Question of the Week: Why are sea sponges considered animals?

By Sarah Gibbs

In that big comparative anatomy lecture hall in the sky, Robert Grant, founder of UCL’s Grant Museum of Zoology, must be smiling. This week’s featured question focuses on Grant’s favourite animal: the sea sponge. Grant’s work definitively proved that sponges are animals, not plants or simple celled organisms.

So, why is a sea sponge more closely related to a dog that a cactus? Read on to find out!

The ever-sage Encyclopedia Britannica informs us that early naturalists classed sponges as plants because, you know, they lack organs, don’t move, and often have branches. Understandable, to be sure. In the eighteenth century, however, scientists began to notice animal characteristics of sponges, including the changes in diameter of their central cavity, and their creation of distinct water currents. Zoologists imagined that sponges occupied an isolated position in the animal kingdom, but molecular testing has since proved that sponges and more complex animals (like humans) developed from a common ancestor; sponges also possess many of the qualities biologists use to distinguish people from plants. For example, bodily composition: the elastic skeletons of sponges are made from collagen, the same protein found in human tendons and skin. Prevailing theories suggest that sponges are early animals which produced no subsequent evolutionary line.

The folks over at Scientific American note that sponges’ specialized cells differentiate them from multicellular protists, creatures which are not animals, plants, or fungus, and which form no tissues. It is the thinness of the sponge body and the fact that its cells are exposed to circulating water—which supplies food and oxygen, and removes waste—that make organs unnecessary. Sponges may have been the first multicellular animals. Multicellularity (which means that cells adhere to one another, communicate, are mutually dependent for survival, and specialize to perform different tasks) is the key to producing more complex organisms. Scientists speculate that sponges emerged, flexing their multicellular muscles*, at least 543 million years ago (*as sponges lack arms, they are sadly ineligible for body building contests). According to Scientific American, sponges were the first filter feeders, tiny Brita jugs of the sea** (**mixed metaphor alert).

So, sponges are in fact the original animal hipster; they were multicellular before it was cool. Let’s close with a few fun sponge facts.

Absorbing (!) Facts:

1. Sponges can range in height from less than one centimeter to two metres tall.
2. Most sponges are hermaphroditic (male and female cells exist in one animal) and reproduce sexually by releasing spermatozoan into the water current to be carried to other sponges, where they interact with eggs. Sponges can also reproduce asexually.
3. Some deep-water sponges are carnivorous. Animals like the ping-pong tree sponge lie in wait for small crustaceans and other hapless sea dwellers to alight on their branches, the hook-like spicules on which prevent escape. Digestive cells migrate to the site of capture and the feast begins. Bet you’ll never look at a loofah the same way again.

 

Sources:

Frazer, Jennifer. “Sponges: The Original Animal House.” Scientific American, 17 Nov. 2011, https://blogs.scientificamerican.com/artful-amoeba/sponges-the-original-animal-house/

Sarà, Michele. “Sponge.” Encyclopedia Britannica. Britannica Academic, Britannica Digital Learning, https://academic-eb-com.libproxy.ucl.ac.uk/levels/collegiate/article/sponge/110257