Studying the Invisible Blob

Scientists can be an amazing lot. Sometimes it seems to me they thrive on difficulty. Take, for example, the work I saw in a poster at the recent Ocean Sciences Conference, which is a joint meeting of the American Geophysical Union and the American Society of Limnology and Oceanography. Three scientists set themselves the problem of figuring out how an invisible animal feeds.

Properly speaking, the tiny marine animal in question isn't invisible. It's just transparent--so transparent that it makes a typical jellyfish seem as obvious as a gob of peanut butter.

The subject of the study conducted by C.C. Morris, D. Deibel, and P.F. Flood is the larvacean tunicate Oikopleura. These tunicates are only a few millimeters long; a big one may reach ten millimeters (still less than half an inch). Most of that largely gelatinous speck is surrounded by a filmy veil of mucous that the animal excretes---the so-called house--which struck some early observer as being like a tunic and so gave the group its name. In effect, the scientists were studying a very small transparent blob surrounded by a larger smear of nearly invisible goo. That would not be my idea of fun.

Yet these difficult creatures are worth study. In high-latitude waters, such as off Alaska, they seem to be important herbivores of the smaller phytoplankton--the tiny photosynthetic cells that thrive on the lower-nutrient waters available after the spring bloom of larger organisms. In turn, the tunicates are eaten by larger animals, including salmon. And, in seasonal spurts, there are lots of them. Figuring out how they eat is a necessary first step in quantifying the food consumption of these oceanic ghosts, and that information matters in the overall understanding of how the sea's food chains work--right up to salmon steaks in the supermarket.

So Morris and colleagues rose to the challenge of finding ways to see the apparently unseeable. First they tried dripping a bit of carmine dye into the tunicates' aquarium, thinking that the animals would ingest particles of dye that then might adhere to their tissues and outline hidden structures. No luck; they reported laconically that the tunicates found the dye noxious and would not take it in.

Next the researchers tried India ink. Worse luck; they found that India ink coagulates in seawater. Tunicates do not chew up large blobs of coagulated ink.

Finally they tried a product that larvacean tunicates might encounter in nature: cuttlefish ink. Bingo! The animals were not only willing to ingest the ink particles, they seemed to accept the ink as a kind of tasty soup. The scientists quickly obtained videotapes of the tunicates' food-concentrating filters in action. Better yet, they found that particles small enough to escape the filters stuck to the animals, outlining folds and features previously unknown. (The greatly enlarged photos they displayed of the inked tunicates looked very artistic. They showed curved and ghostly forms, a little like strangely symmetrical catcher's mitts made out of creased and wrinkled veiling.)

So far, so good. Now the team wanted more information on the real feeding behavior of their subjects. That meant providing the tunicates with something that was highly visible to cameras and human eyes, but of a size and shape that mimicked natural phytoplankton food items.

Their paper did not report which of them came up with the idea, or how many tries they made before they found the right thing to fool the tunicates but satisfy the scientists. (It's hard to believe that the idea came at once, though my marine scientist friends tell me it was easy.) They mixed what they called "fluorescent latex microspheres" into the water. That's right--little, bright-colored rubber balls...sort of like feeding tennis balls to a glass elephant.

Improbable or not, it worked. The captive Oikopleura accepted the artificial phytoplankton, and the scientists got their data. Morris, Deibel, and Flood were able to report that the crucial machinery for a larvacean tunicate's feeding lies in its tail: the animal moves its tail so as to make it act like a kind of sump pump. The water thus circulated helps keep its house structured and the pumping action pulls particle-laden water through the animal's filters. The research team has videotapes of the bouncing balls to prove it.