Snails are highly unusual among multicellular animals in that they move on a layer of costly mucus, leaving behind a trail that can be followed and utilized for various purposes by themselves or by other animals. Here we review more than 40 years of experimental and theoretical research to try to understand the ecological and evolutionary rationales for trail-following in gastropods. Data from over 30 genera are currently available, representing a broad taxonomic range living in both aquatic and terrestrial environments. The emerging picture is that the production of mucus trails, which initially was an adaptation to facilitate locomotion and/or habitat extension, has evolved to facilitate a multitude of additional functions. Trail-following supports homing behaviours, and provides simple mechanisms for self-organisation in groups of snails, promoting aggregation and thus relieving desiccation and predation pressures. In gastropods that copulate, trail-following is an important component in mate-searching, either as an alternative, or in addition to the release of water- or air-borne pheromones. In some species, this includes a capacity of males not only to identify trails of conspecifics but also to discriminate between trails laid by females and males. Notably, trail discrimination seems important as a pre-zygotic barrier to mating in some snail species. As production of a mucus trail is the most costly component of snail locomotion, it is also tempting to speculate that evolution has given rise to various ways to compensate for energy losses. Some snails, for example, increase energy intake by eating particles attached to the mucus of trails that they follow, whereas others save energy through reducing the production of their own mucus by moving over previously laid mucus trails. Trail-following to locate a prey item or a mate is also a way to save energy. While the rationale for trail-following in many cases appears clear, the basic mechanisms of trail discrimination, including the mechanisms by which many snails determine the polarity of the trail, are yet to be experimentally determined. Given the multiple functions of trail-following we propose that future studies should adopt an integrated approach, taking into account the possibility of the simultaneous occurrence of many selectively advantageous roles of trail-following behaviour in gastropods. We also believe that future opportunities to link phenotypic and genotypic traits will make possible a new generation of research projects in which gastropod trail-following, its multitude of functions and evolutionary trade-offs can be further elucidated.
Sexual size dimorphism is widespread among dioecious species but its underlying driving forces are often complex. A review of sexual size dimorphism in marine gastropods revealed two common patterns: firstly, sexual size dimorphism, with females being larger than males, and secondly females being larger than males in mating pairs; both of which suggest sexual selection as being causally related with sexual size dimorphism. To test this hypothesis, we initially investigated mechanisms driving sexual selection on size in three congeneric marine gastropods with different degrees of sexual size dimorphism, and, secondly, the correlation between male/female sexual selection and sexual size dimorphism across several marine gastropod species. Male mate choice via mucus trail following (as evidence of sexual selection) was found during the mating process in all three congeneric species, despite the fact that not all species showed sexual size dimorphism. There was also a significant and strong negative correlation between female sexual selection and sexual size dimorphism across 16 cases from seven marine gastropod species. These results suggest that sexual selection does not drive sexual size dimorphism. There 2 was, however, evidence of males utilizing a similar mechanism to choose mates (i.e. selecting a female slightly larger than own size) which may be widespread among gastropods, and in tandem with present variability in sexual size dimorphism among species, provide a plausible explanation of the observed mating patterns in marine gastropods.
22A key element missing from many predictive models of the impacts of climate change 23 on intertidal ectotherms is the role of individual behaviour. In this synthesis, using 24 littorinid snails as a case study, we show how thermoregulatory behaviours may 25 buffer changes in environmental temperatures. These behaviours include either a 26 flight response, to escape the most extreme conditions and utilize warmer or cooler 27 environments; or a fight response, where individuals modify their own environments 28 to minimize thermal extremes. A conceptual model, generated from studies of 29 littorinid snails, shows that various flight and fight thermoregulatory behaviours may 30 allow an individual to widen its thermal safety margin (TSM) under warming or 31 cooling environmental conditions and hence increase species' resilience to climate 32 change. Thermoregulatory behaviours may also buffer sublethal fitness impacts 33 associated with thermal stresses. Through this synthesis, we emphasise that future 34 studies need to consider not only animals' physiological limits but also their capacities 35 to buffer the impact of climate change through behavioural responses. Current 36 generalizations, made largely on physiological limits of species, often neglect the 37 buffering effects of behaviour and may, therefore, provide an over-estimation of 38 vulnerability, and consequently poor prediction of the potential impacts of climate 39 change on intertidal ectotherms. 40 41
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