9.2: Sponges

Do animals wash your dishes?

Natural sponges, like the one in the picture above, are actually animals taken from the sea! The sponges in your home, however, were most likely never living things. Most sponges used in kitchens today are made from unnatural materials.

Sponges

Sponges (Figure below) are classified in the phylum Porifera, from the Latin words meaning "having pores." These pores allow the movement of water into the sponges’ sac-like bodies. Sponges must pump water through their bodies in order to eat. Because sponges are sessile, meaning they cannot move, they filter water to obtain their food. They are, therefore, known as filter feeders. Filter feeders must filter the water to separate out the organisms and nutrients they want to eat from those they do not.

Figure \(\PageIndex{1}\): The sponges often have tube-like bodies with many tiny pores. There are roughly 5,000 sponge species.

You might think that sponges don't look like animals at all. They don't have a head or legs. Internally, they do not have brains, stomachs, or other organs. This is because sponges evolved much earlier than other animals. In fact, sponges do not even have true tissues. Instead, their bodies are made up of specialized cells (cell-level organization) that do specific jobs. Other animals, including humans, have tissue-level organization because they have tissues with specific functions.

Sponge cells perform a variety of bodily functions and appear to be more independent of each other than are the cells of other animals. For example, some cells control the flow of water, in and out of the sponge, by increasing or decreasing the size of the pores.

Sponges are characterized by a feeding system unique among animals. As sponges don't have mouths, they must feed by some other method. Sponges have tiny pores in their outer walls through which water is drawn. Cells in the sponge walls filter food from the water as the water is pumped through the body and out other larger openings. The flow of water through the sponge is unidirectional, driven by the beating of flagella, which line the surface of chambers connected by a series of canals.

Sponges reproduce by both asexual and sexual means. Sponges that reproduce asexually produce buds or, more often, structures called gemmules, which are packets of several cells of various types inside a protective covering. Freshwater sponges often produce gemmules prior to winter, which then develop into adult sponges beginning the following spring. Most sponges that reproduce sexually are hermaphroditic and produce eggs and sperm at different times. Sperm are frequently released into the water, where they are captured by sponges of the same species. The sperm are then transported to eggs, fertilization occurs and the zygotes develop into larvae. Some sponges release their larvae, where others retain them for some time. Once the larvae are in the water, they settle and develop into juvenile sponges.

Summary

• Sponges are sessile filter feeders.
• Sponges lack true tissues.

Explore More

Use the resource below to answer the questions that follow.

• Sponges: Origins at vimeo.com/37032195 (14:02)

1. How do sponge cells work together?
2. How do most sponges feed? Explain your answer as fully as possible.
3. Where do sponges take in water? Where do they expel water?
4. What evidence do scientists point to when they say sponges may be the oldest type of animal on the planet?
5. What is the "heart" of sponges that controls circulation?

Review

1. What is a sea sponge?
2. How do sponges gain nutrition?
3. What is meant by cell-level organization?
4. Define sessile.