Lin Baldock, Dorset Seasearch Coordinator working with Dorset Wildlife Trust, reveals the incredible, wormy forests that exist just below Poole Harbours waves...
Below the placid surface of some of the sheltered channels of Poole Harbour and especially in the landlocked South Deep are dense forests of the peacock worm (Sabella pavonina). These worms build themselves long flexible tubes over 30cm high binding together particles of mud to provide a protective home into which they snap at the slightest disturbance. These long tubes also raise the feeding fans above the mounds of mud which accumulate around the dense tubes allowing the worms to filter food from the seawater pouring past on each tide.
Picture 1 Forest of Peacock Worms festooned with sponges and sea squirts.
These beautiful fans also provide a safe home for tiny parasitic crustaceans (Copepods) which feed on fats and mucus extracted from their host. These minute animals carefully orientate themselves head down along the filaments so that they are not knocked off when their host retracts into the tube.
Picture 2 Little copepod parasites carefully aligned along their host’s filaments. © Dawn Watson
Peacock Worms occur widely around the UK but these dense stands are quite rare appearing in sheltered estuarine situations where there is a good flow of water through the channels. The stands in Poole Harbour are a very good example of this community. These dense stands add vital three dimensional complexity to the habitat providing cover for small fish such as two-spot gobies, small wrasse and juvenile black bream. Large shoals of juvenile bass hunt in the waters above. The flexible worm tubes also provide an attachment point for all sorts of marine life: bright tasselled sponges, small sea cucumbers and particularly sea squirts (tunicates) which take advantage of the tubes to get up above the smothering soft mud which accumulates around the base.
In June and August a group of volunteer divers under the auspices of Seasearch (http://www.seasearch.org.uk/) carried out a survey of the extent of these peacock worm beds in the southern part of Poole Harbour for Natural England.
We were based in the first instance in the Victorian grandeur of the Villa Wildlife Centre on Brownsea Island owned by Dorset Wildlife Trust (http://www.dorsetwildlifetrust.org.uk/brownsea_island_nature_reserve.html) and our pile of equipment ferried over the water on the National Trust’s Brownsea Seahorse (https://www.nationaltrust.org.uk/brownsea-island). Thank you to both these organisations for making our stay possible.
Picture 3 Diver photographing a Mermaid’s Glove sponge.
Altogether 15 volunteer divers, with the help of several boat crews, surveyed 27 sites around Brownsea Island and South Deep totting up over two days of time underwater. Results showed where the best stands of Peacock Worms were situated in South Deep, Blood Alley Lake and in the Wych Channel to the northwest of Brownsea. An added bonus to the survey effort was the discovery of a male Spiny Seahorse in the Wych Channel by two of the divers Cath Quick and Hugh Waite. There have been occasional sightings of seahorses in this location over a number of years.
Picture 4 Spiny seahorse in Poole Harbour. © Hugh Waite
In addition to the Peacock Worm forests a spectacular array of free living sponges was located in South Deep. Here huge sponge colonies, some almost as big as a football, roll around loose on the seabed making a colourful spectacle in the shallow water at the edge of the South Deep channel. The sponges are attached to the slipper limpet a non-native mollusc which has colonised large parts of the harbour and is the only hard surface available in the otherwise sinkingly soft mud. Sponges include the elegant Mermaid’s Glove and the rare Suberites massa which looks like a bright orange brain sitting on the seabed.
Picture 5. Rare orange sponge Suberites massa in South Deep.