THE MOST COMMON REACTION for divers on seeing jellyfish under water is to avoid them. Some species have tentacles with a painful sting, and some are almost impossible to avoid, being tiny and transparent, but if you swim into them they will leave you with an annoying itch as if you have been attacked by a swarm of mosquitoes.
An exception is the mosaic jellyfish (Thysanostoma thysanura), which is very large and beautiful.
These jellies are normally found in open ocean, most of the time close to the surface. We encounter them often when the currents carry them over or past the reefs on which we dive.
I worked during the diving season (October to May) as the pro photographer for Similan Diving Safaris in Thailand, diving almost every day in the Similan and Surin National Parks and Richelieu Rock.
This gave me a great opportunity to see these beautiful jellyfish through a large part of their life-cycle.
While alive they are often accompanied by small fish – often juvenile trevallies. They hang out with the jellyfish because it provides them with protection – if predator fish such as mackerel come along, they can hide under the jellyfish’s skirt, with the tentacles protecting them from their predators.
Eventually the jellies die, and that’s when the real action starts, as it is a feast time for a variety of creatures.
Rabbitfish are the main species to feed on them, always feeding from underneath. Butterflyfish and bannerfish also take advantage of this food source, though they often have a hard time competing with the rabbitfish.
On one occasion we were lucky enough to see a big green turtle eating a very big jelly. It bit off chunks of the dying jellyfish and swallowed them, and it took almost an hour for it to devour the jelly until there was nothing left but the top of the “bell”, which it left behind.
The turtle was eating it just below the surface, just a metre or two under water. The jelly would keep sinking, so the turtle would pull it back up towards the surface so that whenever the turtle went up for air it could get back to its meal quickly before other fish got hold of it. My series of photos follow the action.