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How do serpulid reefs form? In Loch Creran, the only known British locality where living serpulid reefs occur, reefs are found within a depth range of 1 - 13 m with the most developed reefs being found between 6-10 m. However, in 1887 well-developed serpulid reefs were found living among Zostera beds in the intertidal zone of Loch Creran. Neither the Zostera nor the serpulids occur there now. Biogenic reefs of Serpula vermicularis require a hard substratum on which to form and most commonly occur on the shells of bivalve molluscs. However, within Loch Creran reefs have also formed on bedrock, boulder stones and man-made structures such as fish farm mooring blocks! The Loch Creran reefs form predominantly in areas where there is suitable substratum scattered throughout a muddy or muddy/sand bottom. Reef development occurs by repeated subsequent settling of larvae on adult tubes. Reefs continue to develop this way, reaching a maximum size of approximately 1 m high and 3 m wide. A limited turnover of water in order to facilitate larval retention within the system appears to be a requirement for reef development. Reefs take many years to develop but once they have they act as a substratum for a wide variety of other organisms. Reefs are clearly very fragile, being reported to spread partly by virtue of pieces falling away and then continuing to grow in their new location. This may in part be due to the activities of the yellow boring sponge, How do horse mussel reefs form? The horse mussel Modiolus modiolus is a boreal species with reefs reaching their southerly limit around British shores. Dense aggregations, which would represent biogenic reefs, are restricted to depths between 5 and 50 m. In Loch Creran the reefs are found mainly at a depth range of 13 - 25 m. Horse mussel reefs tend to form on hard substrata such as shell and stones in areas of moderate to strong tidal currents. However, survival of juvenile species is often low due to high predation, and there is evidence that the best refuge for juveniles is in the byssus threads of established clumps or aggregations of larger horse mussels, which also act as a suitable settlement substratum. Horse mussels grow rapidly in the early years, which is thought to be important in enhancing survival to adulthood. Ecological functioning of horse mussel reefs Communities associated with horse mussel reefs are known to be extremely rich and diverse. 90 - 270 invertebrate taxa have been found associated with reef communities. It is likely that this is an underestimate, particularly in terms of sponges and infauna. The diversity of species increases as the horse mussel clump size and number increase. The reef may also act as a nursery ground for other species such as young scallops or clams. Horse mussel reefs may be very extensive and often include many other filter feeders such as sponges, hydroids, bryozoans, soft corals, bivalves and ascidians, and so are probably of great importance in channelling organic material between the plankton and the benthos. Adapted from Holt, TJ, Rees, EI, Hawkins, SJ and Seed, R. (1998) Biogenic Reefs: An overview of dynamic and sensitivity characteristics for conservation management of marine SACs. Vol. IX UK Marine SACs Project |