Have you ever noticed jelly-like blobs attached to a dock on a freshwater lake? At first glance, they appear to be brown fish egg sacks, but what are they?
Typically, when people ask this question, what they have discovered is a colony of the freshwater bryozoan Pectinatella magnifica. Freshwater bryozoans are microscopic aquatic invertebrates that live in colonies that can form into jelly-like clumps, and are often found attached to docks or sticks. Bryozoan colonies can be as big as one foot (30 centimeters) in diameter. The base of each tiny bryozoan is attached to a surface. Its body has an outer sleeve-like structure called the cystid, and a mass of organs called polypide that moves inside of it.
It’s a common sight in ponds and lakes during springtime – strange clusters of clear, jelly-like blobs attached to plants and twigs underwater. These peculiar egg masses belong to amphibians, and are a key part of their reproductive process. But what exactly are these gelatinous sacs containing embryos, and why do amphibians lay their eggs encased in jelly? Let’s take a closer look at this fascinating phenomenon.
Overview of Amphibian Eggs
Many amphibians, including frogs, toads and salamanders, lay eggs in freshwater ponds, lakes and wetlands The eggs are enclosed in a protective, transparent, jelly-like coating that contains nutrients and provides safety for the developing embryos
The jelly suspension allows the eggs to float, preventing them from sinking into debris at the bottom. It also lets in sunlight to nourish the growing offspring. The eggs eventually hatch into larvae like tadpoles which metamorphose into juvenile amphibians ready to emerge onto land.
The Egg-Laying Process
In early spring when water temperatures start rising, amphibians emerge from hibernation to mate. The male grasps the female in a position called amplexus to externally fertilize the eggs as they are laid.
The female then deposits long strands containing hundreds or thousands of eggs encased in jelly, usually attached to vegetation below the water’s surface. This provides support and some protection from predators.
The male releases sperm over the eggs, which fertilizes them internally even as they are enveloped by the jelly coating.
Appearance of the Egg Masses
The egg masses containing developing embryos look like clear clumps of grapes or bubbles. The jelly surrounding each egg capsule is thick yet slimy to the touch.
The size of an egg mass depends on the species – some are small clutches just a few inches across, while others can be more than a foot long containing thousands of eggs. The embryos often appear as dark specks suspended in the jelly.
Protective Jelly Coating
The jelly covering the eggs has multiple important functions:
- Protects the eggs from drying out and provides moisture
- Insulates the eggs from temperature changes
- Allows gas exchange for the embryo to breathe
- Protects from predators like fish and insects
- Contains nutrients and antibiotics to promote growth
- Prevents the eggs from sinking into muddy pond bottoms
- Holds the eggs together in an optimal mass for development
Hatching of Amphibian Larvae
Within the protective jelly cocoons, the fertilized eggs undergo rapid cell division. After a development period ranging from days to weeks depending on species and temperature, the eggs hatch into larvae.
The larvae have gills and tails for swimming but no limbs yet. They eat the remaining jelly and microorganisms until the yolk is fully absorbed. Eventually tadpoles and salamander larvae develop into juvenile amphibians and emerge from the water.
Examples of Amphibians That Lay Eggs in Jelly Masses
Some common amphibians that attach jelly-encased eggs to aquatic plants include:
- Green frogs – Lay glob-shaped egg masses around 2-4 inches wide
- Bullfrogs – Huge clutches containing up to 20,000 eggs in jelly
- Wood frogs – Small clusters attached to twigs and grasses
- Spotted salamanders – Delicate jelly blobs with embryos
- American toads – Long strands of small eggs in jelly
Threats to the Egg Masses
The gelatinous amphibian egg sacs face multiple threats in their aquatic environments:
- Fungi and bacteria can infect the jelly, causing embryos to die
- Predators like turtles, birds and insects feed on the eggs
- Pollution from agricultural runoff can trigger deformities
- Drought dries up ponds, killing unsuspended eggs
- Humans disturbing the masses compromise the protective jelly
So next time you come across these peculiar jelly blobs in a pond or lake, take a moment to appreciate this wonder of nature. The transparent sacs contain the next generation of frogs, toads and salamanders developing to replenish amphibian populations.
Cons of the Jelly-Like Blobs:
They can attach to pond fountains and block the intake on the pump. Filters like the PondSelect Mesh Cage* we sell with a large surface area can be the solution. These filters keep the jelly balls or Bryozoans away from the pumps intake while still allowing water to flow through the filter.
The opening at the top of the cystid allows the polypide to slide out toward the water. This exposes a head-like structure called lophophore topped with tentacles that filter food from the water. The slightest disturbance will cause the polypide and tentacles to retract quickly. The mucous-coated tentacles capture diatoms and other microscopic organisms. The microscopic organisms are then swept into the mouth by tiny hairs called cilia that layer the tentacles.
It’s unclear how bryozoans are related to other invertebrates. Bryozoans are not a close relative of other groups and likely evolved from a marine worm that adapted to a sessile existence. Fossil bryozoans first appear in Cambrian rocks, which is late in the fossil record. They likely existed earlier, but in softer form that did not secrete calcareous exoskeletons that leave fossil evidence.
Pros of the Jelly-Like Blobs:
Bryozoans live in colonies, and they’re filter feeders, so they eat algae and other small creatures.
They’re native to our region – and even better, they require clean water. So if you find them in your lake, it signals that you have a healthy aquatic environment.
Frog spawn surrounded with transparent jelly
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