As part of the HABMAP project surveys were carried out in 5 areas in the summer of 2005. Survey work carried out in Caernarfon Bay showed this area was very diverse in terms of both its seabed features and benthic communities. The location of a well-documented horse mussel (Modiolus modiolus) reef was clearly identified from multibeam bathymetry and backscatter data, and was sampled using video, SPI and grab samples. The area was rich in both infauna and epifauna, with a variety of different biotopes being recorded. Other features of the Caernarfon Bay survey area included an area of cobble and boulder ridges in the southwest with associated tide-swept epifaunal communities, and an area of muddy sediments in shallower waters to the east, with patches of stable cobbles that upported a rich seaweed and epifaunal community.

Biogenic horse mussel (Modiolus Modiolus) reef is an important feature of the Pen Llyn a'r Sarnau SAC. The reef has high biodiversity and is vulnerable to anthropogenic activities, such as benthic fishing. The mussel reef forms an undulating surface that is significantly different to the surrounding seabed and therefore acoustic monitoring methods can be used to monitor changes in extent and identity areas of potential damage. Work is carried out on the north Llyn horse mussel reef in Pen Llyn a'r Sarnau jointly by CCW and University of Wales Bangor. Project is carried out in conjunction with the Across Wales drop-down video project.