Jellyfish Gone Wild — Text-only | Flash Special Report
How They Swim
JELLYFISH: SWIMMING TO EAT
GOING NOWHERE FAST
Jellyfish are captives of the currents, travelling wherever they are carried--sometimes over thousands of miles.
While carried by currents, jellyfish continue to swim and pulsate their bells. But most jellyfish can only move up and down in the water column without making significant horizontal headway against currents. Therefore, the locations of jellyfish swarms are more determined by currents than by jellyfish themselves, which are unable to chart their own courses.
Why are jellyfish such weak swimmers? Because, according to Jack Costello of Providence College, jellyfish muscles are only one-cell thick--hardly the hulk needed to generate Olympic swimming records. In addition, the disk shape of jellyfish bells is probably “one of the least effective shapes for forward progress that we can imagine," says Costello.
DIFFERENT STROKES FOR DIFFERENT GELATINOUS FOLKS
There are two major types of jellyfish strokes:
Small Jellyfish -- The Jet-Set: Most species of small jellyfish (usually too small to be noticed by people) are thimble-shaped with narrow bells. They swim by jetting motions that involve expanding their bells to collect water and then contracting their bells to expel it. The force created by the expelled water propels the jellyfish forward in the opposite direction (modest though this distance may be).
Why do jellyfish jet about? Possibly to escape predators or to reach better feeding areas. Between jetting spurts, small jellyfish remain passively suspended in the water, waiting for prey to touch their killing tentacles, similar to the way that spiders passively wait for prey to stumble into their webs.
Large Jellyfish -- Power Rowers: Most species of large jellyfish have relatively wide, flattened, bells and cannot jet about like small jellyfish. Why not? Because large jellyfish--limited by the one-celled thickness of their muscles--would be unable to muster the strength needed to squeeze out the large quantities of water that would be collected by whole-bell contractions.
So instead of jetting, large jellyfish swim by contracting only the rims of their bells into subtle rowing motions. By doing so, large jellyfish displace manageable volumes of water. They also create small vortices and currents that draw water, and more importantly, entrained prey into their tentacles.
Because the vortices and currents created by pulsating jellyfish bells are relatively subtle and imperceptible to many prey species and because the diaphanous bodies of jellyfish blend practically seamlessly with the water, jellyfish can hide in plain sight from their prey. Most prey are unable to detect even large nearby jellyfish, and so do not flee them.
Therefore, even though brainless jellyfish cannot intentionally target and stalk prey as can brainier animals, they have evolved a stealthily effective hunting method. Indeed, under some circumstances, swarms of large jellyfish--continuously rowing in order to capture enough food to sustain their large bodies--strip fish, fish eggs and plankton from the water almost as completely as plagues of locusts strip vegetation from land.
THE PURPOSE OF THE PULSE
With every pulse of its bell, a jellyfish draws water and entrained prey to its tentacles. A lion’s mane appears here. Credit: Sean Colin, Roger Williams University, and John Costello, Providence College
LIGHTS! CAMERA! JELLYFISH!
Dye is used to outline the wake of a moon jellyfish swimming in a marine lake on the island of Mjlet, Croatia. Credit: Sean Colin, Roger Williams University, and John Costello, Providence College