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Limpets and barnacles are important components of intertidal assemblages worldwide. This study examines the effects of barnacles on the foraging behaviour of the limpet Patella vulgata, which is the main algal grazer in the North-west Atlantic. The behaviour of limpets on a vertical seawall on the Isle of Man (UK) was investigated using autonomous radio-telemetry, comparing their activity patterns on plots characterized by dense barnacle cover and plots from which the barnacles had been removed. Limpet behaviour was investigated at mid-shore level, but two different elevations were considered. This experiment revealed a significant effect of barnacle cover on the activity of P. vulgata. Limpets on smooth surfaces spent a greater proportion of total time active than did limpets on barnacles. Movement activity was also greater in areas that were lower down in the tidal range. In general, limpets were either predominantly active during diurnal high or nocturnal low tides and always avoided nocturnal high tides. Individuals on barnacles at the higher elevation concentrated their activity during nocturnal low water. All the other groups of limpets (smooth surfaces on the upper level and all individuals on the lower shore) had more excursions centred around daylight hours with an equal distribution of activity between periods of low and high water. Inter-individual variability was, however, pronounced.
The rocky shores of the north-east Atlantic have been long studied. Our focus is from Gibraltar to Norway plus the Azores and Iceland. Phylogeographic processes shape biogeographic patterns of biodiversity. Long-term and broadscale studies have shown the responses of biota to past climate fluctuations and more recent anthropogenic climate change. Inter- and intra-specific species interactions along sharp local environmental gradients shape distributions and community structure and hence ecosystem functioning. Shifts in domination by fucoids in shelter to barnacles/mussels in exposure are mediated by grazing by patellid limpets. Further south fucoids become increasingly rare, with species disappearing or restricted to estuarine refuges, caused by greater desiccation and grazing pressure. Mesoscale processes influence bottom-up nutrient forcing and larval supply, hence affecting species abundance and distribution, and can be proximate factors setting range edges (e.g., the English Channel, the Iberian Peninsula). Impacts of invasive non-native species are reviewed. Knowledge gaps such as the work on rockpools and host–parasite dynamics are also outlined.
Land crabs are no different from other crustaceans in that growth can take place only by means of intermittent shedding of the more or less inextensible integument – the process of molting or ecdysis. For aquatic crabs, molting is a time of stress and mortality, resulting both from the dangers inherent in the molt process itself, and from the high risk of predation while the newly molted crabs are soft and relatively immobile. For land crabs there are added complications: The risk of desiccation is greater at this time, and there is the problem of obtaining the quantity of water needed for the postmolt increase in size, which must occur rapidly before the new integument hardens.
Although molting appears superficially as a short and intermittent interlude, it has a pervasive effect on the whole of the life cycle, and the period between molts is one of continuous morphological and physiological change (also see Chapter 5). These changes enable the crab to prepare for molting, and to recover from it. Some basic elements relating to molting and growth of crabs will now be outlined; a more detailed account for crabs in general is provided by Hartnoll (1982, 1983).
The molting cycle can be divided into the following stages.
Premolt (proecdysis). The new integument is being laid down beneath the old, and stored energy reserves are being mobilized to enable the new structures to be formed. Calcium is being resorbed from the old integument and stored in the tissues.
Molt (ecdysis). The old integument is shed, and the crab rapidly increases in size by the absorption of water. The process takes at most only a matter of hours.
At the outset of this chapter two points must be clarified in order to define the limits of coverage: first, which taxa are to be accepted as “crabs, ” and second, which characteristics will qualify members of those taxa for consideration as “land” crabs.
Here, in discussing the taxonomic limits, and in later sections of this chapter, I have followed the classification of the Crustacea developed by Bowman and Abele (1982) in the most recent major review of crustacean biology. This will be convenient, although later in this chapter it will become clear that this classification is certainly not ideal in all respects (see section 4). However, in the currently uncertain state of brachyuran phylogeny an additional different scheme might only increase confusion. The relevant parts of Bowman and Abele's classification are reproduced in Table 2.1. The eleven families of the Potamoidea are listed, but since the subdivision of that superfamily is still to some extent uncertain, the families are not mentioned further in this chapter. A case could be made for restricting coverage to the Brachyura or “true crabs, ” but this would exclude the land hermit crabs: These show terrestrial adaptations very similar to the true crabs, and a comprehensive treatment demands their inclusion. The simplest solution is to extend consideration to the infraorders Anomura and Brachyura. The Anomura is used in a rather more restricted sense by Bowman and Abele (1982) than in some previous classifications.
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