The Incredible Sea Sponge

Sponges – The unnoticed vacuum cleaners of the deep

We see them every time we go diving. Most of the time we don’t even realise that they are animals at all. Even if we do, we rarely appreciate what effect they have on the surrounding waters. I’m taking about sponges… That’s right, that weird, slightly squishy stuff that just sits on the reef like a dull coloured rock. Of all the creatures our customers see underwater, the sea sponge never gets a mention, so let’s put that to rights and shed a little light on this rather intersting animal.

Sponges in general are simple animals thought to have evolved over 500 million years ago. They have diversified into 10,000 different species most of which live in salt water and can live for over 2,000 years. Together, they make up one of the oldest, most primitive groups of animals on Earth… Impressed yet?

Before we get to the cool stuff, let’s get the science bit out of the way…

Kingdom : Animalia (Animals)

Phylum : Porifera (literally means “pore-bearing”)

Classes : Calcarea (Calcium carbonate skeleton – having spicules), Demospongiae (Horn sponge – like a bath sponge), Sclerospongiae (Coralline or tropical reef sponges) and Hexactinellida (silica skeleton or glass sponges)

Anatomy of a Sponge

Still awake? Good, then let us begin…

The body of a sponge has 2 porous layers separated by a gel layer called the mesohyl. Each of the 2 layers are covered with tiny pores called ostia which lead internally to a system of tubes and canals eventually leading to “exit holes” called oscula.

Within the tubes and canals of the sponge are cells called choanocytes or collar cells. The collar cells attach to the sponge walls and use their flagellum (or whiplike tail) to create water movement within the sponge. The other main type of cell in a sponge body is the ameboid cell which transfers food to the other cells within the sponge.

The gel layer between the 2 walls of the sponge is where the structure of the sponge is determined. Whether made from calcium carbonate (spicules), silica (glass) or a flexible protein skeleton (like a bath sponge), sponges can form all kinds of shapes. Tubes, slippers, fans, cups, barrels, blobs and cone shapes are all possible when you have no body organs or tissues to get in the way.

anatomical picture of a sponge

How Sponges Work

Sponge cells don’t have any specialized purposes and can transform to complete the job of any other cell in the body, whether structural or reproductive. This lack of specialization means that sponges do not have tissues like every other type of animal on Earth. Interestingly, in laboratory tests, 3 different sponges were mashed together in a food processor. After being left in standing water for a 48-hour period, they not only started to reform themselves but also reformed the 3 original sponges as the cells swam back together and took on the job needed for recovery.

Picture of a barrel sponge

Barrel Sponge

yellow glass sponge

Glass Sponge

close of of the silica skeleton

Silica Skeleton

close of up calcium sponge

Calcium Carbonate sponge

Types of Sponge

Giant barrel sponges, like we find in Nusa Lembongan, feed by filtering water through the wall of the body by whipping their flagellum. By filtering oxygen and particulate out of the water for food, they make a pretty efficient filtering / eating machines. The materials that they don’t need are excreted into the “bowl” in the middle. The freshly filtered water is the ejected from the osculum, (the large hole at the top).

Remember that the sponge itself is thousands or millions of cells working together to enable this process to happen, each able to change its task as required.

If for example, a clumsy diver kicks or bumps into a barrel sponge, the crushed cells on the sponge may well stop water flow through the wall and whole sections of the sponge will die off. Given time though, the individual cells will find a workaround so that water flow will resume and the sponge will continue to grow.

Giant tube sponges on the other hand are hermaphroditic, producing both eggs and sperm at the same time. While the sponge holds onto the eggs, clouds of sperm are released into the water column and are filtered out by the same process as the barrel sponge uses for food and oxygen. They can then use this filtered sperm (from other sponges) to fertilize the eggs in their own bodies. After the eggs mature they themselves are released to find their way to move on and form new sponges.

Still not impressed? Check out the water flow through a sponge as illustrated by spreading food coloring near the footing of the sponge…

Clip courtesy of Johnathan Birds Blue World

Come on… admit it… you didn’t expect that did you?

So there you have it… The next time you dip under the surface, take a moment to appreciate the humble sponge. They are not only pretty to look at but there’s a hive of activity underneath that calm exterior.

Look at that, I got to the end of the article and resisted the urge to pull a “Sponge blog dive pants” joke 🙂

Darren Warburton

Author Darren Warburton

Aquatic bodyguard, chief tank filler and biscuit dunker.

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