The subject of today’s photo is the ostrich plume hydroid (Aglaophenia sp.), a member of that group of aquatic organisms that employ stinging cells for predation and protection – sea jellies, anemones, and the like. In addition to the stinging cells, these species all share a similar pattern of reproduction, in that they pass through at least two stages in the cycle. For example, the jellies we may see in the ocean or an aquarium exhibit are called the medusa, a free-swimming stage. The medusa releases a larva that settles to the bottom and grows into the polyp stage, which looks like a sea anemone. For species like sea anemones, the reverse is the case. The polyp phase, the anemone we see, is the main form, which reproduces sexually by releasing a swimming medusa phase that can disperse to new habitats. Now, not every single species within this group reproduces exactly like either of these examples. Nature is, after all, nothing if not diverse and variable. Some species skip one phase, or at least the phase has not yet been observed.
Hydroids essentially follow the pattern in which the polyp phase is the most readily observed. But rather than being solitary like sea anemones, many hydroids are colonial, being comprised of hundreds or thousands of individual polyps living joined together. In this joined condition they share a common gastrovascular cavity in which nutrients are circulated among all of the members of the colony.
While looking every bit like a plant or algae species, the ostrich plume hydroid is a collection of animals. If you look closely at the horizontal branches, especially on the plume on the right side of the frame, you will see a series of small studs lining each branch. These are the homes of each individual polyp. Look closely at the branches on the left side of the right-hand plume and you can actually see tiny tentacles extending from many of the studs along the branch. These polyps are feeding.
This photo was taken at Santa Rosa Island off Saint Augustin Canyon 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 in eTTL exposure mode. The exposure was set to 1/45 sec at f/9.5 and ISO 400.