The Isles of Scilly Biodiversity Audit 2008 - Cornwall Wildlife Trust

2.2.1.7 Porifera - spongesOverviewSponges are the least complex of all multicellularanimals; these simple colonial animals vary in formand colour. They are sessile suspension feeders withspecialised cells called choanocytes which drive waterthrough their canals and chambers. They filter largevolumes of water, removing oxygen, fine particles andplankton.Species identification can be difficult and is often onlypossible by microscopic examination of spicules (smallpieces of silica or calcium carbonate which reinforcethe sponges’ structure). Sponges can be found fromthe intertidal zone to deep water habitats. They growon a range of substrates including rocks, shells andseaweeds. Their form may be erect, encrusting orirregular in shape but they are characterised by thepresence of many small water intake holes and fewlarger water outlet holes.Important AreasMany of the rocky reefs in Scilly support rich spongecommunities. Sponges also inhabit the underside ofboulders on the less exposed rocky shores. The circalittoralsponge communities on the sheltered coast ofSt Mary’s, for example are of very high interest andextremely diverse (Fowler and Pilley, 1992)Conservation ImportanceScilly is particularly rich in sponge life (Parslow, 2007),especially on the circa-littoral bedrock (40-65m depth)where there are very high densities of infrequentlyrecorded erect sponges (Munro and Nunney, 1998).this is also a southern species and is nationally scarce.Tethyspira spinosa, a nationally scarce species typicalof exposed sites has been recorded as present in Scillyand, along with Axinella damicornis, has been identifiedin the Natural Area Profile (Natural England) as apriority species for conservation. The nationally scarcespecies Homaxinella subdola and Crella rosea are presentin the Isles of Scilly (Ulster Museum, 2007).Trends and IssuesResearchSponges are recorded during annual Seasearchexpeditions to the Isles of Scilly, however due to thedifficulties of identification in the field not all spongesare recorded at species level.Specimens of many sponges from Scilly have beenadded to the Ulster Museum’s sponge collectionrecently, following survey work by Bernard Picton.DiseaseAlthough there are no reports of sponge disease in theIsles of Scilly, such reports have increased dramaticallyin recent years with sponge populations decimatedthroughout the Mediterranean and Caribbean(Webster, 2007).FishingSponges can be easily damaged by mechanical action,particularly branching or erect forms. Scallop dredgingand trawling can rapidly destroy sponge colonies.PollutionOil spills could be very deleterious to spongecommunities, particularly in intertidal areas.Although none of the sponges in Scilly are BAPspecies the area is rich in sponge life are classifiedunder the BAP habitats ‘fragile sponge and anthozoancommunities on subtidal rock habitats’ and ‘intertidalboulders’. There are several nationally rare and scarcespecies present but it is the diversity and richnessof the sponge communities which is of greatestconservation importance in the Isles of Scilly.Munro and Nunney (1998) found Axinellainfundibuliformis a nationally rare species living incircalittoral areas on western shores in Scilly. In 2005Seasearch surveyors found the nationally scarcesponge Axinella damicornis at four sites, includingone site where it was common. It typically lives onrock faces with moderate wave action below 20mand although it is classified as nationally scarce it isactually quite a common species (Picton, pers. comm.).The nationally rare sponge Desmacidon fruticosuminhabits the same type of surfaces and prefers oceanicwater below depths of about 25m.Seasearch surveys during 2007 found Adreusfascicularis at Peter’s Rock and North Chapel Rock;A yellow branching sponge. Photo: Malcolm NimmoIsles of Scilly biodiversity audit Page 19