Investigation into Musculus discors Beds in Pen Llyn a'r Sarnau (2011) dataset https://portal.medin.org.uk/portal/start.php?tpc=007_NRW_DS115188 This survey was carried out as part of a MSc thesis in September 2011. To provide baseline data on M. discors beds around the LlPeninsula, with four specific aims: Establish the population structure for sample beds from around the LlPeninsula in order to investigate population structure and determine bed longevity. Identify the in-fauna/epi-fauna associated with M. discors beds. Investigate the sedimentation that occurs upon M. discors beds. Provide advice to CCW on whether M. discors should be included within a Local Biodiversity Action Plan for Gwynedd. To determine M. discors population structure and influence on assemblage structure, twenty six, ten centimetre (cm) diameter cores of M. discors, associated fauna and sediment were collected by Seasearch volunteers on SCUBA from four randomly chosen beds in North Wales.were rinsed through a 63 micrometre (sieve with the sediment, M. discors and in-fauna retained. All liquid was retained for later filtration. M. discors were sorted from the sample and length measured to the nearest 0.1 millimetre (mm) using Vernier callipers with epifauna identified and enumerated (Rees et al., 2008). If present, new recruits were sorted from the sample and measured using a calibrated microscope slide (to the nearest 0.1mm). In cases where there were high numbers of recruits, sub-sampling occurred with the abundance and size of a quarter of the total recruit population determined. All associated fauna were identified to the lowest taxonomic level possible. 20110930