Imagine one day that you are casually diving, just snooping around to see what sort of interesting critters are down there. But then you sense that there is something else down there that you were not expecting at all. And that “something else” is cylindrically shaped and BIG! Really big, over twice as big as you are!

Don’t panic. While what you just encountered may be the shape and size of a great white shark, it doesn’t have a gaping, hungry mouth full of teeth. Instead, it has thousands of hungry mouths, pulsating and ready to gobble up whatever they can. And if you try to push this thing away from you, its slimy body might simply emit an intense, eerie glow.

Should you panic now? No, for all that you’ve encountered is a swimming colony of sea squirts.

Now, sedentary or “regular” sea squirts are often hard enough for people to comprehend, let alone explain to their friends. These animals are classified as “Urochordates” or “tailed chordates” because for a fleeting instance in their early development, they have a “notochord”, similar to what becomes a full-fledged backbone in vertebrates (fishes, birds, people, etc.) But sea squirts bear no easily observable similarity to vertebrates. For the most part, sedentary sea squirts are more often than not non-descript gelatinous blobs attached to rocks, piers or boat bottoms, which spend their days sucking in seawater in one of their siphons and out the other.

Yet there is a whole class of sea squirts that don’t live like blah blobs-on-the-rocks, but as chains of crystals with golden accents, roaming freely throughout the open sea. They are a very important ecological feature of the ocean and at times may form aggregations numbering into the billions. But, alas, they are virtually unknown.

The most varieties of this Class Thaliacea, are the numbers of the Order Salpidae. Often called the salps, these swimming sea squirts function as virtual feeding cylinders as they propel themselves through offshore waters. One species, Cyclosalpa affinis, can eat the equivalent of half its body weight in a single day. And Salpa fusiformis has been known to grow 40 percent of its length in one day. As with any living creature, though, when stuff goes in one end, other stuff needs to come out the other, and the voracious eating of salps means a superabundance of waste matter. When all this discarded material subsequently sinks, the process effectively transfers a vast amount of organic matter from the rich upper layers of the sea to the relatively barren depths.

Shearwater TERN

All swimming sea squirts come in two stages: a solitary stage that reproduces asexually and a colonial or aggregate stage where sexual reproduction takes place. This alternation of generations makes identification of these already hard-to-classify animals even more confusing. But classification is sometimes simplified somewhat because the budding florets of the colonial form tend to dangle behind the barrel-like individuals.

One reason that swimming sea squirts are so poorly known is because they are not intertidal dwellers, and hence have been ignored by many marine guidebooks. However, this oversight is understandable. While quite a number of swimming sea squirts are common and widespread throughout the world, the winds and currents only rarely wash them onshore, and when they do, they can appear as formless blobs of jelly.

For people who get out—and in—the water, though, the story can be quite different. On at least one occasion, Thalia demoncratica congregated into a swarm off of Southern California that covered 3,500 square miles! Even when found in smaller quantity, underwater photographers are often able to capture stunning shots of glistening chains of salps—beads of gold radiating against the rich blue of the open sea.

There are well-documented accounts of colonies of another group of swimming sea squirts, the pyrosomes, obtaining the length of over 12 feet! While not a dangerous encounter, certain ones are startling. And also sparkling. Pyrosomes are the most bioluminescent of the swimming sea squirts and their light can radiate through as much as 300 feet of clear water.

No wonder swimming sea squirts have fascinated and delighted seafarers for thousands of years, and are becoming the delight to many divers today.

California Diving News