It’s time for me to blog about one of the most exciting animal groups on the planet. I’ve been saving this one for months, waiting to reveal it to you, my faithful readers. So without further ado, let’s explore the fascinating world of an amazing animal, the sponge.
Sponges, despite what they look like, are actually animals. They are members of the phylum Porifera, which means ‘pore bearer’, and is quite accurate. Basically sponges are collections of cells that have various pore and channels which water flows through. They don’t even have tissues or organs or anything, just specialized cells that perform different functions. So in all honesty, sponges are definitely not exciting. Not by any stretch of the imagination. But they do have some interesting properties, and an entire phylum of animals doesn’t deserve to be ignored just because they don’t have cute faces or run around. Or do much of anything.
Without a respiratory, circulatory, or really any body systems at all, sponges have to rely on diffusion of gasses to survive. Which means they don’t do too well in dry environments. All sponges are aquatic, with most species living in marine environments. Water flows through the channels and holes that characterize a sponge, bringing along life-giving oxygen and food. Choanocytes (also known by the simpler name of collar cells) are cells on the outside of sponges that are lined with flagella. Flagella are little tail-like things, and in the sponge they keep water flowing constantly over the cells.
Food is brought to the sponge in this way, and consists of plankton, bacteria, and other tiny floating particles in the water. Sponges have no ‘mouths’ or any sort of eating implement (they don’t even use forks!). Instead a cell ingests particles by phagocytosis (engulfing the food item), and digestion occurs in individual cells. There’s no sharing among sponge cells. They’re a selfish sort.
Reproduction can occur both sexually and asexually in sponges. To reproduce asexually, sponges create a bud, which breaks off from the parent sponge and becomes its own animal. Some species of sponges can bud internally, creating a gemmule, which is kind of a safety capsule for the sponge. Gemmules can survive extremely harsh conditions, and are viable even if the rest of the sponge dies. Not a bad way to save your genetic data. Just download it into a gemmule.
In sexual reproduction, male gamete are released into the water, and are taken up by collar cells when they reach another sponge. These cells then lose their collars and become transporters, bringing the spermatozoa to the egg for fertilization. It seems like sponge sperm get away with being a lot lazier than our hardworking mammal sperm.
As you can imagine, sponges don’t have the best predator avoidance systems. To compensate for this, most sponges have some sort of toxic substance in their bodies. These substances have been used in human medicine, to treat a number of conditions, like respiratory and cardiovascular disease.
While sponges generally don’t really move, some can crawl very very slowly whole others can contract their entire bodies. I think it would make an awful horror movie: Attack of the Giant Sea Sponge! … That moves at about one millimetre a day. Not really very scary. Still, there are some cool sponges out there, like the carnivorous deep-water ones that trap small crabs with tiny silken threads and digest them over a period of days. So maybe there is potential for a movie here. Who knows?
Cover image By Peter Southwood – Own work, CC BY-SA 3.0