You are right but only regarding one genus of sponges with the common name “boring sponges”. However, even those sponges are not boring. Using a chemical process they can turn coral and shell material into a shelter. During this process they simultaneously produce sediment. Pretty cool, right? (1)
Recently a fellow diver described something he saw during a dive wanting to know what it was. “Sponge” I answered. He wasn’t satisfied by the answer and wanted to know which sponge it was.
Would you know different types of sponges?
I didn’t. Except the barrel sponge. So I looked them up in a fish book (2) and realized that some of them do have extremely funny common names like chicken liver, stinking vase, strawberry or pipes-of-pan sponge. And rather unexpected sponges are actually quite fascinating creatures.
Most people know sponges from cleaning. Today luckily most sponges we use for cleaning are synthetic but I still remember scrubbing the blackboard at school with a natural sponge.
Scuba divers normally don’t pay too much attention to sponges while diving, except when they are part of a pretty scenery like huge barrel sponges which can grow larger than a person.
They are animals
While appearing to be plants sponges are actually multi-cellular animals. They belong to the Phylum Poriphera and are amongst the most primitive animals. Other reef inhabitants like coral are colonies of single-celled organisms whereas a sponge is one single animal composed of specialised cells. No higher animals have evolved from sponges except, of course, for sponge bob.
Sponges are bottom dwelling and attach themselves to the ground, walls or just anywhere they can attach to stay in place while filtering the water for food.
Sponges are hollow
Specific cells work together to move water through the pores of the sponge into channels to filter out nutrients and oxygen.
The water containing waste products and carbon dioxide is then pumped out. The body of the sponge is hollow hold in shape by a jelly-like structure as well as fibre network both made out of collagen.
The effectiveness of the pumping is nicely demonstrated in Jonathan Bird’s blue world video using colored liquid.
Today male, tomorrow female
Sponges are hermaphrodites. So they can be both male and female. Or they just reproduce asexually. When a part of certain sponges is lost it can regrow.
Most sponges time their spawning and all those who decided to be male during this event will release the sperm at the same time. The barrel sponges then look like bone fires.
The females will take in the sperm through the pores like food but transport them to different cells and most retain the fertilized eggs till they hatch.
I can swim
Sponge larvae can swim! This is the only time in their life they can actually move freely but only for a few days. Then they sink and crawl until finding a substrate they can attach to and start growing.
They produce food for other organisms
Sponges can filter small organic matter from the oceans which couldn’t be used by most other organisms. These are then transformed into small cells which are replaced often. When flushed out of the sponge the cells can be consumed by larger animals (3).
And they offer a house or shelter for other animals
Like the hairy squat lobster and the cryptic sponge shrimp as well as frogfish who camouflage perfectly in with the sponge.
Turtles like sponges too
Sometimes you see sponges where some parts have been nibbled out. This was most probably done by a turtle. Sponges don’t have many natural enemies besides turtles and some fish. No worries, the sponges might not mind too much as they don’t have a nervous system.
Sponges are much older than dinosaurs
Sponges already exist over 500 million years while dinosaurs lived between 230 and 65 million years ago.
(1) Cliona spp. In Smithsonian Marine Station at Fort Pierce. Retrieved June 8, 2016 from http://www.sms.si.edu/irlspec/Cliona_spp.htm
(2) Eugene H. Kaplan (1999) “Coral Reefs Caribbean and Florida” Houghton Mifflin Harcourt; 1st edition. ISBN-10: 0618002111
(3) Sponges Recycle Food for Reefs. In New York Times. Retrieved June 8, 2016 from http://www.nytimes.com/2013/10/08/science/sponges-recycle-food-for-reefs.html?_r=0
(4) Sponge. In Wikipedia. Retrieved June 8, 2016 from https://en.wikipedia.org/wiki/Sponge