May 31, 2019

Blue Hydrozoans

Posted by: Cohen Carpenter

Blue Hydrozoans

We’ve had some unusual organisms washing ashore on Little St. Simons Island lately due to several days of heavy winds pushing in from the east. Among them is Sargassum, a brown algae “seaweed” that forms large, free-floating mats in Sargasso Sea. These mats are ecosystems adrift at sea, supporting a variety of life, some of which have evolved to exist exclusively in this floating world. Many of the sea turtles that nest on Little St. Simons likely spend some part of their young lives in the Sargasso Sea, where food is abundant and protection from predators is easier to come by than in the open ocean.

Sargassum can be expected at least a couple of times per year on the beaches of Little St. Simons Island, but some of the other recent visitors occur less frequently. I’m going to focus on two of these in this post.

The Portuguese Man O’ War (Physalia physalis) shows up occasionally as its gas-filled “sail” allows strong winds to carry it great distances. Named because its sail-like gas bladder resembles a small warship at full sail, the same feature renders it similar to a plastic bottle to the beach-goer’s eye. This is one bottle you wouldn’t want to pick up though, as they have long tentacles, each containing many nematocysts or “stinging cells”. The venom is highly painful, but rarely deadly. As cool as they are, we’re lucky they aren’t a resident of Georgia beaches. These colonial organisms prefer to live a little further out in coastal regions and on the open ocean. Did you catch that… colonial organisms? That’s right, they form colonies. But not like your thinking. Each “individual” Man O’ War is comprised of many, much smaller organisms called “zooids” or “polyps”. These tiny organisms can’t exist alone and must form a colony with other zooids. This is the same phenomenon that makes coral reefs. In the case of the Man O’ War, one polyp forms the sail, others function together as the stinging tentacles for prey capture and protection, another grouping forms the feeding tentacles, and others exist as reproductive polyps. They are all bound together by a gooey tissue they create. They feed primarily on small fish, but also take advantage of any smaller animals like shrimp and other crustaceans floating in the water column. They can cover a lot of vertical distance in their fishing efforts, as their tentacles extend down an average of 30 ft. but can reach up to 165 ft.!

So they are jelly fish, right? Wrong, they are “Hydrozoans,” whereas true jellyfish are in the class “Scyphozoa”. True jellyfishes always exist as a single organism, and as noted, this is not always the case in Hydrozoans.

The Man O’ War is not the only Hydrozoan that has washed up recently, though. Even more rare is the Blue Button (Porpita porpita). The last reports of this on Little St. Simons were roughly 15 years ago! The Blue Button has a very similar anatomy to the Man O’ War, with the “button” being a single polyp that keeps it afloat, with a colony of other polyps below for feeding and reproduction. They are much smaller though (only about 2 inches in diameter). They feed primarily on zooplankton like larval shrimp and crabs. Blue Buttons do have nematocysts like the Man O’ War, but these are much less toxic and in my experience, completely harmless.

To leave you with a little more confusion about this curious group of animals, some biologists consider colonial hydrozoans like the Man O’ War and Blue Button to be individuals, in which the polyp groupings exist as “organs” for the individual. This line of reasoning has potential implications for our understanding of the evolution of complex organisms from simpler ones.

*Above: Blue Button (Porpita porpita); photo by Cohen Carpenter

*Below: Portuguese Man O' War (Physalia physalis); photos by Robby Brannum



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