What Is a Sponge?

Have you ever looked closely at a living sponge? Although they may look more like plants or fungi, sponges are actually animals. Sponges live in waters all around the world, from the deep seas of the Arctic and Antarctic to the shallow and warm tropical seas, as well as in rivers, streams, ponds, and lakes [1]. Adult sponges do not move around—they are fixed to the bottom of the seafloor, on rocks, or on other sandy or hard underwater surfaces (Figure 1).

Unlike humans, sponges do not have stomachs, organs, backbones, or blood. Still, they are multicellular animals, which means that they are made up of many cells that work together, like in the human body. If you put a sponge in a blender, the cells can recognize each other afterwards and reform into tiny sponges! Sponges are the only animals that can rebuild themselves if they are torn apart!

Sponges have many different sizes and shapes. They can be huge and round, or make thin crusts on rocks, or even look like vases or cups (Figures 1A–F). Larger sponges can also serve as homes for other animals like fishes, turtles, and shrimps (Figure 1G).

The sponge body is made of holes and channels through which water is pumped and filtered to obtain particles of food. Because they filter the water, sponges are called filter feeders [2]. Some sponges can filter up to 50,000 times their own volume in a day. This would be like an average-sized person drinking 3,500 l (or 925 gallons) of water in a day! Due to this water flow through their bodies, sponges are very important in marine ecosystems.

How do sponges filter so much water? Within their tissue, they have chambers that are lined with special cells that have tail-like appendages called flagella. These tails all wiggle around together to create a water current through the sponge body. This current makes the water flow through the sponge and provides the sponge with food and oxygen. This explains why most sponges are full of tubes and holes: they allow the water to flow around inside the sponge [2].

What Makes a Sponge Spongy or Rock-Like?

Some sponge species have lots of tiny, hard parts called spicules that assemble into a skeleton and make their bodies stiff. Spicules are made of either calcium carbonate (similar to limestone) or silicon dioxide (quartz/glass), and they come in a dizzying variety of shapes and sizes (Figure 1H). A single sponge can contain several different types of spicules [2]. Spicules are what make some species “spongy” and others “rock-like.” Sponges without spicules are generally the most squishy and spongy, like our bath sponge (Figure 1F). Although most modern kitchen and bath sponges are made of plastic, sponges without spicules, like the Mediterranean bath sponge Spongia officinalis, have been used by humans for thousands of years as cleaning tools, among other uses. These are the species that kitchen sponges try to mimic. Other sponges have evolved dense skeletons packed with hard spicules. Some of these spicules can form an interlocked network that makes the sponge hard as a rock, and they are unsurprisingly known as rock sponges (Figure 1D) [3]. These spicules are very strong, but they can still be bent, and light can pass through them. This combination of properties makes spicules very interesting to both materials scientists and biologists.

Sponges can look very similar when we hold them in our hands, so scientists called taxonomists need to look closely for unique characteristics to tell species apart from one another. Spicules play a very important role in this process because they are unique and can be used to identify different species. Spicules can even be used to identify totally new species of sponges. There are over 9,350 known sponge species in the world. Each of these species has either a distinctive set of spicules or, if the sponge is soft, no spicules at all [3].

The Curious Case of Carnivorous Sponges

In addition to making some sponge species rock-like, evolution has also driven other changes in sponges over many millions of years. Although most sponges are filter feeders, certain species have developed techniques to eat larger prey, such as larvae and crabs (Figure 2). These are called carnivorous sponges, meaning that they eat meat. Carnivorous sponges have lost the ability to filter food from the water.

Most carnivorous sponges live in the deep-sea at depths of more than 4,000 m (13,123 feet). In some regions, these sponges are the most common animals, surviving the cold, dark, crushing pressures of the deep ocean. Scientists think that eating meat is a tactic certain sponges use to survive when other ways of finding food do not work. A similar strategy is used by some plants, like the Venus fly trap, which evolved carnivory to live in food-poor habitats.

Carnivorous sponges generally have long fishing-rod-like “arms” lined with hook-like spicules that are used to capture prey. The spicules work like hook-and-loop fasteners, like Velcro or the seeds that get stuck to you when you walk outside. Prey animals caught in these hooks are slowly digested by the sponge. Sponges catch their food slowly, and it can take them several days to digest their catch.

It is not clear when this unique feeding mechanism evolved, but scientists think that carnivorous sponges have been on Earth for at least 60 million years, meaning that they appeared just after the dinosaur age ended. Since then, at least 150 sponge species have evolved to be carnivorous to survive in extreme, food-poor environments such as the deep sea [4].

Sponges Have Been on Earth for More Than 550 Million Years!

It is likely that sponges were the first animals that evolved on Earth. However, not all sponge groups have survived since then. Some species went extinct, while others adapted (like the carnivorous sponges) to survive harsh climatic conditions such as decreased oxygen in the water and increased temperatures caused by global warming. To date, Earth has experienced five big extinction events, and one of the biggest occurred 250 million years ago, before the dinosaurs even existed. This extinction is known as The Great Dying. During The Great Dying, 90% of all ocean life went extinct. It was a harsh time for sponges, and only a handful of species survived. One of the surviving groups was the rock sponges.

Paleontologists that work with fossils know that rock sponges made it through The Great Dying because scientists have found the skeletal remains of these sponges from the Cambrian Period (about 550 million years ago) onwards throughout the Earth’s history. In the fossil record, the skeletons of rock sponges remain largely whole and can be used to identify rock sponges even after millions of years (Figure 3). This is very unique among sponges. Most other “spongier” sponges that have only a few loose spicules or no spicules at all are less preserved in the fossil record because they do not have a formed skeleton. This means that even if paleontologists do find the spicules of these softer sponges, it is much more difficult to identify the species—a bit like finding just a little finger bone instead of a whole skeleton [5].

Conclusions

The species of sponges that we humans use and are most familiar with are just a few examples of the numerous diverse groups of sponges that exist in nature. Sponges have adapted to lots of different habitats and environmental changes over hundreds of millions of years. These adaptations helped them outlive not only the dinosaurs, but also many other plant and animal species.

Although sponges have various body shapes, colors, and spicule shapes, they still are some of the simplest animals that exist. Nevertheless, from the fossil record, we know that sponges were quite abundant in the past and they are still plentiful today. Like many other animals, sponges are affected by human activities. This means that sponges can be harmed by climate change, pollution, and over-fishing. As we take steps to protect our oceans from these man-made threats, we should be sure to remember the sponges and include these ancient and fascinating animals in our conservation plans.

Funding

We thank the Villum Fonden Grant No. 16518. Part of this work was made possible by the European Society for Evolutionary Biology (ESEB) outreach fund granted to AS.

Glossary

Filter Feeders: ↑ Are animals that feet on food particles from the water. Examples are sponges, baleen whales and sharks.

Flagella: ↑ Wildly whipping tail structures keeping the water moving inside the sponge body.

Spicules: ↑ A tiny structure made up of minerals that serves as a piece of the skeleton of sponges and other marine or freshwater animals.

Taxonomists: ↑ A scientist who groups organisms into categories and studies the relationships between various types of organisms.

Carnivorous: ↑ If an animal feeds on other animals tissues, meaning eating its meat, it is called a carnivorous animal.

Paleontologists: ↑ A scientist who studies fossils and the history of life on Earth.

Fossil Record: ↑ Remains of animals and plants that lived in the past (fossils) and their placement in the rock formation (record).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank Samuel Rarau (12 years) for giving us helpful feedback on our figures. We thank Shirley Pomponi from the Harbor Branch Oceanographic Institute (HBOI, Florida, USA) for providing the underwater sponge pictures of the rock sponge in Figure 1, Lisa Eckford-Soper from the University of Southern Denmark for the bath sponge picture, Andrzej Pisera from the Polish Academy of Sciences (Warsaw, Poland) for the picture in Figure 3C and Prof. Dr. Jean Vacelet from the Marine Station of Endoume, Marseille, France for the carnivorous sponge pictures in Figure 2. We want to thank our young reviewer Juniper for her excellent feedback to our article.