Biogenic reefs
Horse mussel reef – Pen Llŷn a’r Sarnau SAC supports one of only two remaining horse mussel reefs in the Irish Sea (a third, damaged reef also exists in Northern Ireland). The horse mussel reef in the SAC is approximately 4 km offshore from the north Llŷn coast. Here the mixed sediment and cobble seabed has been covered by a dense bed of horse mussels Modiolus modiolus. The reef is at least 150 years old but is likely to be much older. Horse mussels are long-lived animals. Samples from the north Llyn horse mussel reef have shown that individual mussels are over 50 years old but with also juvenile mussels appearing to be recruiting onto the reef.
The mussels attach together by special (byssus) threads and these trap fine silt and the waste ‘sediment’ material produced by the mussels (pseudofaeces) and so build up the reef structure. The mussel reef comprises undulating waves on the seabed created by the mussels themselves – the bulk of the live mussels living on the crests of each wave and the troughs comprising empty shells.
The combination of substrata in the form of the hard mussel shells, various grades of sediment and the opportunities for cryptic species to find shelter amongst the mussels provides a broad range of sub-habitats - this supports a high biomass of a wide variety of species living in amongst the matrix of the mussel bed (infauna) and on the surface of the mussel reef (epifauna). Soft coral, molluscs, echinoderms, sea anemones, crustaceans and fish are some of the more conspicuous and larger species associated with the horse mussel reef. Biogenic reefs of this kind are thought to be highly productive and likely to play an important role in the marine ecosystem.
Musculus mussel reef - Musculus discors is a very small mussel species that occasionally lives in dense aggregations attached to rock and gravely sediments where it consolidates the sediment surface by binding it and a thick layer of pseudofaeces together with its byssus threads. Musculus discors is a common species but there are very few examples of dense beds of this species in the British Isles and most of the records to date are from the reefs off the north coast of the Llŷn Peninsula. Several large patches (perhaps hundreds of metres long) of Musculus bed are found encrusting the bedrock and boulder reefs along the north coast of the Llŷn Peninsula off Porth Ysgadan and Porth Colmon.
Honeycomb worm reef - The worm Sabellaria alveolata forms and lives in tubes made of sand grains. Under certain conditions the worm forms clusters of these tubes to form reefs that look like a giant honeycomb attached to rock. Honeycomb worm, or Sabellaria reefs in the SAC have high associated species richness; this is due to the fact that the reef structures stabilise and often forms rock pools in what would otherwise be more mobile and free draining shores. Wales has a large proportion of the UK’s honeycomb worm reef resource with a significant amount of this in the Pen Llŷn a’r Sarnau SAC.
