A few nights ago, my daughter and I walked the beach at sunset. She was heading back to college the next day, so we soaked in one last summer beach memory. As we walked, we chatted about the detritus in the wrack line—algae-covered driftwood, interesting shells, big pieces of sand dollars. At one point, I looked down and exclaimed, “Porpita porpita!”
It’s an occupational hazard for a lot of field biologists that sometimes the scientific name for an organism comes to mind before the common one. We laughed as I remembered (several minutes later) that it’s also called a blue button—a tiny “jellyfish.” Blue buttons are not true jellyfish, but like Portuguese Man ‘O Wars, they are a collection of individual organisms—hydrozoan polyps—living together in an interdependent community. Groups of polyps serve different roles in making the entire organism function, including feeding, reproduction, and fending off predators.
Physically, they possess a distinctive beauty. Like their “button” namesake, they are nearly flat, small disks about the size of a quarter (1” radius). Up close, the center, or “float” looks like the cross-section of a tree, with rings and rays radiating out from the deep blue center. Also called a pneumatophore, it is filled with air and serves to keep the whole miniature floating commune at the water’s surface.
Fringing the circumference of the disk are dozens of short green tentacles composed of the “defense” dactylozooid polyp colony. They have stinging nematocysts on the tips, which aren’t terribly painful but can irritate the skin. Underneath the float are reproductive hydroids (gonozooids), which are white stringy structures that are oddly reminiscent of piles of teeth or lump crabmeat in appearance.
Living as free-floating plankton, blue buttons are vulnerable to the whims of currents and waves. They feed on smaller plankton and larvae, but are fed on by sea turtles, bubble-raft-forming purple storm snails, and wild-looking sea slugs called blue dragons. The tiny blue dragons take in venom from their prey, then concentrate it so intensely that they can paralyze prey 300 times their size.
Many observers have noticed that a the blue button provides a protective shelter for juveniles of an Indo-Pacific species of fish called the Malabar jack. The young kingfish is immune to the venom, and benefits from being physically hidden beneath the blue button’s float and its ability to sting would-be predators. An observation and experiments on this phenomenon in the 1960’s found that the fish are highly attached to their particular blue button protector. When the sheltering blue button was removed, the fish exhibited severe stress, then quickly returned to its chosen bodyguard when put back into in a tank with multiple jellyfish.
Blue buttons are “simultaneous hermaphrodites,” meaning they possess both male and female reproductive organs that function at the same time, and can therefore reproduce completely on their own.
With no “sail” structure like Portuguese man ‘o wars or by-the-wind-sailors, blue buttons are less likely to wash up on shore. Therefore, these tiny little invertebrates are somewhat rare on our beaches, often arriving during stormy weather or late summer “blooms” with other jellyfish species. This was the only individual we saw on the entire stretch of beach. Generally, blue buttons spend their days floating on open seas, particularly in tropical areas where they grow in huge numbers. They may travel the Gulf Stream as far north as the North Carolina coast, or to central California from storms moving through their home territories in the Pacific or Indian Oceans.
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