Foraging in the limpet Patella vulgata: the influence of rock slope on the timing of activity

Abstract

Preliminary observations of limpet activity at Lough Hyne, in south-west Ireland, showed that individuals on steep slopes were primarily active at night, when emersed; while those on near-horizontal rocks were often active during daytime submersion. Observations of limpet populations over an 11 d period of limpet populations on a near-vertical and a near-horizontal site, only 45 m apart, confirmed that animals on the near-vertical site were active on nocturnal low tides, whilst those on the near-horizontal site were active on daytime high waters. A short-term survey at ten sites, which had limpets on both extremes of slope (i.e. either near-vertical or near-horizontal), showed that limpets on near-horizontal surfaces were, on average, more active at daytime high waters than those on near-vertical faces. In 1996 and 1997 surveys of activity at daytime high, and nocturnal low waters were conducted at sites (14–15) with varying rock slopes (∼3–87°). In all cases, limpets on more steep slopes were active at nocturnal emersion whilst animals on more gentle slopes were active on daytime submersion periods. In most cases these trends were significant and explained between 22–40% and 37–44% of the variation in activity with site in 1996 and 1997 respectively. Analysis of the head orientation of limpets on their home scars showed that animals orientated in a down shore direction at all sites (1997 data) suggesting that limpets do perceive and respond to slope. Whilst slope does appear to influence the timing of limpets' activity (and especially on very steep or gently sloping sites) it does not account for a large degree of the variation in activity and, on sites with slopes between 30 and 60°, is likely to work in combination with other factors.

Up or down? Limpet orientation on steeply sloped substrata

“…If orientating the head downwards confers benefits, it must be expected that there are also costs for doing so, for if there were no costs, the majority of limpets should be so orientated. In the published accounts of limpet orientation there are no reported tests of hypotheses related to costs of patterns of orientation (Gallien 1985, Iwasaki 1993, Williams et al 1999. As there were no differences in orientation between solitary and aggregated limpets, this suggests that the possible benefits and costs of a specific orientation are not dependent on an individual's group status.…”

“…Bartholomew 1966, DeWitt et al 1967, Lustick et al 1978, Brodsky & Weatherhead 1984, Fortin et al 2000. In addition to models around thermoregulation, another proposed explanation is that a downward orientation decreases desiccation stress or allows for a greater capacity for water storage, as the head area is wet for a longer period of time (Gallien 1985, Williams et al 1999. Alternatively, a downward head orientation may allow for greater flushing of the nuchal cavity to remove waste products such as faecal matter and CO 2 (Williams et al 1999).…”

“…The study sites consisted of a series of steeply sloped rock surfaces and/or large boulders and varied in size (minimum size: 2 m 2 ). Steeply sloped surfaces were defined as having a slope of ≥60°, excluding overhangs; this angle was chosen because previous observations on limpet orientation were made on surfaces with slopes between 60 and 95°from the vertical (Williams et al 1999). The weather was not atypical for the study periods (Australian Bureau of Meteorology, www.bom.gov.au), and as no hypotheses on a possible relationship between weather conditions and limpet head orientation had been proposed, there was no need to test for this.…”

“…The ability of an organism to adopt a particular orientation at a given time/place may have survival and health implications. Specific orientations have been suggested to confer benefits in terms of thermoregulation (Bartholomew 1966, DeWitt et al 1967, Lustick et al 1978, Brodsky & Weatherhead 1984, Fortin et al 2000, predation success (Walguarnery et al 2009), vulnerability to predation (Coleman et al 1999), waste disposal (Williams et al 1999), hatchling success ) and health (Goodenough et al 2008). Often these orientations function through varying exposure of the body to the stimulus of interest, to increase or decrease the surface area sensitive to the factor.…”

“…Iwasaki 1993) or downward (e.g. Gallien 1985, Williams et al 1999) direction. Differences in orientation have been noted between aggregated and solitary Colisella digitalis, such that aggregated limpets were positioned at a 90°angle as opposed to an observed downwards orientation of solitary limpets (Gallien 1985).…”