An Ode to Sponges

What comes to mind when you hear the word “sponge”? Do you think of the cleaning objects we use at home? Or do you imagine desserts, behaviours, textures, or aquatic animals? The Greeks first gave the name “spongos” to this magnificent, prehistoric, and prolific marine animal group we still call sea sponges.

Pick up a kitchen sponge and notice its soft, bouncy, and sometimes fluffy texture and appearance. Although cleaning sponges in urban India are probably made of polyester or vegetable cellulose, they closely resemble their marine animal namesakes. Sea sponges have porous internal skeletons held together by calcareous or siliceous fragments known as “spicules” and networks of fibres called “spongin”. Their survival depends on constant water circulation in and out of their bodies. Centuries ago, humans identified numerous valuable ways in which the water-retaining habits of cushiony sea sponges could be used: absorbing, cleaning, softening, protecting, etc.

Since sea sponges (phylum Porifera) first evolved (possibly 890 million years ago), their body plan and lifestyle have changed very little. On one hand, this might suggest that sponges today are some of the simplest and most primitive animals on Earth. On the other, it shows that a sessile (attached to the sea bottom) plankton feeder, without elaborate nervous and circulatory systems, has been extremely successful in tiding over the trials of geological time. Today, sponges are found in tropical and temperate waters around the world, among shallow coral reefs and seagrass meadows, all the way to the poles, thousands of metres deep in the Arctic and Antarctic.

Sea sponges are an integral part of marine ecosystems across ocean basins. Predominantly filter-feeders, they ingest microscopic plankton that enter their body cavity with incoming volumes of water through pores known as ostia. They send their digestive waste with the outgoing water through larger openings called the osculum. This process alone profoundly impacts the cycling of nutrients in the ecosystem, making nutrient-rich particulate matter available to other inhabitants across the marine food web. 

Depending on the species, sponges can be predators, competitors, symbionts, or prey. Sea turtles, reef fish like bannerfish and angelfish, and some sea slugs actively seek out sponges as food. Sponges also have chemical defences, which pose stiff competition to other sponges, corals, algae, sea squirts, and various marine life on the sea floor. Some species of sponges can eat their way through other animals, including live coral, dissolving, boring, and tunnelling through calcium walls. Sponge tissue also houses a thriving microbiome of symbiotic bacteria, cyanobacteria, and microalgae, providing their host with supplementary food and metabolites for defence.

One group of sponges popular among SCUBA divers for its picture-postcard looks is the barrel sponge. They are relatively easy to find, and their sizes are often staggering. On a typical day, a barrel sponge can be a hotbed of animal interactions: spongivorous fish come to feed on it; smaller fish and invertebrates seek shelter on it; and they can be fish- and shrimp-run cleaning stations. Barrels are, however, just one of nearly 9,000 different kinds of sponges. A different community of sponges emerges if you swim away from a coral reef to a sandier habitat, where stable structures and hiding places are hard to come by.

In the most extreme kind of marine environment, the intertidal zone, everything from water level, dissolved oxygen, salinity, and water temperature fluctuates every few hours with the changing tides. Sponges thrive here, too, being able to withstand hours of exposure to air while also taking the force of lashing waves.

Regardless of where sponges are found, other animals have found ways to make the most out of the presence of their filter-feeding stationary neighbours. For instance, frogfish and some sea cucumber worms mimic the colour, texture, and patterns of sponges, while the decorator crab carries a small sponge colony on its head as a disguise. Each of these animals tries to masquerade as a sponge, either to escape predators who are not fond of sponges or because they are ambush predators themselves who want to blend in their environment and remain unseen by their prey.

When we think of marine conservation and the mounting pressures on our oceans in the 21st century, it has become common knowledge that overharvesting and overfishing are some of the biggest and scariest threats. Marine species higher up in the food web are rapidly declining worldwide. We tend not to notice how invertebrate species lower down in the food web, but just as crucial, are impacted too. A combination of destructive fishing practices, climate change, and disease is tipping the delicate balance of these benthic communities. There is a decrease in sponge numbers in warming waters and a rapid increase in numbers in areas where spongivorous fish are overfished. 

Sea sponges like the Spongia and Hippospongia have had a long history of being harvested for human use from the time of the early Greeks and the Roman Empire. The demand for sponges in domestic and industrial sectors has steadily grown through the centuries. By the mid-to-late 20th century, sponges were declining, especially in the Mediterranean. Many countries today practice sponge aquaculture, notably Fiji, Micronesia, and the Solomon Islands. Sponges are thought to be easy to grow in shallow lagoons, and as filter-feeders, they are thought to inherently benefit the ecology of their local habitats. Sponge farming in Zanzibar is carried out by fisherfolk, primarily women, who find it more sustainable than fishing.

For city dwellers who find it progressively harder to relate to nature and the wildlife in our neighbourhoods, a mundane kitchen sponge can take us on a roller coaster journey — of evolution, human history, and biomimicry — and ground us right back to this glorious watery planet.