Freshwater sponges: our native species and their inhabitants

Sponges, despite being arguably our largest freshwater invertebrates (albeit colonial), are strangely overlooked in Britain. They sometimes appear as mere lichen-like cushions, but they are capable of forming massive growths festooned with green finger-width tentacles, like an art-installation of gloved hands protruding from a river or canal waterway wall – unforgettable when seen, but rarely identified or recorded.