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Biology of the gastropod family Littorinidae. II. Role in the ecology of intertidal and shallow marine ecosystems
McQuaid, C.D. (1996). Biology of the gastropod family Littorinidae. II. Role in the ecology of intertidal and shallow marine ecosystems. Oceanogr. Mar. Biol. Ann. Rev. 34: 263-302
In: Oceanography and Marine Biology: An Annual Review. Aberdeen University Press/Allen & Unwin: London. ISSN 0078-3218, more
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    Biology; Marine molluscs; Littorinidae Children, 1834 [WoRMS]; Marine

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  • McQuaid, C.D.

    Littorinid gastropods are cosmopolitan and ubiquitous in intertidal and shallow marine ecosystems. In many cases they have a profound influence on the structure of these systems. There is an extensive literature on the ecology of littorinids, dealing in greater or lesser detail with the autecology of a large number of species. This paper summarizes the literature on community interactions, concentrating primarily on the best studied examples of the main feeding types. The effects of predators are described for each case. Although predation is often cited as central to selective processes in the literature on the evolutionary biology of littorinids, in most of the ecological literature predation is not seen as controlling population densities.Individual species of littorinids can feed on many different types of food, but most fall reasonably well into one of four categories. These are: detritivores (that eat dead angiosperm material); epiphytic grazers (that eat the periphyton of algae and angiosperms); macroalgal grazers; and epilithic grazers (that brush microalgae and sporelings from the rocks). Community interactions can be complex and are extremely variable from place to place and time to time, but it is possible to make generalizations. The effects of littorinids on community structure depend primarily on feeding type. Species with strong direct effects on the distribution and abundance of other organisms fall into the last two categories, species in the first two categories have weak direct effects.Neither detritivores nor epiphytic grazers have been shown to influence community structure strongly. Detritivores have no recorded effect on species densities or distributions, but can be very important in nutrient cycling in saltmarshes. They eat both angiosperm detritus and the fungi colonizing dead plants, making refractory angiosperm material available as faeces to other organisms. Some, for example Littoraria irrorata, are important prey for predators such as conchs and crabs and provide hermit crabs with shells, that can control their population structure. Epiphytic feeders have no direct effect on macroalgae. Several authors have commented on the negative effects of epiphytic plants on macroalgae (increased shading, drag and frond breakage) and it has been suggested that epiphytic grazers may indirectly influence community structure by removing epiphytes and enhancing the fitness of their host plants.The situation can be quite different for the remaining two feeding types. Littorinid macroalgal grazers mostly occur on the low shore or subtidal and can control algal biomass directly. While macroalgal grazers of other taxa have been found to influence algal zonation, it is not clear that littorinids do so. Feeding behaviour and preferences reflect algal defences, both chemical and physical. Both types of defence deter generalist grazers, but chemical defences can be overcome by specialist grazers that are immune to the chemicals produced by a particular alga. The amount of damage that macroalgal grazers do depends not so much on grazer density as on fine details of the interactions between grazer and alga pairs. A grazer may devastate one species of alga, but do little damage to another, even at similar grazer densities.Lastly, epilithic grazers have the most dramatic influence on community structure. Whereas macroalgal grazers affect algae by reducing biomass or survival of mature plants, this group can control the recruitment of algae. Epilithic grazers occur throughout the shore and their effects vary with height on the shore. This is because the consequences of grazing depend not only on grazing pressure, but also on algal growth rates and algal competitive hierarchies. Because the effects of epilithic grazers depend on the balance between grazing and algal growth or recruitment, they are chiefly density-dependent. Grazing effects operate mainly through the removal of macroalgal sporelings so that mature plants represent sporelings that have "escaped" grazing. These grazers may exclude mature macroalgae where particularly dense (as on sheltered shores), or have no appreciable effect where algal settlement or growth are particularly high. Epilithic grazers may also eat ephemeral algae. Whether their grazing increases or decreases algal species richness depends on the position of their preferred food in the algal competitive hierarchy.The primary reason for the complexity of these interactions is the fact that littorinid grazing rates, algal growth rates and algal competitive hierarchies are all influenced by the same factors: height on the shore, wave exposure, habitat and season, i.e. grazers and algae react to the same factors and interact among each other.Epilithic grazers can also affect sedentary animals very strongly. The complexity and importance of interactions are clear when we consider epilithic grazers in combination with either algae or barnacles. Algae or barnacles can be dense enough to inhibit grazers (either directly, or indirectly by harbouring predators), or grazers can control recruitment of algae and barnacles. The direction of the interaction depends on the relative densities of the species involved. As densities largely reflect recruitment success, a good understanding of recruitment processes is essential if we are to be able to predict the results of species interactions.

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