The Isles of Scilly Biodiversity Audit 2008 - Cornwall Wildlife Trust

2.2.1.25 Tunicates – Larvaceans,thaliaceans and ascidiansOverviewTunicata (or Urochordata) is a subphylum of thephylum Chordata which means that these species havea dorsal nerve cord (a notochord), like the vertebrates.Most members of the Tunicata only show thischaracteristic in their larval stages and it disappears inadult stages. There are 60 species of tunicate in Britainand approximately 50 have been recorded aroundCornwall and the Isles of Scilly (Gainey, 1997).Seasquirts are the most noticeable members of theTunicata and are often seen on rocky substrate orgrowing on algae. Seasquirts may be solitary orcolonial and are sessile benthic species which attach tohard surfaces and filter feed by passing water throughtheir siphons.Both the Larvacea and the Thaliacea are freeswimming planktonic members of the Tunicata. Rowe(1972) studied these groups in the Isles of Scilly andfound one species of Larvacean Oikopleura dioica to bevery common in the plankton at night. The ThaliaceanDoliolum nationalis was also present in the nightplankton and was the only member of the Thaliaceapresent in Scilly.Important AreasAdult seasquirts colonise rocky areas therefore thesheltered intertidal boulders and subtidal rocky reefsare likely to be important for this group. Lightbulbseasquirts Clavelina lepadiformis and star ascidiansBotryllus schlosseri are attractive species which arepopular with divers and have been recorded onseveral reefs around the Isles of Scilly. They also growon seaweeds and their holdfasts.Conservation ImportanceThe orange lights seasquirt Pycnoclavella aurilucenshas been recorded in the Isles of Scilly and is anationally scarce species. This species has a southwesterndistribution in Britain and is a southernspecies thought to extend to the Mediterranean. Twoother nationally scarce sea squirts occurring here arePhallusia mammillata and Molgula oculata (Barne et al,1996).Trends and IssuesResearchRowe (1972) published a list of the Enteropneusta(acorn worms), Ascididiacea (seasquirts), Thaliacea(salps), Larvacea (larvaceans) and Cephalochordata ofthe Isles of Scilly in the Journal of Natural History.Lightbulb seasquirts Clavelina lepadiformis. Photo: Malcolm NimmoIsles of Scilly biodiversity audit Page 38