A sponge is another easily recognized marine creature.  But if asked what type of creature it is – most would not be 100% sure.  It “seems” like a plant – lives on the bottom, does not move – but it is not green.  It is NOTHING like an animal – animals walk, crawl, and have brains… well… most of them have brains.  The other option is a fungus – and for many of my first-year marine science students, that was the direction they would head – a fungus.

So, what is the difference?  What exactly is a sponge?

We begin with the question whether it has a cell wall surrounding their cells.  Plants and fungi do; animals do not.  What separates the plants and fungi is that plants have chlorophyll, fungi (unless they have symbiotic plant partner) do not.

Sponges… they do not have cell walls – this makes them animals.

This is not what most people think.  Again, animals move, leave poop piles, and can sense their world via a brain.  Sponges lack all of these.  Another twist to this, the tissue of animals begins with germ layers.  There are three of them.  The ectoderm (on the outside) develops into skin and the nervous system.  The endoderm (on the inside) develops into the digestive tract, some of the internal organs, and some of the endocrine glands.  The mesoderm (in the middle) develops into the skeletal system, muscular system, circulatory system, and some of the internal organs.  Sponges lack germ layers – thus, they lack all of the systems and organs mentioned above.  This is another reason people do not connect them with the animal kingdom.

Sponges are obviously made of many cells, but it is more like a living “condo” of specialized cells.  Some, the choanocytes, focus on food gathering.  To do this they extend a hair-like structure (called a flagella) which circulates generating a current.  The body wall of the sponge colony is full of pores which lead to numerous canals in which the choanocytes exist.  Their circulating currents draw water and food (plankton) into the canals where they, and the amebocytes, can feed.  The water drawn into the canals eventually leads a cavity in the center of the sponge – called a gastrovascular cavity – and the mass of water then exits the sponge through a large opening at the top of the animal called an osculum.

Amebocytes are responsible for reproduction.  They do this by combining sperm and egg producing a fertilized egg, which is then encapsulated in a shell and called a gemmule.  These gemmules are then released into the current and into the environment via the osculum.  Of course, sponges can reproduce asexually by fragmentation of their body parts.  Lacking all know forms of organs and tissue – this can easily be done.

So… there you go… sponges are animals.  Many did not think that.

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