Coming up next on Jonathan Bird’s Blue World,
Jonathan explores the biology of sponges, and you might be surprised at what he finds! Hi, I’m Jonathan Bird and welcome to my world! 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! The ultimate test is a hefty barrel sponge!
What will a big monster like this do? It takes a few seconds for the dye to work its way
through the sponge…but then it pours out like smoke from a chimney. That’s pretty good
pumping from those tiny little collar cells! 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. Sponges might not be very exciting and they
certainly don’t have much personality, but they’re an ancient animal that has been living
in the oceans for at least half a billion years! They can’t crawl around, or swim, but
they are very good at reproducing and feeding themselves by pumping water. So, chances are,
sponges will continue to populate the oceans longer than people will populate the Earth.

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