Intertidal species are useful indicators of marine biodiversity response to climate change.
The trochid species Gibbula umbilicalis and Osilinus lineatus were studied at their eastern
limit in the English Channel to determine the factors setting distribution limit and if
climatic controls can be detected. The abundance of trochids on rocky shores was measured
using a catch per unit effort method of repeated three minute counts. Limitations in this
method have been identified and a new method to estimate population abundance of
trochids on rocky shores has been proposed (see appendix 1). Trochid populations have
extended their range eastwards along the English Channel. Populations of G. umbilicalis
now inhabit as far east as Beachy Head, east Sussex. This is thought to be the result of sea
defence structures at Elmer, Sussex increasing the connectivity of suitable habitat between
the Isle of Wight and Sussex. The 0. lineatus population at Portland Harbour, Dorset has
increased in abundance and is potentially the main source of recruits at White Nothe and
Osmington Mills, Dorset. The gonad development in trochid populations in the western
basin of the Channel (milder winter temperatures) is larger than in the eastern basin (lower
winter temperatures), suggesting some temperature/ climatic control. Regression analysis
revealed that there is a strong relationship between the population density and shell size of
G. umbilicalis, but not in O. lineatus. Changes in the reproductive cycle of trochids after
the 2005/6 winter were detected but it is not clear if reproductive success was affected.Faculty of Scienc
