Although we do not have coral reefs in Welsh waters, there are a number of marine plants and animals whose hard skeletons and tubes form biogenic (living) reefs. The honeycomb worm Sabellaria alveolata is one such species. S.alveolata is a tube-dwelling, filter feeding polychaete worm that forms colonies consisting of thousands or even millions of individuals.
They can be found on the lower intertidal areas of the north eastern Atlantic and Mediterranean Oceans and are widespread in areas with stable sediment supply. In Britain, honeycomb worm reefs are most abundant on the south and west coasts with isolated records from the south-east and east coasts. Their northern limit is the Outer Hebrides.
Although these reefs may often be overlooked or even trampled by beach users, the reefs are a UK Biodiversity Action Plan (BAP) priority habitat and a feature of the Cardigan Bay and Pen Llŷn a’r Sarnau Special Areas of Conservation (SAC). The worms act as important ecosystem engineers, creating stable intertidal reef substrate which increases biodiversity by providing habitat for other shore-dwelling animals such as anemones, snails, shore crabs and seaweeds such as sea lettuce.
The worms are very specific in their requirements for forming reefs – as well as needing a hard substrate to attach to, they also need a supply of sand for tube-building so they are found on moderately exposed shores where there is sufficient water movement to bring a sand supply from nearby. Their tubes are made by gluing sand and shell fragments together with mucus.
Reefs are formed when dense aggregations of these worms occur and their tubes cement to each other like cells in a honeycomb, creating a distinctive reef structure which can be up to a metre deep. The formation of reefs is assisted by a clever biological mechanism. Honeycomb worm larvae drift around in seawater and could settle anywhere to grow into adults. However, the presence of existing worm colonies, or their dead remains, strongly stimulates any passing larvae to settle, helping to ensure that the reef continues to grow.
Being at the northern end of their range in Britain, honeycomb worm reefs are vulnerable to storm damage and extreme cold weather, after which they can die back for several years. They are also susceptible to large-scale changes in sediment supply as a result of storms, sea defence and beach replenishment work. Although they can tolerate burial for several weeks, prolonged burial will cause them to die.
Other threats include trampling, pollution, incidental damage from mussel fisheries and bait digging and they may also compete for space with mussels which destabilise the reefs. The good news is that over the past few years, honeycomb worms have made a significant comeback to the North Wales coast at Llandulas, which is an encouraging sign about the health of the environment along that part of the Welsh coast.
So next time you hear or think of the word reef, don’t automatically think of tropical coral reefs in clear blue waters, remember we have our own equally important and fascinating reefs right here in Wales.
Places to see these reefs: Cei Bach, New Quay; Llanddulas, Conwy; Newport Sands, Pembrokeshire. Please be careful not to trample them!