While not the most attractive of ocean creatures, sea sponges have both a history and a future in scientific research. Dr Michael Ellwood from the Research School of Earth Sciences takes us through the basics of these simple animals, assisted by PhD students Jill Sutton and Andrea De Leon.
Are sponges animals or plants?
Despite looking like a plant growing from the ocean floor, sea sponges do not photosynthesise, but rather consume organic matter from the ocean around them, which makes them an animal. Sponges are classified into three species based on their skeleton, whether its made from glass (silicon), calcium carbonate or organic matter. In the Research School of Earth Sciences, research is focused specifically on silicon skeleton sponges that can be found in fresh or saltwater at any depth, anywhere in the world.
How long have these multi-cellular organisms been around?
Sponges evolved somewhere between 540 and 600 million years ago, and that’s just the ones we know about. “One of the difficult aspects with dating these organisms is that is you can only date the ones that leave fossil skeletons,” Ellwood says. “And plenty of species, particularly the organic ones, don’t leave a fossil record so we don’t know for sure if they were there.” Here in Australia we have existing species dating back more than 35 million years.
Some sponges still retain exactly the same morphology as they did hundreds of thousands of years ago because there haven’t been any pressures on them to change, with a simplistic cellular structure and glass skeleton that acts as a deterrent for predators.
Would a sponge survive being put through a blender?
The simplistic structure of sponges means that each cell is autonomous and is able to perform all the cellular functions common to the sponge. “In humans, once you’ve laid down a liver cell, that’s all it knows how to be. But in a sponge, each cell knows how to do everything that a sponge does,” Ellwood says.
Thanks to the autonomy of the cells, you can cut a single sponge into three, eight or 20 pieces and have three, eight or 20 smaller sponges in its place. The researchers have even heard of experiments that have seen sponges added to blenders, only to have the cells join together in a new formation to make a new sponge.
A sponge can be as small as a single cell and, theoretically, as large as a building. And while they have what’s called indeterminate growth (no set point where they stop growing or die) most sponges don’t grow more than half a metre in height, the size where it becomes difficult to get enough food to maintain their size.
How do they eat?
Sponges feed on organic matter that floats through the water around them, remnants of plankton or small algae. Their sides are porous and they draw in water, process the organic matter it contains and push the clean water back out, similar to the filter in a fish tank. Whereas a plant can fix carbon and nutrients, via photosynthetic processes, into individual molecules, sponges must source their food from the material that they filter from water. They (sponges) can be discerning feeds with the ability to discard anything that they can’t feed on, rocks and the like.
What can we learn from sponges?
Because sponges are generally soft and stationary, one might expect them to be easy prey for other organisms. However, based on that knowledge and the fact that sponges are ubiquitous, we can assume that they possess physical or chemical protection. Physically, their skeleton provides a good defence however it does not offer complete protection. Subsequently, sponges have developed chemical defence mechanisms to deter predators.
Like the human liver, sponges have chemical defence mechanisms to convert toxins into less harmful materials but there are differences in these defences. It’s hoped that further study of their biochemistry will find a biomedical compound that can help cure a human disease. Already, the compound Spongistatin 1 has been found to stunt the growth of cancerous tumours.
What can sponges tell us about ocean climates?
Through sponges we can look at the interactions between the ocean, the atmosphere and climate over long periods of time. As the sponge grows over time, the various chemical signatures from the ocean, such as pH levels or oceanic silicon, are recorded in the skeleton. This skeletal history, like the rings of a tree, can provide a timeline of the climatic conditions the organism has experienced.
With a focus on ice ages, the researchers can look at those chemical signatures during the glacial and interglacial periods and understand how the ocean changes through these warm and colds periods. And if we understand how ocean cycles work and how they change over those climate periods, we can also look with greater insight at how they might influence or react to climate now and in the future.
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