Seamats and Reefs

Tufts of ‘seaweed’ on rocks on the seashore and beneath the waves are often not what they seem and are in fact animals.

The Booth Museum's collection of Seamats and Reefs
The Booth Museum’s collection of Seamats and Reefs

They can form such a dense covering that the growth is known as ‘animal turf’. Included in the inhabitants of this turf are creatures known as sea mats, horn wracks and lace corals. These colonial creatures belong to a group of animals called Bryozoa, also known as Ectoprocta or commonly as Moss Animals.

The Booth Museum of Natural History has a substantial collection of these creatures, many of which were found on or just below the Brighton shoreline. Such specimens provide important information about the past distribution of animals. An example of one is Chartella papyracea (Ellis & Solander, 1786) [previously known as Flustra papyracea].

This species can be found around the British Isles and ranges from the southern North Sea through the English Channel, the south and west coasts of the UK, the Irish Sea to the northern coast of Spain, and the north and west coast of France.

It forms colonies on the shallow subtidal hard substrates, but may also colonise overhangs on rocky shores, near the low water mark. Fresh colonies are brown or light grey and grow to up to 10 cm in height and may live several years.

BV400020
BV400020

The specimen to the right (BV400020) was collected off Brighton about 80 years ago by C.T.A. Gaster of Hove who largely collected geology. More recently specimens have been collected as part of survey work along the Sussex coast. The data from this work has been fed into the Balanced Seas Project , one of the projects around the United Kingdom involved with setting up Marine Conservation Zones to protect our marine environment.

Fresh specimen of Chartella papyracea; now in the Booth collection
Fresh specimen of Chartella papyracea; now in the Booth collection

One place that this species can form an extensive cover together with other seamats such as Flustra foliacea and the seafir Hydrallamia falcata is on the Marina Reef, also known as Measor’s Rocks. This reef is to be found about one km SSW of the entrance to Brighton Marina at a depth of 9-13 m below chart datum. It is about 50 m long, forming a gradual curve in plan view. Its form varies considerably along its length, the width of the raised bedrock forming the reef ranging from about 15-50 m.

Close-up of individual polyps each tucked within its little protective box. The tentacles that are extended when the animal feeds can just be seen
Close-up of individual polyps each tucked within its little protective box. The tentacles that are extended when the animal feeds can just be seen

It rises to a maximum of 3 m above the surrounding seabed. Essentially the reef consists of an angled narrow stratum of chalk (only about 30 cm thick) with a softer grey layer beneath it. The clay is being eroded at a faster rate than the chalk, and the lower part of the reef consists of chalk slabs and boulders which have broken off following erosion of the clay base. In places the chalk bedrock has been dissected into blocks, with numerous small gullies in between, which in places form steps and ledges. It resembles a slightly over-cooked bulging sponge with cracks.

Marina Reef is one of Sussex’s Marine Sites of Conservation Interest, a none-statutory designation but one that highlights the fact that the area is significant. This designation was justified on the grounds that sublittoral chalk exposures are unusual within the British Isles, with most being found in the South East. Marina Reef is a good example of a sublittoral chalk reef.

Edge of the reef with a Goldsinny feeding amongst Chartella papyracea
Edge of the reef with a Goldsinny feeding amongst Chartella papyracea

Whilst not being particularly spectacular in its form or in the marine communities it possesses, the reef nonetheless forms an oasis of life surrounded by vast expanses of relatively barren sand.

Tompot Blenny
Tompot Blenny

Side of Marina Reef with the hydroid Hydrallmania falcata (top) and seemat Chartella papyracea lower down

The sponges Polymastia mamillaris and Ciocalypta penicillus, not often recorded elsewhere in Sussex waters, have also been noted from here. Piddocks, Pholas dactylus, holes are present where both the chalk and the clay are exposed. Fish associated with the reef include Bib Trisopterus luscus, Pollack Polachius polachius, Leopard-spotted Goby Thorogobius ephippiatus, Goldsinny Ctenolabrus rupestris and Tompot Blennies Parablennius gattorugine.

Leopard-spotted Goby at the base of the reef

pebbles. Other regularly seen marine life in this habitat includes hermit crabs and occasional flatfish including Plaice Pleuronectes platessa, and Sole Solea solea.

Gerald Legg, Keeper of Natural Sciences