Up or down? Limpet orientation on steeply sloped substrata

Abstract

Variation in the distribution of animals across the landscape, within and between habitats, can occur at a variety of scales. At the smallest scale, individuals can either be positioned randomly or can be orientated in regard to variables. Differences in orientation can, for many animals, directly affect their reproductive success and/or survivorship, and these effects may have downstream ecological consequences. Organisms may orientate themselves in a specific way in response to varying environmental conditions or habitat properties; alternatively they may consistently orientate in a specific direction. The present study investigated patterns of orientation during low tide in the intertidal marine limpet Cellana tramoserica on steeply sloped rocky surfaces (> 60°to the vertical). We also examined whether individual limpets consistently orientated in the same direction or if orientation was dependent upon their orientation during the previous low tide. There was a downward bias in orientation. Mensurative and manipulative experiments measuring limpet orientation over different days showed that individual limpets did not consistently orientate in the same direction and that their orientation was independent of their orientation during the previous low tide. Thus, whilst limpets on slopes often orientate with their heads downwards, there is much inherent individual variation in the orientation patterns exhibited by resting limpets. Simple mechanistic explanations such as responses to desiccation are unlikely to be sufficient to explain why limpets choose different orientations at the end of foraging excursions. These findings have implications for understanding limpet distributions on rocky shores and downstream ecological consequences.

Inter-individual variation partially explains patterns of orientation on steeply sloped substrata in a keystone grazer, the limpet Cellana tramoserica

“…Limpets which were not found on a steeply sloped surface at least six times were excluded from the data set. Animals were defined as facing downwards if their orientation was between 135 and 225°where 0°was straight up (Fraser et al 2010). At any given time, a limpet has a 1 in 4 chance of facing downwards as the range defined as downwards is equal to of all possible orientations.…”

“…The orientation of limpets was measured along the anteroposterior axis of each limpet using a spirit-level protractor, where the asymmetric peak of an individual is its head end (Fraser et al 2010). and approximately every fortnight for 4 months (8 sample times in total), the orientation of all labelled limpets was measured.…”

“…The exclusion of limpets from an area results in a proliferation of algae and a change in spatial variability in assemblages (O'Connor and Crowe 2005;Coleman et al 2006;Moore et al 2007). Several species of patellogastropod limpet display clear biases in head orientation whilst stationary during low tide on steeply sloped substrata ([60°) (Iwasaki 1993;Williams et al 1999;Fraser et al 2010;Fraser 2014b). In New South Wales, Cellana tramoserica (Holten 1802), an inconsistent homing limpet (Mackay and Underwood 1977), on steeply sloped substrata exhibit a downwards bias in head orientation, with approximately 39 % (±2 %) of limpets orientated downwards at any given point in time, but there is also inter-individual variation (Fraser et al 2010).…”

“…For example, snowy owls display different patterns of orientation depending on their plumage colouration, with a greater number of less spotted and white individuals orientating into the sun compared with heavily spotted individuals (Bortolotti et al 2011). The orientation of intertidal limpets during low tide is also highly variable (Fraser et al 2010), but relatively little is known of the processes that contribute to these patterns of behaviour at a population level (see Fraser et al 2014Fraser et al , 2015. Before investigating the causal basis for variations in orientation within and between populations of a single species, it must first be established whether observed differences are a result of intrinsic differences between individuals or as a consequence of variations in the environment or habitat (Chapman 2000).…”

“…Several species of patellogastropod limpet display clear biases in head orientation whilst stationary during low tide on steeply sloped substrata ([60°) (Iwasaki 1993;Williams et al 1999;Fraser et al 2010;Fraser 2014b). In New South Wales, Cellana tramoserica (Holten 1802), an inconsistent homing limpet (Mackay and Underwood 1977), on steeply sloped substrata exhibit a downwards bias in head orientation, with approximately 39 % (±2 %) of limpets orientated downwards at any given point in time, but there is also inter-individual variation (Fraser et al 2010). This variation in orientation may be because different individuals consistently orientate in the same general direction, i.e.…”