Introduction to dog whelk density

Introduction to dog whelk density

Introduction

The aim of my investigation is to determine whether the density of Dog Whelks at 3.00m above chart datum on a rocky shore in Dale Fort, Pembrokeshire, Wales, are different on a sheltered and exposed rocky shore (the sites under investigation being Frenchman’s Steps and Castle Beach).

Dog Whelks are carnivorous and they eat barnacles, limpets and mussels. They have a groove in the shell lip and this allows water in for breathing.   This is necessary when they are boring for their food, which takes many hours, e.g. barnacles take 48 hours to ingest. They are very adaptable feeders, changing their diet according to availability (www.theseashore.org.uk).

Found on exposed to sheltered rocky shores from the mid-shores. “May be abundant in areas exposed to extremely strong tidal stress. They are gregarious and common amongst barnacles and mussels on which they feed.” (http://busybee.mba.ac.uk/species/Nucellalapillus.htm).

The environment changes in abiotic (non-living) factors across the habitat. One factor, desiccation, is the result of the tide. As the water slowly drops so the shore is exposed to drying (called emersion, the opposite of immersion).   Clearly this emersion becomes less the further down the shore you are. On neap tides it may be that days occur when the lower shore never dries out. Likewise, on neap tides the upper shore may never get water on some days. This is an environmental gradient of desiccation. The tide is a primary factor, as it not only affects the amount of water present but temperature, salinity and many others.

The Lower Shore has an 80% immersion time for the top of the zone and a 99% immersion time for the bottom of the zone.   This makes it theoretically the best zone for inter-tidal organisms due to the optimal abiotic conditions.   For example, animals and plants here have an abundant water supply, minimal temperature fluctuations and minimal light fluctuations.   The Upper Shore, however, is a very...