If you thought sponges only live in the ocean, I have a surprise for you. The freshwater sponge (Spongilla lacustris) lives in large rivers, lakes, ponds and wetlands. Like their marine cousins they feed by filtering nutrients from the water and stay put on one spot for life.
As you can imagine, a freshwater environment can see more extreme changes than the ocean environment. By nature, the ecosystem where freshwater bodies of water find themselves in can change drastically. Events such as droughts, floods or an extremely cold winter can put the freshwater sponge survival in peril.
The mere presence of these sponges in a given body of water reveals a source free of pollutants along with pristine conditions (National Parks Service).
Looks Can Deceive
Perhaps you have spotted a freshwater sponge when you thought you were seeing an alga. That’s because Spongilla are green. This is due to a symbiotic relationship the freshwater sponge enjoys with a single-cell algae; the sponge is provided with oxygen and food, while the algae is given a place to live (Montana Outdoors).
Unlike algae, freshwater sponges are not slimy but have a coarse texture. Besides, they are not plants as is the algae, but are instead animals.
Batten Down The Hatches
Unlike their marine cousins, Spongilla face more unpredictability and must wrestle with a greater impact from a relatively unstable climate. As you are well aware of terrestrial environments are prone to floods, droughts and extreme cold for example. This tough sponge has evolved an ingenious way to withstand these natural assaults.
The tougher layers and spiny spicules evolved to allow the freshwater sponge to withstand more extreme weather and disperse at great distances – it’s designed to disperse (Italian Journal of Zoology). With the help of these internal pods the freshwater sponge is able to genetically survive most environmental extremes – including lack of oxygen.
Canary in The Pond
If a freshwater sponge is found in a given body of water, then you can be sure the water is very clean and free of pollutants. This is because sponges are very sensitive to pollution.
This sensitivity is directly connected to the fact that these animals are filter feeders and will strain microscopic bits of plankton and bacteria out of the water. If there happens to be pollutants in the water, the sponge will filter it out as well; it will enter the sponge’s body, where it’ll cause havoc.
I Get Around
Freshwater sponges (found mainly in the northern hemisphere) are significantly more widely dispersed than their marine counterparts. This is due to the ever-changing nature of freshwater ecosystems and the fact that terrestrial bodies of water are naturally fragmented – unlike the continuous and generally uninterrupted marine environment.
Flood waters, for example, can carry a sponge’s gemnule far away from its host. This can result in the new sponge being born a considerable distance from its parent. It’s not unusual to find freshwater sponges that go back a hundred million years in a body of water that’s only tens of thousands years-old.
When water itself is not the vehicle of transport, the spiny spicules found on the gemnules allow for over land migration of these freshwater sponges; increasing their reach across many parts of continental waters (Italian Journal of Zoology).
You probably know that many organisms have life-cycles that are divided into different stages. Well, Spongilla are no different in this respect. The order and length of these different phases are an adaptation to whatever environmental conditions happen to be prevailing at the time.
Gemnule hatching followed by sponge growth generally takes place continuously during the spring and summer. Deterioration of the sponge and subsequent release of the gemnule happens gradually from August to November. The dormant stage extends from December until April; at this time the ground is usually covered with gemnules (ResearchGate).
Freshwater sponges do not produce gemnules as a result of an imminent threat from the weather, but rather as a measure of preparedness. Some produce gemnules even when the long term outlook favors their survival. This means that the instinct to be prepared and avoid extinction must be controlled by innate biological factors (The Italian Journal of Zoology).
The Spongilla is highly adaptive, and thus can also invert its life cycle in response to the prevailing climatic conditions. For example in the northern latitudes above 40 degrees latitude, where the land is subject to intolerable icing and floods, this sponge will go into stasis. Whereas in habitats prone to drought and extreme high temperature one can find this organism in the growing stage.
A Tough Sponge
The Freshwater Sponge mainly lives in the northern latitudes and can be confused with algae because of its green color. The color comes from an alga that calls the sponge home.
The freshwater sponge belongs to the same species as all other marine sponges. In order to survive and avoid extinction it must overcome many more challenges than its marine cousins. It has evolved tougher gemnules in order to withstand the onslaught of the terrestrial climate, such as floods, droughts and ice over.
Like all sponges, Spongilla are filter – feeders; as such they are sensitive to pollution. If you spot one you can rest assure that the water is pristine and free of pollutants.
Because they must contend with more drastic changes from extreme weather than sea sponges, spongilla have evolved tougher gemnules. These have also evolved spiny spicules that attach to an unsuspecting animal that will provide the method of transport when water is not available.
Freshwater sponges go through several life stages. First they hatch from the gemnule and grow, followed by death of the sponge and release of a new generation of gemnules.
Spongilla don’t respond to climatic conditions in order to get geared-up for survival. Their drive to avoid extinction is driven by inner biological factors. This means that they will grow and disperse gemnules even if the weather outlook is good for them.
Freshwater sponges are a tough sponge. They must contend with significant threats that marine sponges do not. For this reason they command our respect.
Have you ever spotted a freshwater sponge? What part of the world was it? How did the water look? Leave you comment below.
Thanks for stopping by,