In the Secret Cities of the Sea exhibition at the Natural History Museum in London, which I discovered while listening to an Inside Science program, there were various displays about the inhabitants of the reefs. One such example was this stunningly preserved sea fan. The sea fan is a type of coral that grows out of the continental shelves at the edge of tropical and subtropical reefs. They build out into large fan-like structures that help them to catch plankton and other tiny organisms that are brought up from the deep waters of the Twilight Zone by strong currents. These same currents are what feed the reef and keep a large supply of nutrients available to the creatures that live there, and the sea fan is simply seating itself closer to the source.
Like other corals, often contain zooxanthellae, tiny algae that are allows to grow inside and around the coral giving them protection in return for photosynthesising for the coral. This symbiotic relationship is essential for keeping reefs alive, since most reefs are fairly near the surface. These algae are also what give the coral its colour, which is why coral bleaching occurs when a reef is sick, since the stress of illness makes the coral jettison its cargo of zooxanthellae. With changes in acidity and temperature in coral reefs across the globe (mainly due to what humans are doing to the ocean) more and more reefs are beginning to bleach and turn white, which will eventually kill them. Some sea fans, that live deeper down in the shelf wall do not have zooxanthellae, since it is no longer beneficial due to the lack of light, and so they must rely entirely on capturing enough plankton to eat.
Many species live in the winding mazes that sea fans create, jutting out of the shelf walls, including starfish, kinds of pipefish, and colonies of bryozoan, which are actually several zooids living together. Bargibant’s Pygmy Seahorse and Denise’s Pygmy Seahorse are two species of the tiny pygmy seahorse that live especially on sea fans and their relatives. They are perfectly camouflaged to the coral, coloured and with bumps and knobs in lighter colours over them. Large groups of these seahorses can be found over a sea fan, where they eat the plankton that filter through from the deeps. The Bargibant’s Seahorse will likely never leave the coral that they were born on, leading to large families of seahorses living in the same place for generations.
These sea fans are extremely interesting, now being looked into by the medical industry for the chemicals they produce, and I am glad that I saw them here at the museum.