These creatures are common all round the UK
BEST PLACE TO SEE: Common all round Britain.
LIKELY TO APPEAR: Sea firs appear in late winter, grow rapidly in spring, and die back in autumn.
DISTINGUISHING FEATURES: Sea firs are pale, bushy, plant-like animals. The sea beard Nemertesia antennina has unbranched main stems, often orange or yellowish, with fine side branches. Nemertesia ramosa is very similar, but the main stems are branched.
SIZE: Nemertesia antennina up to 25cm tall; Nemertesia ramosa up to 20cm.
Sea firs (hydroids) resemble fine, greyish seaweeds, but their pallid colour is a clue that they are not plants but animals – seaweeds are always red, green or brown. Sea firs are carnivorous, with hundreds of polyps (little mouths surrounded by tentacles) that catch small planktonic animals. The polyps are all joined at the base, so each polyp contributes food to the whole colony. Sea firs grow rapidly in March, ready to feast on the spring plankton bloom.
The sea beard Nemertesia antennina is a big and easily recognized British sea fir (its close relative Nemertesia ramosa is very similar). However, there are many other kinds growing around our coasts, and they make great sheltered homes for other little critters. I have a friend who takes a magnifying glass underwater to reveal the hidden world of strange mini-beasts that live among sea firs. Here graze tiny sea slugs (nudibranchs), many of them small, drab-coloured and hard to spot, but look for their white egg ribbons coiled around the sea fir, and the adult sea slugs will often be nearby. Others have bright warning colours that say ‘Trust me – eating me is definitely not a good idea’. Some sea slugs eat the sea fir’s stinging cells, then re-use them second-hand as part of their own defences!
Many sea slugs are picky eaters and will only eat one kind of sea fir. The little brown Doto pinnatifida, for example, eats only the sea beard, while Doto cuspidata eats only Nemertesia ramosa. One you can’t miss, the big and beautiful Eubranchus tricolor (see photograph, right), eats a variety of sea firs.
Another unwelcome critter from the sea fir’s point of view is the sea spider Pycnogonid – not related to land spiders. This charming creature (one of the few in British waters that gives me the creeps) inserts a syringe-like proboscis into the sea fir and sucks out its living tissues. Sometimes, in summer, you might find an unfortunate sea fir infested with dozens of sea spiders – eech!
Other small crustaceans don’t eat sea firs, but use them as a perch and cover for their own brand of hunting. On current-swept sea beds, weird skeleton or ghost shrimps (caprellids), less than 2cm long, crowd onto sea fir branches. They behave like tiny praying mantises, grabbing passing prey with claws out of all proportion to their anorexic bodies. Chameleon prawns adopt camouflage colours, with see-through sections as a further disguise. Tiny scallops, newly settled from the plankton, attach to sea firs for a while before growing big enough to lead an independent life on the sea bed.
Like tigers in a jungle, the biggest inhabitants of sea firs are often the hardest to see. Spider crabs can make themselves almost invisible by sticking bits of sea fir onto their carapace and legs. In fact, once well covered, the crab can leave the sea fir and sit on the sea bed looking just like another sea fir. Some of these ‘decorator’ crabs even sway gently from side to side, doing an excellent imitation of a sea fir in a swell.
Animals that live fixed to the sea bed need a mobile phase in their life cycle if they are to colonize new areas. Many sea firs do this by budding off tiny jellyfish which drift and feed with the plankton, eventually producing eggs and sperm which combine to form a tiny swimming larva. This settles onto the sea bed to start a new sea fir colony. However, the sea beard has cut out the free-swimming jellyfish stage, producing swimming larvae directly from little pouches on its branches. Perhaps it should be renamed the kangaroo sea fir!