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.