A photo of sponge, anthozoa and mytilus mussel. A Bathymetric map of Pembrokeshire.

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Nature still surprises - even the ecologists!

We made a surprising habitat discovery when undertaking survey work in support to the first ever Welsh National Marine Plan.


Last summer, ABPmer made a surprising discovery whilst undertaking surveys on behalf of Welsh Government.

The Sustainable Management of Marine Natural Resources project, funded by the European Maritime and Fisheries Fund,* aims to expand the environmental evidence base with which decisions will be made relating to tidal stream, wave energy and aquaculture development in Welsh waters.

As part of this work, members of ABPmer carried out a series of bathymetric and drop-down video (DDV) surveys in Welsh waters.

Unexpected findings

Firstly, when we created bathymetry maps using the results of multibeam (backscatter) surveys, we found what looked to be shallow rocky outcrops off Pembrokeshire coast.

Following drop-down video surveys to ground truth the multibeam findings, some unexpected results were discovered during DDV analysis by ABPmer and MarineSpace ecologists.

Sabellaria reef discovered at Anglesey

Sabellaria reef discovered at Anglesey

Among the habitats recorded at Pembrokeshire were sponge communities and several potential biogenic reef structures created by Sabellariid worms.

These communities have previously been recorded nearshore to the Pembrokeshire coastline; however, the DDV surveys identified these communities further offshore, in water up to 45m depth. Around this depth several stations recorded a mosaic of Sabellariid worm and blue mussel aggregations.

Sabellariid worm structures were also seen in waters off the west coast of Anglesey. Sabellaria reef has been previously found around the coast of Anglesey; however, our study area has been largely un-surveyed so provides new insight into locations of these features.

Recent surveys elsewhere around Anglesey have recorded the coexistence of S. alveolata and S. spinulosa in these subtidal features, despite S. alveolata often being associated with inshore and intertidal environments, indicating that both species contribute to form these subtidal structures.

Sponge, anemone and Mytilus mussel at Pembrokeshire

Sponge, anemone and Mytilus mussel at Pembrokeshire

What this means

Sabellaria reef tends to occur in shallow waters where tidal/wave disturbance mobilizes sea bed sediment, which Sabellaria spp. require for building their tubes.

The occurrence of Sabellaria structures in deeper waters off Pembrokeshire and Anglesey possibly reflects the high energy nature of these tidal stream environments, with sediment disturbance occurring at considerable depths. The energetic nature of the system also likely explains the occurrence of blue mussels at depths of up to 50m. Water turbulence causes mixing of water column and transports plankton (which would normally be in surface waters) to the sea bed providing adequate food supply for filter feeding mussels.

Sponge and anemones at Pembrokeshire

Sponge and anemones at Pembrokeshire

Biogenic reefs are important habitats that can have a positive impact on their environment, often stabilising sands, gravels and stone, providing hard substrata for attachment of sessile organisms, and being associated with a diverse fauna and flora, often elevated from the surrounding environment.

What next?

The discovery of these reefs is interesting from a conservation standpoint and highlights areas which may be susceptible to anthropogenic pressure, but more research needs to be done to fully understand the extent of the reefs and the role they play in the local ecosystem.

Prepared by Vicky West, Ecological Surveyor


ABPmer regularly provides support to regulators, policymakers and developers across a project’s development, offering the full complement of ecological, environmental, hydrographic, metocean and topographic survey and reporting capability.

* This project has received funding through the European Maritime and Fisheries Fund, which is funded by the European Union and the Welsh Government.