The first true love of my life was the underwater world. I was obsessed with everything aquatic, especially fish. My obsession with fish gave way to a collection of home aquaria that tested the limits of my parents patience. Most of my aquariums were landscaped with a variety of aquatic plants whose variety boggled the mind. The underwater world is full of incredibly varied habitats that are home to a myriad of different organisms. Entire ecological communities exist, often unnoticed, just under the waters surface. This fact was not lost on me this past week as I was paddling my way around the Bog River in the Adirondacks. One group of organisms that I became especially enamored with were the river's freshwater sponges.
Though it is not readily apparent, sponges are animals. They aren't a single animal either. What functions as a single unit is actually a collection of individual organisms working in unison. The entire body of the sponge consists of these microscopic individuals connected by living tissue and held rigid by tiny rods made out of silica. I know what you're thinking, this is not a plant, why am I writing about it? The answer lies in the green color of this sponge.
There are many species of freshwater sponge throughout the world. Here in North America we have somewhere around 30. They are an indicator of clean, clear water. If you see sponges then you know it must be a healthy ecosystem. The freshwater sponges come in many different shapes, colors and sizes. Even within a species there can be a lot of variety between each colony. Pictured here is a species of Spongilla. Not all Spongilla are green though. Many are brown. The green coloration comes from algae living symbiotically within the tissue of the sponge. Similar to lichens, the algae photosynthesize and provide food to the sponge in return for a safe place to grow.
Though not technically a plant, the need to photosynthesize has pushed these sponges to grow into shapes not unlike what is seen in the plant kingdom. Depending on water clarity, temperature, and light levels, sponge shapes range from prostrate, creeping forms to upright branching structures. Also similar to plants, sponges can reproduce both sexually and asexually. As the water begins to cool in the fall, the sponges produce what are known as gemmules. These little packets of dormant cells are quite hardy, resisting pretty much anything the environment can throw at them. When the water begins to warm in the spring, the gemmules will grow into new sponge colonies. During the warm summer months, sponges reproduce sexually. Males release sperm into the water in hopes that it will come into contact with receptive females. This is similar to what we see many wind pollinated tree species do in the spring.
The idea that two completely different branches on the tree of life have converged onto similar biological strategies is a very exciting idea. Indeed, the similarities are striking. I went a long time before I knew that these freshwater sponges even existed. The fact that I live in the Great Lakes region and never encountered them tells me just how poorly we have treated our local waterways.
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