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A salty pretzel but hold the mustard

Weirdly, on this diving day, as I briefly hovered above the silty seafloor at about 60 feet deep, floating in the water before me was a tangled, gelatinous strand. Could it be a tentacle somehow lost by a jelly animal? I captured some images with hopes of figuring it out in front of my laptop. 

It doesn’t look like there’s much of a connection to us but each individual cell that makes up this colonial radiolarian, Solenosphaera collina, has the same complex organelles as those found in our own cells. c. 2019

Back on land, someone from my cnidarian Internet group comes to my aid, shedding light on the opaque, gooey-looking noodle. It turns out I’ve eyed a really rad radiolarian, a protozoan that resides in the kingdom Protista. Protists are living beings that don’t fully fit the definition of animal, plant, fungus, or bacterium. The radiolarian in question is the colonial polycystine radiolarian Solenosphaera collina,  a mouthful to pronounce. For readability, let’s call it as I see it: the pretzel radiolarian.  

There isn’t a flood of information about this specific genus and species of radiolarian but know that although it is jellylike on the outside, it has a hard, internal skeleton made of silica. Individually, you’d need a microscope to suss out each single-celled S. collina but because it is a colonial critter (many individuals living together as one), the pretzel was easily visible at about 2 inches across. 

All radiolaria are devoutly open-ocean planktonic species so it’s no wonder I’ve never before seen one in my decades of diving, which is not to say others may have enjoyed a rare nearshore sighting in La Jolla’s submarine canyon. Some radiolarian species occur to the deepest depths but the pretzel claims the shallower layer where it is found in the nutrient-rich, sunlit waters of the California Current. In general, radiolaria are greatest in number and diversity in warm waters near the equator; after then, they decline with increasing latitude. 

Mentioning “sunlit” waters is no afterthought as biologists have found dinoflagellates, tiny photosynthesizing plankton, taking up residence in the pretzel. It is a good situation for both in that the dinoflagellate guest gains an ideal living environment within the pretzel host’s cell. Here, the photosynthesizer makes food for itself from the most basic of ingredients (carbon dioxide, trace nutrients, and sunlight), which the pretzel cannot do. Fortunately, the symbiotic dinoflagellate churns out more simple carbohydrates than it needs, and these are absorbed by the pretzel host for sustenance. The pretzel may also use other means to capture food for itself, possibly by having tinier plankton prey like copepods bungle into the pretzel’s sticky outer layer. 

As passive voyagers lacking an independent means of locomotion, the pretzel’s reproductive success is unreliable. To date, this aspect of a polycystine radiolarian’s life remains an enigma because no one has yet to get them to reproduce in the lab or even nail down the specifics of their reproduction strategies. Of course, first you have to keep them alive in culture, so there’s that.

That radiolarian species like the pretzel have a solid silica skeleton explains why these antediluvian protists make up a huge presence in the fossil record. In fact, radiolarians are second only to diatoms (think diatomaceous earth) as a chief source of silicate found in ocean sediments. Sediment cores containing these so-called microfossils aid scientists in understanding early climate conditions. How early? Since radiolarian skeletons have been present on Earth for 600 million years, they have myriad stories to tell. As for my own simple quest, I hope it won’t be such a wait for my next pretzel sighting. 

— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to