Jaws--With Filters
"The thing about field work," said my friend Beth Bergeron recently, "is that you rack up experiences stay-at-home types wouldn't believe." She was back on solid ground after weeks aboard the university's research vessel Alpha Helix, where her technical expertise keeps numbers flowing accurately from an array of sensitive electronic equipment.
The ship had passed the Barren Islands on its way home to Seward when, according to Bergeron, "the bridge called down and said there were fins off to starboard. Big fins. Way off. Not killer whales."
With that kind of bait, both the scientific and ship crews quickly agreed that a little change in course was appropriate. The vessel's decks grew crowded with people all humming the theme from the movie "Jaws." Those were big fins indeed, and they were unmistakably the proper shape for shark fins--the arch of a dorsal, the curve of a tail lobe.
The Alpha Helix had found a shark, all right, but not one deserving sinister theme music, even though it was about the biggest fish anyone aboard had seen alive in the ocean. The observers got a good estimate of its size because the helmsman could virtually lay the vessel alongside the animal. It was about eight meters--better than 25 feet--in length.
Actually, that's not an unusual size for an adult basking shark, a fish with about the same wariness and wit as a typical floating log. It's also just about as dangerous. Basking sharks eat plankton, not people. And they eat nearly continuously, even when watched by a shipload of gawkers. It takes a lot of plankton to feed a 25-foot fish.
Fortunately for people like me, who wonder about such things, someone has figured out how much plankton. According to the March 1990 issue of Scientific American, a big (10 meter) basking shark filters through 1,850 cubic meters of water every hour. From that steady effort, it will glean 540 liters of zooplankton a day; that's millions of tiny animals. (Think of it as processing the contents of 242 ten-yard dump trucks full of seawater every hour, and eating the contents of 567 quart jars packed with miniscule critters during 24 hours.)
In the sea, many giant animals live by this tedious method, technically known as ram feeding. The largest animals of all, blue whales, are intermittent ram feeders. So are all the other baleen whales. That means they open their huge mouths and surge forward, engulfing tons of water as their pleated throats expand. They force the trapped water back out through the baleen fringes, which snare edible particles in the process. Whales don't make this mighty effort unless the water holds plenty to eat.
Basking sharks are continuous ram feeders. Oversized jaws agape, they swim for hours through thin and thick swarms of zooplankton. Because their oceanic prey sometimes is found near the surface during daylight hours, these sharks looked to sailors as if they were basking in the sunshine. That behavior gave them their name, and also explains why the unusual visitor was easily spotted from the bridge of the Alpha Helix. Its mouth was in an animal soup while its fins jutted above the surface.
Unlike whales, ram-feeding fish have no baleen. Instead, they trap their food with gill rakers, comb-like structures that sieve edible particles from the water stream before it passes into the respiratory portion of the gills. The experts believe the main process is inertial impaction: particles slightly denser than the water strike the feeding structure and stick there while the water flow passes around the structure. That would prevent the gill rakers from clogging and blocking the water flow, an important consideration for an animal with feeding and breathing processes so intimately connected.
Ram feeding sounds inefficient, but that it works very well is proved by the continuing presence of some sea-going giants--including one basking shark that strayed near Gulf of Alaska shores just long enough to be glimpsed by an appreciative audience. That's the kind of bonus for scientific field work that the IRS can't tax.
