Nick Owen celebrates an underappreciated marine habitat, revealing a delicate world of sponges and invertebrates
COMPETITION
Axinella dissimis sponge community
The best bit
Nick Owen celebrates an underappreciated marine habitat, revealing a delicate world of sponges and invertebrates
Briefing: The Isles of Scilly: white sandy beaches, wrecks and walls, plus stunning swathes of jewel anemones. Sunlit seagrass beds with stalked jellies like tiny jewelled bells, sunset cup corals and pink sea fans. The latter may be adorned with the sea fan anemone Amphiathus dohrnii, the sea fan nudibranch Duvaucelia odhneri and the under-recorded slender peacock worm Sabella discifera, with its small grey tubes and lovely blue and yellow tentacle fans. Hour-long dives starting deep and ending shallow; then good food in a small-island setting. All good reasons to go, but hardly anyone sees the very best bit.
Sponges can thrive in a silty habitat
The Dive: Down past 25m there are beautiful‘ sponge ledge’ communities that are stunning but completely different to the‘ usual’ Scilly diving. Start your dive at the deepest point – don’ t get distracted, you’ ve not got long! You’ re diving on air unless you’ ve made special arrangements for nitrox. Watch your depth, in the usual clear Scilly viz it’ s easy to reduce your‘ no deco’ time to nothing.
Phakellia ventilabrum cup sponge
The ledges you’ re looking for are scattered along the faces of huge granite walls which make for grand scenery. 30m is a good target depth if you’ re qualified and dived-up. You can often hover by your chosen ledge and look down into inaccessible depths. Down here in current shadow, horizontals and gentle slopes are covered in sponges. Some are large and occur in a range of shapes, colours and textures, alongside pink sea fingers and pink( and white) sea fans. You will need a good torch to really enjoy the colours.
The sponges support a range of different species that feed on them or are in symbiosis – such as the yellow trumpet anemone Parazoanthus axinellae, which grows on the sponge Axinella damicornis. Look for these on little less-silty verticals. Your buoyancy control will need to be good, as the silt is very fine and easily stirred up. But the silt is the reason these communities are here: these are silt veneer communities.
The silt blankets the hard granite rock. During the very worst winter storms, it gets stirred up and the silt‘ clears’ the rock so species can settle. The silt smothers many species, leaving those species that can cope( such as the sponges and sea fans) in assemblages that are unique to these ledges. During a gradual ascent up the granite reef and staying in no-deco diving, you can watch the community change as you‘ lose’ the silt.
Seaweeds and jewel anemones come back in as you ascend.
Why it’ s special: These sponges may be many decades old. They’ re beautiful and very fragile but it’ s totally impractical to trawl or dredge here. Potting in the Scillies needs a lot of skill and with dropoffs and rocky‘ traps’ it’ s easy to lose a string. These ledges are an experience well worth the time and trouble. If you can write a Seasearch form of your visit, so much the better: most surveying in the Scillies misses these communities. Veneer communities are also underrecorded and contain‘ rare’ specialist species. For more details about veneers start here: https:// tinyurl. com / siltcommunity
42