide and rapid changes of temperature and salinity that occur on the shore surface during low tide require wide eurythermy and euryhalinity in the exposed population (Cornelius, 1972; Southward, 1958). They must also be capable of making appropriate adjustments of behaviour in response to changes in their surroundings. The limpet (Patella), if wetted with freshwater, pulls its shell hard down and remains still; but if repeatedly splashed with seawater it begins to wander about (Arnold, 1972). The lugworm, Arenicola marina, before commencing the intermittent irrigation cycles which replace the water in its burrow, moves backwards up the rear end of the burrow and appears to test the quality of the surface water with its tail, modifying the subsequent sequence of irrigation activity accordingly (Wells, 1949). Animals that live among algal fronds are usually coloured to match their surroundings, and some, such as the sea scorpion (Taurulus bubalis) have considerable powers of colour change to match different backgrounds.
Appropriate changes of activity are required to meet the profoundly different conditions of submergence and exposure to air. Movement, feeding or reproduction is only possible for many littoral species during the periods when they are covered by water. When uncovered, the heart rate slows and they become more or less inactive. In this quiescent state their respiratory needs are reduced, water is conserved and there is less danger of attracting the attention of seabirds and terrestrial predators. Restriction of movement as the shore dries during low tide may also help to confine free-living forms to their appropriate zones. Some regulate their activity according to wind conditions, only moving about over the shore surface in calm or light winds but in strong, drying winds sheltering in crevices or under stones, e.g. the top shell Monodonta lineata (Courtney, 1972). Many free-living forms show seasonal changes of level, usually moving slightly downshore during the coldest part of the winter and ascending in spring.
The tiny estuarine mud snail Hydrobia ulvae is very common on the surface of mudflats, usually mainly above mid-tide level (see references in Graham, 1988). When the snails are first uncovered by the tide the majority are found crawling on the surface of the mud. In areas where the mud surface remains wet, the animals continue their active browsing on the surface throughout the tidal cycle; but in places where the surface becomes dry the majority of the population then burrow just below the surface, remaining buried until they are covered by the returning tide. Newell (1962) describes a behaviour cycle related to feeding. As the tide ebbs, the snails follow the water down, feeding as they go. When the tide starts to flow back in, they float to the surface and are carried back to their starting level. At high water, numbers of floating Hydrobia can sometimes be skimmed from the sea surface, but this may not be a regular cycle of activity (see Graham, 1988).
Free-living shore creatures are in some danger that their own movements may carry them out of their proper zones into levels too high or too low on the shore, or into positions too exposed to wave, wind or sun. Studies of their behaviour reveal some of the mechanisms whereby they find and keep within suitable parts of the shore. Many shore animals, e.g. the crab Carcinus maenas, the polychaete Eurydice pulchra, the blenny Blennius pholis, and the prawn Palaemon elegans, display cyclical changes of activity having a tidal frequency even when removed from the shore, apparently controlled by endogenous rhythms (Alheit and Naylor, 1976; Naylor, 1958, 1976; Palmer, 1973; Rodriguez and Naylor, 1972). In various small crustaceans which burrow in intertidal sand (Corophium volutator, Synchelidium, Eurydice pulchra) an endogenous rhythm evokes emergence from the sand for swimming mainly during the ebb tide. This pattern of activity avoids the danger of stranding too high up the shore but promotes swimming at the stage of the tide when food and oxygen have just been replenished.
A notable feature of the movements of the adult limpet (Patella vulgata) is its 'homing' behaviour, usually returning to a particular site or 'home' on the rock after foraging for food, often over a distance of several feet. Patella feeds chiefly by scraping the surface with its long, toothed radula, rasping off the microscopic film of algae which forms a slimy coating on the rocks. During the day it seldom moves about unless submerged, but at night Patella can often be found crawling on moist surfaces while uncovered. Homing behaviour is most strongly developed in individuals living high on the shore. On returning to its home, Patella always settles in the same position and the margin of the shell grows to fit the rock surface very accurately. On soft rocks the home is often marked by a ring-shaped groove, the 'limpet scar', conforming to the shell margin and presumably worn into the rock by slight movements of the shell. Homing obviously ensures that the animal maintains its zonational position, and the exact fit between shell and rock reduces water loss during exposure, and also lessens the danger of the shell being prised off the rock by predators.
It is uncertain how Patella finds its way back to its home (see references in Graham, 1988), but in some animals it is evident that the direction of their movements is related to factors such as light (phototaxes), gravity (geotaxes), lateral contact (thigmotaxes), humidity (hydrotaxes) or direction of flow of water (rheotaxes) (Fraenkel and Gunn, 1961). This is a complex field of study because animal behaviour is seldom altogether consistent, varying from one individual to another and sometimes changing at different stages of the life-history or reproductive cycle. It can also be modified or even reversed by alterations in the condition of the animal or the environment; for example, by changes of temperature or salinity, the animal's need for food, its state of desiccation or its previous experiences. Nevertheless, it has been demonstrated that some shore creatures display patterns of movement which carry them into situations for which they are well suited and enable them to remain in appropriate zones despite their need to move about over the shore in search of food or for mating.
The movements of Lasaea rubra, for instance, are influenced by light, gravity, and contact (Morton, 1960; Morton et al., 1957). This tiny bivalve is widely distributed throughout the littoral zone, extending to a high level and occurring mainly in the protection of crevices, empty barnacle shells and tufts of Lichina. It makes temporary attachment by means of byssus but is capable of moving freely over the surface by using its extensible foot to crawl on a mucus film. On a level surface Lasaea moves away from light, but on a sloping surface it climbs even against the light. Its response to lateral contact overrides the effects of both light and gravity, causing the animal to move into crevices and small holes. Laboratory experiments have shown that Lasaea will crawl into a narrow hole even downwards towards bright light.
The periwinkle Littorina obtusata is numerous under cover of middle-shore algae, where it usually matches the weed in colour and often also with respect to size of air bladders (Gill etal., 1976). On level surfaces it usually moves away from light. L. saxatilis extends high in the littoral fringe, showing a strong tendency to move towards light and to climb. L. littorea is widely distributed across the middle shore in both sheltered and moderately exposed situations. The direction of wave action is one clue whereby this snail, if displaced, moves towards its original level (Gendron, 1977; Williams and Ellis, 1975). Its movements have been studied on the Whitstable mud flats (Newell, 1958a,b), and found there to be related to the direction of the sun. In this locality the feeding excursions occur mainly during the periods shortly after the winkles are uncovered by the receding tide, or submerged when the tide returns. The majority of winkles on the mud move at first towards the general direction of the sun, but later they reverse their direction. They therefore tend to retrace their course, their overall movement following a roughly U-shaped path which brings them back approximately to their starting-point. These animals, accustomed to a horizontal surface, show no geotaxic responses, but experiments with other specimens collected from the vertical faces of groins demonstrate responses to both light and gravity, these too moving over a U-shaped track, at first downwards and later upwards. Looped tracks have been reported for a number of other shore creatures, in some cases orientated to light, in others to gravity, and such behaviour has obvious advantages in enabling free-living animals to range about over the shore without moving far out of the levels in which they find favourable conditions.
The adults of Melaraphe (Littorina) neritoides occur high in the littoral fringe, but their eggs and larvae are planktonic, and settlement is mainly on the lower shore. The attainment of adult zonation is brought about by a combination of responses to light and gravity, modified by immersion (Fraenkel and Gunn, 1961). This winkle is negatively geotactic, and climbs upwards on rock surfaces. It is also negatively phototactic, and so will move into dark crevices. But the reaction to light reverses if the animal is immersed in water while it is upside down. If its crevice becomes submerged, Littorina neritoides therefore tends to crawl out along the ceiling towards the light, and then climb higher on the shore.
Some shore creatures have sufficiently well-developed vision to be able to see nearby objects and direct their movements by sight. An example is the sand-hopper Talitrus saltator, which burrows in upper-shore sand in the daytime and emerges at night during low water to feed on the surface. These feeding explorations carry it well down the shore, but it eventually finds its way back to high-water level. If removed from its burrow during daytime and released lower on the shore on a firm, unbroken sand surface, irrespective of slope of the beach or direction of sun or wind, it tends to move over the surface towards the back of the beach, where it burrows on reaching the drier, looser sand. If both eyes are covered, the movements of Talitrus released low on the shore are haphazard and show no tendency to carry it back upshore. Experiments both on the beach and in the laboratory suggest that Talitrus is capable of seeing shapes (form vision), that certain shapes attract it, and that its movements towards the top of the beach are probably associated with its ability to see the line of the backshore, even in dim, night-time illumination.
Some experiments with periwinkles on a rocky shore have demonstrated that they too direct their movements in relation to what they see of their surroundings (Evans, 1961). From an examination of the structure of the eye of Littorina littorea (Newell, 1965), it appears that this can form sharp images of distant objects in air, and can probably accommodate to bring near objects into focus.
Pardi (1960) and others have studied the movements of Talitrus from various parts of the coastline when placed above high-water level. They have demonstrated that the animals generally move in a direction which would carry them towards the sea in the localities from which they were taken, even when removed to areas remote from the sea. Their orientation is based on their sight of the sun or, in shade, on their perception of the polarization of light from the sky. Even in animals bred in captivity in uniform light, which have never had previous sight of the sun or contact with the shore, the orientation shows once they are exposed to sunlight.
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