237 4 Adaptations to Shore Life CONTENTS 4.1 Introduction 238 4.2 Ecological Niches on the Shore 238 4.2.1 Introduction 238 4.2.2 The Environment 238 4.2.3 Environmental Stress 239 4.2.3.1 Desiccation 239 4.2.3.2 Thermal Tolerance 243 4.2.4 Ecological Niches 244 4.2.4.1 Introduction 244 4.2.4.2 The “Envirogram” Concept 245 4.2.4.3 Weather 247 4.2.4.4 Resources 249 4.2.4.5 Other Organisms 249 4.2.4.6 Disturbance and Patchiness 251 4.2.4.7 The Importance of Recruitment 251 4.3 The Establishment of Zonation Patterns 251 4.3.1 Reproduction 251 4.3.1.1 Developmental Types in Marine Benthic Invertebrates 251 4.3.1.2 Development Types in Marine Algae 252 4.3.1.3 Reproductive Strategies 252 4.3.1.4 A Model of Nonpelagic Development Co-adaptive with Iteroparity 254 4.3.2 Settlement and Recruitment 255 4.3.2.1 Introduction 255 4.3.2.2 Distinction Between Settlement and Recruitment 256 4.3.3 Settlement 256 4.3.3.1 Introduction 256 4.3.3.2 Settlement Inducers 257 4.3.3.3 Settlement on Rock Surfaces and Algae 258 4.3.3.4 Avoidance of Crowding 259 4.3.3.5 Settlement on Particulate Substrates 260 4.3.3.6 Variation in Settlement 261 4.3.4 Recruitment 261 4.3.4.1 Introduction 261 4.3.4.2 Components of Recruitment 261 4.4 The Maintenance of Zonation Patterns 263 4.4.1 Introduction 263 4.4.2 Elements of Behavior in Littoral Marine Invertebrates 263 4.4.3 Behavior Patterns in Representative Species 265 4.4.3.1 Movement Patterns and Orientation Mechanisms in Intertidal Chitons and Gastropods 265 4.4.3.2 Interaction Between the Siphonarian Limpet Siphonaria Theristes and Its Food Plant Iridacea Corriucopiae 265 4.4.3.3 Maintenance of Shore-Level Size Gradients 266 4.4.3.4 Behavior Patterns in Sandy Beach Invertebrates 267 0008_frame_C04 Page 237 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC 238 The Ecology of Seashores 4.4.4 Clock-Controlled Behavior in Intertidal Animals 271 4.4.4.1 Introduction 271 4.4.4.2 Behavior Rhythms, Tidal Oscillations, and Lunar Cycles 273 4.4.4.3 Locomotor Rhythms and Maintenance of Zonation 273 4.1 INTRODUCTION In previous chapters, it was shown that the plants and animals on the shore occupy distinct zones or habitats in which they can survive and obtain the resources they require for growth and reproduction. They thus occupy a specific ecological niche. The ecological niche concept will be explored in the succeeding section. While a limited number of animal species exhibit direct development in which the juveniles hatch directly from the egg, other species have pelagic larvae that need to settle at an appro- priate level on the shore in order to maintain viable pop- ulations. For sessile species, the choice of a settlement site is irreversible. Hence, such species have evolved behaviors that will ensure that they will settle at the right level on the shore. Other species such as mussels and limpets settle low on the shore and subsequently migrate to occupy the zone in which the adults are found. Most algae reproduce by forming microscopic life cycle stages that are released into the water column and later settle on rocky substrates. If they settle at the appropriate level, they will grow to give rise to the adult plant. Many motile shore animals on both hard and soft shores have evolved behavioral strategies that enable them to both evade extreme environmental conditions and to undertake feeding migrations in order to utilize available food resources. These behavioral strategies will be dis- cussed in detail later in this chapter. 4.2 ECOLOGICAL NICHES ON THE SHORE 4.2.1 I NTRODUCTION It is obvious from the preceding chapters that plants and animals on the shore are not randomly distributed, but occupy distinct vertical zones, and are often restricted to microhabitats within these zones. Basic to an understand- ing of shore ecology is a knowledge of the ways in which organisms are adapted to the environmental conditions they are subjected to, and the particular functional role or ecological niche that they occupy in the ecosystem of which they are an integral part. Here we will consider the twin concepts of “environment” and “ecological niche” in some detail. The history of the niche concept is well known and documented (see reviews by Whittaker and Levin, 1977; Diamond and Case, 1986). More recently, Price (1980) has reviewed niche and community concepts in the inshore benthos with particular reference to macroalgae. The niche concept includes the ideas of ecopotential, fundamental niche, and realized niche. Ecopotential can be considered as the unexpressed individual, breeding group, or local population potentiality to occupy a particular role in a community. The fundamental niche is the unconstrained expression of that ecopotential in the presence of only those limitations that derive from interactions between the ambient physical environment and the population of the species under consideration. The realized niche is the totally constrained living relationships of the population of a species within its delimited community. 4.2.2 T HE E NVIRONMENT The term “environment” is not an easy one to define since organisms, populations, and communities form interacting systems within their environments. For the individual organisms, substrate, physical and chemical conditions, its disease organisms, parasites, symbionts and commensals, its associated organisms, competitors and predators, its food resources and other phenomena, all form part of its environment. The environment of a population is more difficult to define, since the individuals within a population do not all respond in the same way to a particular environ- mental factor. However, it is useful to consider the envi- ronment of a population as the sum of all those phenomena to which the population as a whole and its individuals respond. Communities, on the other hand, modify and con- trol the physical and chemical conditions and resources of the areas in which they are found to such an extent that separate consideration of the environment and community is of little value. It is best to view the community and the sum total of the environmental conditions of an area in which it is found as the components of an ecosystem. It is useful to break down the environment of an organ- ism into its component factors, all of which must remain within tolerable limits if the organism is to survive. Any one of these factors may become limiting in the sense that, if it exceeds the tolerable limits for the individual, it will die, although the other factors remain suitable. We will consider this concept of limiting factors in some detail later. In addition to limiting environmental factors we also need to consider regulatory factors that control the size of the population, e.g., disease, competition, or predation may prevent the population from expanding, but it does not threaten its continuous existence. The environment, then, is a term used to describe in an unspecific way, the sum total of all the factors of an 0008_frame_C04 Page 238 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC Adaptations to Shore Life 239 area that influence the lives of the individuals present. There have been numerous attempts to classify the impor- tant environmental variables, ranging from very general ones (e.g., biotic vs. abiotic), to habitat-specific schemes (e.g., for a rocky shore mussel community, these include tidal emersion and immersion, wave action, water move- ment, water and air temperatures, salinity, substrate, aspect, etc.). However, a better classification reflecting causal relationships is needed. Perhaps the most useful one is that of Andrewartha and Birch (1954), who sepa- rated the environment into four major divisions: weather, food, organism of the same and different kinds, and a place to live. A modified subdivision of these categories as they apply to the shore is given in Table 4.1. 4.2.3 E NVIRONMENTAL S TRESS 4.2.3.1 Desiccation There is a considerable body of literature on the responses of intertidal communities and individual species to gradi- ents of emersion/submersion (tidal height) and wave expo- sure. Nevertheless, many of the conclusions reached on the effects of emersion during low tide do not provide completely satisfactory explanations for littoral zonation, species distributions, and abundance patterns (Chapman, 1973; Underwood, 1978a,b; 1985; Underwood and Den- ley, 1984). During periods of emersion (exposure to air), desicca- tion (water loss and temperature stress) may affect the photosynthetic capacities of plants (Schonbeck and Norton, 1979; Dring and Brown, 1982; Smith and Berry, 1986), the nutritional performance of algae (Schonbeck and Norton, 1979), and the ability of animals to grow and carry out the normal functions of feeding and reproduction. The amount of water lost by algae depends on the duration of exposure to air, the atmospheric conditions (solar insolation, temperature, cloud cover, humidity, etc.), and the surface-to-volume evaporation ratio of the plant (Dromgoole, 1980). While a brief exposure of an alga would have little impact, prolonged exposure could be severe. The higher up the shore that a species grows, the longer it is exposed to desiccation effects. However, des- iccation can be minimized by growing in favorable habi- tats, e.g., under overhangs, in shade, in rock pools, or beneath the canopy of larger algae. Some algae (e.g., fucoids) tolerate desiccation rather than having the ability to avoid stress (i.e., by maintaining a high water potential). Moreover some algae have the ability to harden to drought conditions (Schonbeck and Norton, 1979). Emersion from the marine environment exposes mac- roalgae to increased osmotic stress because of tissue water loss (desiccation), increased irradiances, and elevated thal- lus temperatures as tissues dry. Desiccation stress reduces photosynthetic capacity (Dring and Brown, 1982), as well as altering respiration rates. Increased thallus temperatures are typically associated with emersion stress increases, photosynthesis, and dark respiration rates with a Q 10 of ca. 2.0. Photosynthetic rates reach temperates above which they rapidly decline. Many experiments have tested the recovery of algae from emersion, usually by measuring the rates of photo- synthesis or respiration (see review of Gesner and Schramm, 1971). Some representative results shown in Figure 4.1 illustrate the recovery of Fucus vesciculosus (mid-intertidal) and Pelvetia canaliculata (high intertidal). The latter, as expected, was able to withstand longer peri- ods of desiccation. If relative humidity is experimentally maintained at a level high enough to prevent desiccation, the photosynthetic rate may be maintained for long peri- ods, as found in Fucus serratus by Dring and Brown (1982). These authors assessed three hypotheses that might explain the effects of desiccation on intertidal plants and zonation: (1) species from the upper shore are able to maintain active photosynthesis at lower tissue water con- tent than are species lower on the shore (this was refuted by the experimental data); (2) the rate of recovery of photosynthesis after a period of emersion is more rapid in species on the upper shore (this was also refuted by the available data); and (3) the recovery of photosynthesis after a period of emersion is more complete in species from the upper shore (this hypothesis was supported by Dring and Brown’s data). Beach and Smith (1997) have studied the ecophysiol- ogy of the Hawaiian high-tidal, turf-forming red alga, Ahn- feltiopsis concinna . They found that the capacity to recover TABLE 4.1 Classification of Environmental Factors A. Weather 1. Immersion 2. Emersion and water loss 3. Temperature — heat and cold 4. Wave action 5. Salinity 6. Gases — oxygen and carbon dioxide 7. Light 8. Water currents 9. Nutrients and organic constituents B. Resources 1. Food 2. A place to live C. Other Organisms 1. Intraspecific interactions 2. Interspecific interactions (a) Competition (b) Parasitism (c) Predation (d) Commensalism (e) Mutualism 0008_frame_C04 Page 239 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC 240 The Ecology of Seashores photosynthetic activity from emersion stresses varied between algae from microsites separated by <10 cm. Algae from canopy microsites that were regularly exposed to a greater range of irradiance, temperature, and osmotic stress than algae from understory microsites had greater capacity to recover from these stresses alone or in combination compared to tissues from understory microsites. Net pho- tosynthesis was enhanced by 20% water loss or exposure to 2,150 MosM. kg –1 media compared to values for algae that were in a fully immersed state. The temperature optima for net photosynthesis was 33°C, while the upper performance threshold was 40°C. Highly responsive stress acclimation capacity, coupled with microclimate benefits of a turf form, substantially contribute to the ecological success of A. concinna as an ecological dominant at high tidal elevations in the Hawaiian archipelago. The situation concerning the consequences of algae drying out is further complicated in that there is evidence it may be accompanied by an increase in the rate of exudation of organic matter (Siebruth, 1960). The amount of carbon released in 10 minutes by Fucus vesciculosus after resubmergence increased in relation to the duration of exposure (and hence the amount of water lost). Algae from higher on the shore lost more water and released less carbon. Unless rocky-shore animals have special mechanisms to combat water loss, they lose water to the air. If this occurs for extended periods, they eventually die from desiccation. Death of animals on the shore due to desiccation may be due to disturbances in the metabolism resulting from an increasing concentration of the internal body fluids or more usually from asphyxia. For those organisms that respire by means of gills, a constant water film must be maintained over the respiratory surfaces. In addition to water loss by evaporation, animals also lose water by excretion. Most marine animals excrete ammonia as their principal nitrogenous waste product, but it is highly toxic, requiring a very dilute urine and the passage out of the body of a large volume of water. Some littoral species of gastropods have been able to reduce their excretory water loss by excreting appreciable amounts of uric acid, which is a soluble and less toxic product requiring less water for its excretion. In British gastropods, those living highest on the shore have the greatest uric acid concentration in their nephridia. Desiccation stress, of course, varies with position on the shore in relation to the amount of exposure to air over a tidal cycle, as well as to the periods of continuous emer- sion. Animals on hard shores are much more vulnerable to drying out than those on soft shores, and for the latter the problem is more acute on sandy than on muddy shores. Mudflats rarely dry out, but the upper regions of sandy beaches can become quite dry. However, on sandy beaches, the inhabitants avoid desiccation by burrowing. On muddy shores, surface dwellers such as some mudflat snails bur- row into the surface sediments when the tide is out. On hard shores the animals found in the eulittoral can resist desiccation inside an impervious shell or tube that can be tightly closed up (barnacles and mussels), sealed off by a horny membrane (many gastropods), a calcareous operculum (serpulid tubeworms), or closely pressed to the rock surface (limpets). In many of these species there is a correlation between shell thickness and position on the shore, animals living higher on the shore having, in gen- eral, thicker shells than those lower down. Some attached soft-bodied forms, such as anemones, produce a copious secretion of mucus that assists resistance to drying out. In FIGURE 4.1 Recovery of photosynthesis in two intertidal fucoids, Fucus vesciculosus (a) and Pelvetia canaliculata (b), following desiccation for several days. Upper curve in (a) is rate in a thallus resubmerged immediately after reaching 10 to 12% of the original water content. Photosynthetic rate, as O 2 output, is expressed as percentage of the rate in undehydrated control plants. (From Lobban, C.S., Harrison, P.T., and Duncan, M.J., The Physiological Ecology of Seaweeds, Cambridge University Press, Cambridge, 1985, 170. Based on Gessner, F. and Schramm, W., 1971.) 0008_frame_C04 Page 240 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC Adaptations to Shore Life 241 addition, a number of physiological adaptations have developed to enable animals to withstand the risk of des- iccation. Since water loss results in an increasing concen- tration of the body fluids, an efficient osmoregulatory sys- tem is required. Also, since desiccation is usually accompanied by an increase in body temperature, toler- ance of high temperatures is also required. It must be pointed out, however, that the latent heat of evaporation helps to cool an animal that is losing water, and this may be a significant factor in reducing body temperature. On New Zealand shores, littorinids and trochids form a useful series (Figure 4.2), with overlapping vertical ranges and distinctive midpoints, for studying the effects of desiccation on intertidal animals. Rasmussen (1965) has tested the relative amount of desiccation these four species can tolerate by determining the 50% mortality point when they were exposed in sunlight at 35°C. The results are given below: He also carried out a series of experiments to test whether there was a differential susceptibility to desiccation with increasing age. The distribution curve for Melagraphia aethiops is shown in Figure 4.3. It can be seen that there are four definite size classes and possibly a fifth. Mela- graphia spat settle over the entire intertidal range and then migrate as they grow toward a central vertical zone. Those that do not reach this zone perish. The first-year class remains well sheltered from desiccation in runnels, pools, and under rocks. Older individuals are found on the open rock surface. Desiccation experiments (Figure 4.3) indi- cate that there is a definite increase in desiccation toler- ance with size and age. Broekhuysen (1940) studied a series of gastropods ranging, from the upper shore Littorina africana knys- naensis through Oxystele variagata, Thais dubia, O. trgina, and Burnupena cincta to the low shore species O. sinensis (Figure 4.4). Broekhuysen (1940) compared the relative tolerance of the six gastropods to desiccation by measuring both the percentage water loss and mortality in the gastropod over a range of temperatures. However, as pointed out by Brown (1960), the water loss was expressed as percentage of total wet weight including the shell, whereas most investigators express the rate of des- FIGURE 4.2 The vertical distribution of littorinid and trochid gastropods on New Zealand shores. Littorina cincta >120 hours Littorina unifasciata >120 hours Turbo smaragda 60–65 hours Melaraphia aethiops 40–60 hours 0008_frame_C04 Page 241 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC 242 The Ecology of Seashores iccation as water loss per unit dry weight including the shell. Brown repeated Broekhuysen’s experiments to give the results shown in Table 4.2, where desiccation is expressed as water loss per unit dry weight including shell. From the table, two general conclusions can be drawn. First, there is a correspondence between zonational level and the percentage water loss causing 50% mortality. However, some species, such as L. africana knysnaensis , are less tolerant of water loss than would be assumed from their level on the shore, while others such as Burnupena cincta are apparently more tolerant than would be indi- cated by their position on the shore. Second, a tolerance of between 15 and 37% water loss is characteristic of the species series before 50% mortality occurs. The reasons for the exceptions in the zonational sequence are twofold. Burnupena cincta lives in a drier sit- uation on the open rock surface compared to Oxystele tig- rina, which is restricted to damp situations and pools. Sec- ond, L. africana knysnaensis , in common with other high tidal species, can cement the rim of the shell to the substra- tum with mucus and thus limit water loss. Other species including the gastropod Nerita and limpets can retain extra- FIGURE 4.3 A. Size class numbers of the trochid, Melagraphia aethiops. B. Size classes of the catseye, Turbo smaragda . C. Percent survival of the various year classes of Melagraphia aethiops exposed in sunlight at 35°C. D. Response to temperature stress of Littorina cincta , L. unifasciata, and Melagraphia aethiops. E. The percentage of individuals of Melagraphia aethiops with the operculum closed (a measure of desiccation stress) on a hot windy day and a cloudy day. (After Rasmussen, N., The Ecology of the Kaikoura Peninsula, Ph.D. thesis, University of Canterbury, Christchurch, New Zealand, 1965.) 0008_frame_C04 Page 242 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC Adaptations to Shore Life 243 corporeal water under the shell for much of the intertidal emersion period and thus reduce desiccation effects. 4.2.3.2 Thermal Tolerance The temperature tolerance of an intertidal organism is an important factor in determining the upper level at which a particular species can survive when the tide is out. How- ever, the situation is complicated by the interaction of a large number of variables. The magnitude of the temper- ature stress is dependent on season, the time of day when emersion occurs, the duration of the exposure to air, and other factors. Its effect may be modified by factors such as shape and color of the organism, body size, and the magnitude of the water loss. Desiccation may modify the effects of temperature stress in a variety of ways. For example, each gram of water evaporated from the tissues at 33°C removes 544 calories of heat, and this value increases with temperature so that it represents an impor- tant potential method of facilitating heat loss. Many inter- tidal organisms have evolved structural and physiological adaptations that minimize the impact of thermal stress such as shell shape and the retention of extracorporeal water in the mantle cavities of molluscs. Much of the extensive literature on the thermal toler- ance of intertidal and subtidal organisms has been reviewed by Kinne (1971), Somero and Hochachka (1976), and Newell (1979). The most detailed of these early studies relating the temperature tolerances of inter- tidal animals to their zonational position on the shore was that of Broekhuysen (1940). He demonstrated that the sequence of thermal death points of a series of South African gastropods showed a general correspondence with their zonational position on the shore much as described for their desiccation tolerance (see Section 5.1.4.2 above) (Figure 4.5). The highest species on the shore, L. africana knysnaensis , had the highest upper lethal temperature (48.6°C), while the lowest species, Oxtstele sinensis , had the lowest (39.6°C). Since then, numerous studies have confirmed and amplified such sequences in the thermal tolerances for a variety of taxa. FIGURE 4.4 Graph showing the relation between the distribution of gastropods on the shore at False Bay, South Africa, and tidal level. The curve shows the percentage exposure at the various tidal levels. A. Littorina africana knysnaensis B. Oxystele variegata . C. Thaisdubia. D. Oxystele tigrina . E. Burnupena (=Cominella) cincta. F. Oxystele sinensis . (Redrawn from Newell, R.C., The Biology of Intertidal Animals, 3rd ed., Marine Ecological Surveys, Faversham, Kent, 1979, 125. After Broekhuysen, 1940. With permission.) TABLE 4.2 The Range of Distribution (Height in Feet above Datum) and Water Loss Required to Induce 50% Mortality in a Series of Gastropods from Cape Peninsula, South Africa Species Mean Zonational Level % Water Loss for 50% Mortality Littorina africana knysnaensis 12.3 33.17 Oxystele variegata 9.7 37.61 Thaia dubia 9.2 34.88 Oxystele tigrina 8.0 24.40 Burnupena cincta 7.5 32.03 Oxystele sinensis 5.2 15.87 Source: After Brown, A.C., Porta Acta Zool., 7, 1960. 0008_frame_C04 Page 243 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC 244 The Ecology of Seashores 4.2.4 E COLOGICAL N ICHES 4.2.4.1 Introduction As Warren (1971) points out in referring to animal species, “Each species has evolved as part of an ecosystem: an ecosystem in which it occupies certain spaces during cer- tain times; and ecosystem in which it can tolerate the ranges of physical and chemical conditions; an ecosystem in which it utilizes some of the species for energy and material resources and in which it is utilized by other organisms; an ecosystem in which it has many kinds of relations with different species, and in which it can satisfy its shelter and other needs.” In considering the ecological niche of a species, we concern ourselves with what a species does, what activities characterize its life, how and where it carries out these activities, why or for what purpose they are carried out, and when they occur. The concept of the niche is thus a functional one. The ecological niche of a species can be described by considering: 1. The interaction of the species populations with the environmental factors listed in Table 4.1 2. The structural, physiological, and behavioral adaptations that enable the species to survive and reproduce in the environment it inhabits 3. The times at which the interactions occur 4. The effects of the species’ activities on the eco- system of which it is a part While a complete description of the ecological niche of a species is not usually possible, the concept is never- theless a useful one in that it enables us to gain an under- standing of the role of a particular species in the ecosystem in which it is found. Species can be categorized (Vermeij, 1978) as: 1. Opportunists : Such species show high repro- ductive output, a short life history, high dispers- ability, reduced long-term competitive abilities, and generally occupy ephemeral or disturbed habitats. 2. Stress-tolerant forms : These can tolerate chronic physiological stress, exhibit low rates of recolonization, tend to be long-lived with slow growth rates and, consequently are gener- ally poor competitors. 3. Biotically competent forms : These generally live in physiologically favorable environments, have long life spans, are good competitors, and have evolved mechanisms to reduce predation. In the rocky intertidal zone, stress-tolerant forms are characteristic of the upper intertidal habitat, whereas biot- ically competent forms are prevalent in the lower inter- tidal. Opportunistic forms appear ephemerally on dis- turbed or newly available substrates. Andrewartha and Birch (1984) make three proposi- tions concerning the way in which the environment works. The first is that the environment can be considered as a FIGURE 4.5 Graph showing the relationship between upper zonational limit (height in feet above chart datum) and upper limit of thermal tolerance of the series of gastropods illustrated in Figure 4.4. (Redrawn from Newell, R.C., The Biology of Intertidal Animals, 3rd ed., Marine Ecological Surveys, Faversham, Kent, 1979. 146. Data from Broekhuysen, 1940. With permission.) 0008_frame_C04 Page 244 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC Adaptations to Shore Life 245 centrum of components that act directly on a species together with a web of indirectly acting components that affect those in the centrum (Figure 4.6). The second is that the centrum consists of four divisions here modified to include (1) resources, with two components ( food and a place to live ), (2) other organisms (competitors and predators), (3) weather , and (4) disturbances (accidental events that eliminate an organism or population). The third proposition is that the web is a number of systems of branching chains; a link in the chain may be a living organism (or its artifact or residue), or inorganic matter or energy. 4.2.4.2 The “Envirogram” Concept According to Andrewartha and Birch (1984), activity in the directly acting components is the proximate cause of the condition of an individual of a species that affects its chance to survive and reproduce. But the distal cause of an individual’s condition is to be found in the web, among the indirectly acting components that modify the centrum. A modifier may be one or several steps removed from the centrum, and the pathway from a particular modifier to its target in the centrum may be joined by incoming pathways from other modifiers that may be behind or alongside the first one ( n steps away from its target in Figure 4.8). The envirogram is a graphic representation of these pathways. An example of an envirogram for the food resource of a limpet, Patelloida latistrigata , is given in Figure 4.7. The food of this limpet on the rocky shores of southern Australia comprises the spores and young stages of algae that it scrapes from the rock. The envirogram depicts the web of effects determining the supply of food and, hence, indirectly affecting the limpet. Nearby mature algae are the source of the spores and the water currents are required to carry them onto the shores. Another limpet, Cellana FIGURE 4.6 The environment comprises everything that might influence an animal’s chance to survive and reproduce. Only those “things” that are the proximate causes of changes in the physiology or behavior of an animal are placed in the centrum and recognized as “directly acting” components of the environment. Everything else acts indirectly, that is, through an intermediary of chain of intermediaries that ultimately influences the activity of one or other of the components of the centrum. All these indirectly acting components are placed in the web. (Modified from Andrewartha, H.G. and Birch, L.C., The Ecological Web: More on the Distribution and Abundance of Animals, University of Chicago Press, Chicago, 1984, 7. With permission.) 0008_frame_C04 Page 245 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC 246 The Ecology of Seashores FIGURE 4.7 Part of the envirogram of the limpet Patelloida latistrigata on the coast of New South Wales, showing only the interactions that lead to food for the limpet. (Redrawn from Butler, A.J., in Marine Biology , Hammond, L.S. and Synnot, R.R., Eds., Longman Chesire, Melbourne, 1994, 156. Adapted from Andrewartha and Birch, 1984. With permission.) FIGURE 4.8 Basic algal life cycles. (Redrawn from Hinde, R., in Coastal Marine Ecology of Temperate Australia, Underwood, A.J. and Chapman, M.G., Eds., University of New South Wales Press, Sydney, 1995, 127. With permission.) 0008_frame_C04 Page 246 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC [...]... over the tops of the barnacles) so barnacles indirectly influence the food supply of Patelloida The density of the barnacles in turn is determined by the suitability of the substrate for settling, predation by the whelk Morula marginata, and “bulldozing” of newly settled barnacle by Cellana Other 2n to 5n factors affecting the supply of food for Patelloida are also shown 4. 2 .4. 3 Weather The various weather... combination of factors involving the following: 1 The ability of the larvae of animals and the settling stages of algae to select an environment that is suitable for adult life 2 The presence of a suitable substratum or sediments of a suitable grain size in the particular zone occupied 3 The interaction of the plants and animals with the various environmental factors (Table 4. 5) 4 The availability of the right... in the lives of burrowing species However, the degree of illumination is an important factor in the photosynthesis of benthic microalgae 0008_frame_C 04 Page 249 Monday, November 13, 2000 9:53 AM 249 Adaptations to Shore Life Water currents: Perhaps the most important role of currents in the life of shore organisms is in the distribution of larval stages of animals and the sporelings of algae Much of. .. behavior consists of a series of responses to interactions between swash-backwash processes on the one hand and beach face slope, water content, and accretion/erosion on the other Most of the patterns of animal movements and distribution are the accumulated results of these individual responses © 2001 by CRC Press LLC 4. 4 .4 4 .4. 4.1 CLOCK-CONTROLLED BEHAVIOR INTERTIDAL ANIMALS IN Introduction The behavioral... youngest part of the fronds (Stebbing, 1962) Stebbing cut discs from the length of a blade of Laminaria, arranged them evenly in a circular vessel containing Spirorbis larvae, and noted that the number of larvae settling was greatest on discs cut from the growing end of the frond (1 to 3 in Figure 4. 13) The adaptive value of this is that the youngest part of the frond has the least dense growth of attached... is clear that the bacteria on the sand grains was the source of attractiveness Furthermore, Gray (1966) showed that the degree of attractiveness was not due so much to the numbers as to the kinds of bacteria The presence of particular species of bacteria, rather than bacterial numbers, has been shown to be of importance in the distribution of the sand-dwelling harpacticoid copepod, Leptastacus constrictus... shell of the bivalve, Paphies donacina, on sand beaches • Endoecism There are large numbers of commensals that lurk in the burrows, tubes, or dwellings of various animals Many polynoid worms are such commensals They include the polynoid Lepidasthenia aecolus, found in the burrows of the lugworm, Abarenicola assimilis; the short, rather broad and flat Lepidastheniella comma living in the tubes of terebellids,... some species, provided the body surface remains wet, can breathe atmospheric air The meio- and microfauna of soft shores, on the other hand, must cope with anaerobic conditions if they live below the RPD layer Light: The amount of light on the shore varies widely with the rise and fall of the tide Excessive illumination can be damaging due to the ultraviolet and infrared rays, which can be lethal to some... number of recruits differed at the three sites Factors affecting the density of juveniles and adults: Connell (1985) examined the relationship between the density of larval settlement, the density of recruits, mortality up to the end of the period of settlement, the mortality between settlement and the age of 1 year, and the density of adults for Semibalanus balanoides in Scotland England and Massachusetts,... mortality during the first few weeks after settlement acted independently of the initial density of settlers; thus, the density of recruits was a direct reflection of that of the settlers However, the FIGURE 4. 16 Variation in recruitment of the first year classes of the clam, Trivela stultorum, on three sand beaches at Pismo Beach, California; all years are included in which some clams in the first year were . 239 4. 2.3.2 Thermal Tolerance 243 4. 2 .4 Ecological Niches 244 4. 2 .4. 1 Introduction 244 4. 2 .4. 2 The “Envirogram” Concept 245 4. 2 .4. 3 Weather 247 4. 2 .4. 4 Resources 249 4. 2 .4. 5 Other Organisms 249 4. 2 .4. 6. 0008_frame_C 04 Page 237 Monday, November 13, 2000 9:53 AM © 2001 by CRC Press LLC 238 The Ecology of Seashores 4. 4 .4 Clock-Controlled Behavior in Intertidal Animals 271 4. 4 .4. 1 Introduction 271 4. 4 .4. 2. combination of biological and physical factors and the presence of chem- ical cues (Table 4. 4, Figure 4. 12). They include the speed of fluids (especially close to the sediment surface), the contours of the