For years, some observant Westborough neighbors wondered about the strange, large round balls that mysteriously appeared every year at the end of the summer on the bottom of the nearby frog pond.

They occasionally poked them with a stick, but the balls just sat there. The balls were 6 to 12 inches wide. After someone pulled one up on shore and discovered that it was jelly-like inside, the neighbors dubbed them jelly balls. The ball dried up overnight when it was left out of the water, leaving next to nothing behind.

Vegetable, animal, or mineral? Or something from a horror movie involving alien pods from outer space?

Seen through the water, the jelly balls look almost like rocks, dark and slimy and covered with white spots. Upon closer inspection, the white spots look like little clusters of “roots” sticking out. So – definitely not rocks, but alive. Perhaps strange underwater plant parts?

The surprising answer to the mystery of the jelly balls isn’t mineral or vegetable, but animal. The jelly balls are actually colonies of simple animals known as bryozoans, or “moss animals.”

The answer came from biologist Scott Shumway, professor at Wheaton College, and author of the forthcoming field guide, Naturalist's Guide to the Atlantic Seashore. He’s a Westborough resident and a Westborough Community Land Trust director.

The particular moss animals that form the jelly balls are known in scientific circles as Pectinatella magnifica, but they have no common name. Maybe “jelly ball animals” would do. Or, to borrow from the Latin name, perhaps “magnificent jelly ball animals.”

Like so many creatures from jellyfish to worms to insects to starfish, they are animals without backbones, otherwise known as invertebrates. Most moss animals – about 4000 different kinds – live in the oceans, but Pectinatella magnifica and about 50 other kinds live in freshwater.

The jelly ball animals are a normal part of the life in a pond or slow-moving river. They are native to North America and were first recorded in Massachusetts in the 1860s. Besides Westborough, they’ve been reported in Lake Cochituate in Natick, the Charles River near Needham, and the Connecticut River. Scott Shumway has seen them in Fisherville Pond in Grafton and the reservoir at Hopkinton State Park.

So what are they like, really? Each jelly ball is a colony, and many individual animals make up the outside of the ball. All the individuals in a colony are genetically identical.

The animals are tiny, less than half an inch long. They produce the “jelly,” which fills the inside of the ball-shaped colony and also covers and protects the animals. The jelly is firm, but it’s 99 percent water.

What about white spots that look like clumps of little roots on the outside? These “roots” are actually the tentacles of the individual animals. The jelly ball animals are filter feeders – they eat by straining algae, microbes, and tiny plants and water creatures out of the water. Their tentacles wave in the water, creating currents and catching food. When the animals are disturbed, they pull in their tentacles.

Other small water creatures often hang around near the tentacles and snatch some of the prey that drifts in the currents. Still others may feed on the jelly ball animals.

Jelly ball animals are simpler than most animals we know, but their bodies have at least some of the same basic equipment. Each one has a mouth – surrounded by a horseshoe of tentacles that feed it – and a digestive tract, muscles, and nerves. The animals have no lungs or gills, but they absorb oxygen directly from the water, with help from their swaying tentacles.

Each individual produces both eggs and sperm, instead of one or the other (as most animals we know do). In early summer, eggs and sperm combine to create genetically different offspring, which pass through an immature stage as swimming larvae on the way to becoming mature adults. To form a colony, a new adult multiplies itself by budding.

If you’re inclined to search for jelly balls, look in shallow, warm water that’s a few feet deep. The colonies are typically attached to underwater sticks or logs, wooden docks, or sometimes water-lily stems, at least a foot underwater.

Ponds that are well fed with nutrients, or even overfed, are promising places to look. Located on private property, the frog pond where neighbors found Pectinatella magnifica is an example. It’s abundantly supplied with nutrients from runoff containing fertilizers from the nearby golf course, according to neighbors. This enrichment leads to lots of algae and other minute pond life for the jelly ball animals to filter out of the water.

The jelly ball animals don’t grow in contaminated water.

The season for jelly balls will soon end. As fall approaches and the pond water cools below 62 – 68 degrees, the colonies will fall apart and disappear.

But that’s not the end of the jelly ball animals. The jelly in the middle of the colony contains large numbers of small black dots, visible to the human eye. These dots are tiny capsules containing groups of dormant cells.

The capsules can survive winter temperatures, as well as drying. They’ll start to grow again in the spring when conditions are right. One capsule grows into an individual animal, which then forms a colony as identical individuals bud from it.

The capsules also help to spread the jelly ball animals. The capsules float and are covered with small hooks that catch like burrs on fur and feathers, so pond visitors such as dogs and ducks sometimes give them a ride to a new pond. Maybe magnificent jelly ball animals will be living in a pond near you.