Marine Sponges - What is a sponge?

Marine Sponges - What is a sponge?

Many people when they see a sponge, think it is as a plant, not an animal. 
Sponges are underwater invertebrates with no heads, arms, legs or internal organs.
Adult sponges don't move from place to place.
They attach themselves to rocks or plants and stay in one place all their life. It's no wonder that even scientists once thought sponges were plants!

What made scientists decide that sponges were animals?

Scientists discovered that sponges get the food they need in the same way as other animals do—by eating other living creatures.
This is an important difference between animals and plants.
Plants can make their own food and do not need to eat other living creatures.

There are about 5,000 different species of sponge. 
Sponges grow in many different shapes and sizes. Some species of sponge are tiny and are no bigger than a large coin, but others grow to be more than 40 inches (1 meter) wide.
Most sponges live in the salty water of warm oceans.
A few live in fresh water, such as lakes and rivers.
If you see a sponge that is bright yellow or orange or brown, it is sure to come from salt water.
If it's a green sponge, it will come from fresh water.
But purple or gray sponges can live in either salty water or fresh water.

Even though they may be shaped differently, all sponges function in the same way, they take their water through small pores, or holes, called ostia.
The water contains oxygen and lots of tiny plants and animals for the sponge to eat.
After the sponge has absorbed the oxygen and food that it needs to live, it pushes the water out through one large pore called an osculum.
The water takes with it waste products the sponge no longer needs.

Some species of sponge start life as an egg, while others begin as small knobs called buds. 
The egg is a female sex cell that starts to grow inside the body of the parent sponge.
Before it can grow into another sponge, the egg must be fertilized by a male sex cell, or sperm.
Some species of sponge can produce both the egg and the sperm.
These species can fertilize their own eggs.
Other species produce only eggs or only sperm. The male of the species releases sperm into the water. Then the sperm enter the female through the holes, or pores, in her body and fertilize the eggs.
Each fertilized egg grows into a round larva covered with tiny hairs called flagella. The larva uses the flagella to swim out of the parent sponge's body and into the water. After a few hours, or even days, the larva attaches itself to the sea¬bed and grows into a new sponge.
In some species, the small knob-like buds from the parent sponge grow into new sponges.
Sometimes they grow while still attached to the parent, and sometimes they break away and grow separately.
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