Dive Dry with Dr. Bill

#670: The "Blue Button"

Just before I took off for the Egyptian Red Sea last August, I stopped by to chat with the SCICo boat crew to find out what they were seeing in Lover's Cove. They're out on the ocean every day and often have interesting sightings that I find intriguing. On that day they described a critter that looked like the by-the-wind sailors (Velella velella) we were seeing months back, but they didn't have a "sail" on top. After asking a few questions, I was able to offer the answer... the crew was seeing Porpita porpita, a relative of Velella. For those of you who prefer a common name, it is the blue button.

Porpita is actually a colony of hydroids rather than a single individual. Both the blue button and the by-the-wind sailor are members of the same phylum that coral, sea anemones and jellyfish belong to... the ones with stinging cells (nematocysts) known as the Cnidaria. Some of you "old timers" like myself remember them being called Coelenterata, but the times they dun changed. Some refer to Porpita as the "blue button jellyfish," but of course it isn't a jellyfish and jellyfish aren't fish, either!. Don't tell the PC crowd or they'll try changing the name of sea horses to something ridiculous. Fortunately the blue button's sting is not powerful, but may cause irritation for some humans.

Velella and Porpita are both members of the subgroup chondrophores which are similar to the siphonophores such as the Portuguese man-of-war and our local species in the genera Apolemia and Praya. Previously, scientists grouped the chondrophores and siphonophores together. Taxonomists, scientists who specialize in naming species, are always fiddling with the critter IDs. Being an old geezer, I have trouble keeping up with these changes. As my UCSB colleague Dr. Adrian Wenner told me in grad school, when it comes to the human brain, "first in, last out."

Porpita lives at the surface of the ocean, drifting with the currents in tropical and subtropical seas. It lacks the sail found on Velella which gives them their common name of by-the-wind sailor. There are two main structures to the blue button. The float or button is a round, nearly flat golden-brown chitin-like structure up to 1-1.5" in diameter that differs from the oval-shaped float of Velella. It contains gas which keeps the colony at the water's surface. The colorful "tentacles" extending from the float is the colony itself. It is often a beautiful turquoise or blue but apparently can also be yellowish. Each "tentacle" ends with clusters of the stinging cells.

Although the blue button is a colony of hydroids, it has a single mouth. Most hydroids are polyps with tentacles and an individual mouth. This species feeds mainly on crustaceans, including planktonic copepods and larvae. Because there is just this single opening, it also serves to dispel wastes following digestion. I prefer two ends to my digestive tract. In my research I found some sources state there are feeding polyps or zooids while others defend the colony and some reproduce. In turn, the blue button is fed upon by planktonic sea snails such as Glaucus and Janthina.

Blue buttons are hermaphrodites and have both male and female structures in their reproductive polyps. Not sure that would be much fun. Eggs and sperm are released into the surrounding water. Upon fertilization, larvae develop and eventually become individual polyps. These polyps then divide and become specialized. Not too terribly exciting if you look forward to my columns as the "pornography of the sea."

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The blue button Porpita porpita with close-ups taken under my microscope.

This document maintained by Dr. Bill Bushing.
Material and images © 2015 Star Thrower Educational Multimedia