Of all of the species of marine organism that possess stinging cells used for immobilizing prey, sea jellies are likely the most well-known. Drifting placidly with the currents, collecting food along the way, jellies have come to fascinate visitors to aquariums displaying them. But there are many other species in this group of animals we call cnidarians (nye-dare’-ee-ans, the letter c is silent).
We know jellies primarily from their floating medusa stage, but they also go through a stage of life as a polyp that is attached to the bottom. And within the cnidarians, there are many species that spend their entire life as polyps attached to a hard surface and never exist in the floating medusa form. Among these are familiar sea anemones, and a group known as hydroids, which are the feature of today’s post.
Hydroids are colonial organisms in which all of the individuals are connected to a common living tube called a stolon that allows food collected by the individual feeding polyps to be shared among all members of the colony. In addition to the feeding polyps, the colonies will also produce polyps specialized for reproduction.
Some hydroids form feathery branches that look for the entire world like a plant or algae. Others, like today’s subject, form less-branched polyps that look like sea anemones with extremely long bases. Regardless of the specific form, all share a common process of feeding and reproduction, and all have stinging cells in the tentacles of the feeding polyps.
The species seen in today’s photo is called the pink-mouth hydroid (Ectopleura crocea). And as described above, the individual polyps in the photo share a common base that allows the flow of materials from one to the rest of the colony. In this case, that common base, or stolon is anchored to the steel structure of an oil drilling platform located off the coast of southern California. Constantly bathed by sea water flowing by, these hungry filter feeders get a never ending supply of tiny planktonic organisms on which to snack. Seems like an ideal habitat for a hydroid.
Today’s photo was taken with a Canon EF100 mm f/2.8 macro lens on a Canon EOS 5D Mk. III in an Ikelite underwater housing. Lighting was provided by an Ikelite DS161 strobe set to eTTL exposure. The exposure was set to 1/45 sec at f/9.5 and ISO 200.