It's a sweet time for jellies

Pulsing as large as pie plates, jellyfish have invaded Boston Harbor, slapping against pilings, pooling in Fort Point Channel, and lolling in gentle waves while they slurp microscopic prey near the USS Constitution.

Off Long Wharf, clumps of the pinkish-white blobs have made it look as if sand were floating on the blue- green water. "I thought the bottom was coming up, they were so thick," said Lawrence Cannon, owner of Waterboat Marina.

While no one has taken a census, scientists say that, based on observers' accounts, the population of gelatinous creatures known as moon jellies appears to have spiked. The jellyfish seem to be thriving because of a variety of factors, including a strong reproductive season last winter, calm seas, warm water, and an abundance of plankton and other food, due to depleted fish stocks.

"I think people are noticing larger ones earlier this year because of optimal conditions," said Steven L. Bailey, curator of fishes for the New England Aquarium.

The brainless, bell-shaped organisms don't sting like the venomous Portuguese man-of-war, but they can clog intake valves on marine air conditioners and generators and have caused larger problems. In 2000, a bloom of moon jellies in the Philippines shut down a coastal power station in Manila.

In Boston, a steady coastal breeze has kept many jellies within the inner harbor, but some of the goopy creatures have shown up on beaches. They will disappear when temperatures drop in autumn.

"People tend to be freaked out by jellyfish generally, but these moon jellies are pretty harmless," said Lawrence P. Madin, senior scientist at Woods Hole Oceanographic Institution. "They are pretty elegant animals that are beautiful, really."

ANDREW RYAN