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Jonathan Bird's Blue World S4 • E17

Sponges! | JONATHAN BIRD'S BLUE WORLD

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Published on Mar 11, 2014

A sponge might not look like much, but these simple animals with no brain or ability to move have lived on Earth for hundreds of millions of years. They can hunt prey and spawn, and Jonathan demonstrates how in this fascinating segment about the biology of sponges!

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When people think of a sponge, they usually imagine something like this: a cleaning sponge for washing the dishes…or for washing the car. Cleaning sponges are synthetic these days, but they used to come from the ocean.

Back in the days before we had synthetic sponges for housework, cleaning sponges did come from the ocean. They were harvested by hard-hat divers walking along the ocean floor in heavy weighted boots with a rake-like tool to pluck the sponges off the bottom. It was a dangerous and difficult job.

Bagfuls of sponges were lifted to the boat above. Immediately, the crew set to work cleaning the sponges. Then they were hung to air dry. After days or weeks at sea, when the boat was full of sponges, they were taken to shore, where they were auctioned to the highest bidder. Finally they were trimmed, sorted and sold to the public.

Natural sponges are still harvested in nearly the same way today, but the synthetic sponge has spared the lives of countless ocean sponges!

While an ocean sponge looks something like a weird plant, it's actually an animal.

In fact, sponges are among the simplest multi-cellular animals on Earth. They live on the bottom of the ocean, attached to a surface and never moving because they can't walk or swim. Some are quite colorful, while others are drab. They also come in all shapes and sizes. There are tube sponges, vase sponges, barrel sponges, rope sponges, encrusting sponges and many other types. Sponges live from the frigid waters of the arctic and Antarctic, to the tropics. On many coral reefs, sponges dominate the sea floor and the drop off.

One of the most common sponges on coral reefs is the barrel sponge. Barrel sponges grow to epic proportions, getting larger than a person!

Although sponges can't walk or swim, they can feed. They do it by filtering tiny plankton from the water.

A sponge is covered with small pores, called ostia, which lead to a system of internal canals and eventually out to one or more larger holes, called oscula. Within the canals of the sponge, chambers are lined with specialized cells called choanocytes, or collar cells. The collar cells have a sticky, funnel shaped collar and a hairlike whip, called a flagellum. The collar cells serve two purposes. First, they beat their flagella back and forth like fans to move water through the sponge. The water brings in nutrients and oxygen, while it carries out waste and carbon dioxide. Second, the sticky collars of the collar cells pick up tiny bits of planktonic food brought in with the water.

Sponges are very effective filter feeders, since they are able to capture and eat particles as small as bacteria as well as much larger particles. They might not look like they are doing much, but a simple demonstration shows how effectively sponges can pump water.

On a reef in the Caribbean, I make a dive with a syringe filled with a non-toxic dye called fluorescein. By squirting it around the base of some sponges, we can observe how the water is moving by watching what the dye does.

Within only seconds, the dye is pumped through the sponges along with the water. As you can see, a sponge is a pretty good water pump, and also a good strainer. Any plankton that goes in with the water, won't come back out through the osculum.

Tube sponges are even more spectacular to observe. They pump the dye so furiously that they look like a collection of miniature smoke stacks!

Since sponges can't get together to reproduce, they spawn in a way similar to coral. The sperm is released into the water column by the male sponge and finds its way to the female sponges, where fertilization occurs internally. Eventually, the planktonic larvae are released from the female sponge and float around in the water column as plankton for only a few days. They then settle down and start growing.

Sponges don't have many predators. There is not much nutritional value in a sponge and they're hard to digest. Hence, very few animals can eat sponges. But something was clearly eating this sponge! A sea turtle is the culprit. Sea turtles are one of the principal predators of sponges, along with a few species of fish and some invertebrates like nudibranchs.

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