What are the most surprising facts about sea sponges?

Here are the facts that surprise people most, grouped by what makes sponges strange.

How much water can a sponge filter?

A lot. Up to about 24,000 litres can be pumped through a 1 kg sponge in a single day, a figure from Steven Vogel’s classic 1977 study and repeated in later reviews.

Flagellated collar cells called choanocytes line internal chambers and beat in unison, drawing water in through thousands of tiny pores and out through larger openings called oscula. As it passes, bacteria and fine organic particles are trapped and digested inside individual cells. Sponges remove a large share of the bacteria from the water around reefs, and on some reefs they recycle dissolved organic matter into food other animals can use — the so-called “sponge loop.” Read more in what does a sea sponge eat.

Are there sponges that eat meat?

Yes. Carnivorous sponges, mostly in the family Cladorhizidae, capture small crustaceans with hook-covered filaments instead of filtering water.

How are sponges useful to people?

Beyond bath sponges, sponge chemistry has given medicine some important drugs.

Many of those chemical defences exist because sponges cannot move or hide. Toxic or foul-tasting compounds keep most fish away, which is why only a handful of specialists — hawksbills, some angelfish and certain nudibranchs — feed on them regularly.

How is a sponge built without organs?

A sponge is a living water pump. A thin skin of cells and a lining of collar cells sandwich a jelly-like layer called the mesohyl, and almost every job is done by cells that can move around and change roles.

The outer surface is covered by flat pinacocytes, and water enters through thousands of tiny pores called ostia. Inside, chambers are lined with choanocytes. Each has a single whip-like flagellum surrounded by a collar of microvilli. The flagella beat and drive the current, and the collars catch bacteria and fine particles. Between the two layers lies the mesohyl, a collagen-rich gel full of wandering cells. Lophocytes lay down collagen fibres. Sclerocytes build spicules. In demosponges, spongocytes secrete the tough protein spongin that makes a bath sponge springy. Archaeocytes are amoeba-like cells that are totipotent, meaning each can turn into any other cell type. They also carry food around the body and clear grit that blocks the canals.

Sponges solve the size problem with plumbing. The simplest design, called asconoid, is a single tube lined with collar cells, and it can’t grow much beyond a few millimetres, because volume outgrows the lining. Syconoid sponges fold the wall into pleats and reach a few centimetres. Most sponges are leuconoid: the body is filled with a network of small choanocyte chambers linked by canals, which lets them grow to more than a metre across and take almost any shape, from thin crusts to barrels and branching trees. In every design, the chambers have a far larger cross-section than the inlet and outlet canals. Water slows right down where the collar cells feed and then leaves fast through the osculum. That is how a small leuconoid sponge such as Leuconia can draw water through more than 80,000 inlet canals.

Particles larger than about 50 micrometres can’t get through the pores and are engulfed by the skin cells. Particles from about 0.5 to 50 micrometres are trapped in the narrowing inlet canals. The finest bacteria are caught by the choanocyte collars. Oxygen and waste simply diffuse in and out of the passing water. Many tropical species also host symbiotic microbes. In some sponges, cyanobacteria make up as much as a third of the living tissue and supply roughly half or more of the sponge’s energy.

Why are sponges classed as animals?

Sponges are animals because their cells lack walls, they eat other organisms, they make sperm and eggs, and they develop from embryos. DNA confirms they sit at the base of the animal family tree.

For most of history, sponges were filed somewhere between plants and animals. Their status was only settled in the early nineteenth century, after the Scottish anatomist Robert Grant watched water currents flowing through living sponges. He later named the phylum Porifera, “pore bearers”, in 1836. The strongest clue to their place came from the collar cells. Choanocytes are almost identical to the single-celled choanoflagellates, the closest living relatives of animals. Many biologists therefore see sponges as a window onto what the first animals were like.

Whether sponges are the oldest surviving animal branch is a live debate. One camp places sponges first. The other argues that comb jellies (ctenophores), which have nerves and muscles, branched off even earlier, which would mean nervous systems either evolved twice or were lost in sponges. A 2023 study in Nature compared the order of genes along chromosomes and supported ctenophores as the earliest branch, but the question isn’t closed. Either way, sponges carry genes resembling those used in nerve synapses, which are switched on in the “flask cells” of swimming larvae. That hints at how neurons may have originated.

The fossil and chemical record agrees that sponges are very old. A steroid called 24-isopropylcholestane, made mainly by demosponges, turns up in rocks laid down before the end of the Marinoan glaciation about 635 million years ago. Well-preserved sponge-like fossils come from the roughly 580-million-year-old Doushantuo Formation in China. Unambiguous siliceous spicules appear in the Cambrian. In the Late Jurassic, glass sponges built a discontinuous reef belt about 7,000 km long along the northern edge of the Tethys Ocean, larger than any reef system alive today.

How do sponges reproduce?

Most sponges are hermaphrodites with no gonads at all. They can also clone themselves by fragmenting, budding or making dormant survival capsules.

In sexual reproduction, whole choanocyte chambers can turn into sperm cysts, while eggs develop from archaeocytes and grow by swallowing “nurse cells” for yolk. When reef sponges spawn, many release sperm in a dense plume from the osculum, so a spawning barrel sponge looks as if it is smoking. Most species keep the fertilised eggs inside and release swimming larvae. A few broadcast the eggs into the water. The larvae are balls of ciliated cells that swim for a few days, then sink and crawl in search of a settlement site. Many have simple light-sensitive spots without any nerves. After settling, most larval cells turn into archaeocytes and rebuild themselves as a miniature adult.

Asexual reproduction is just as important. Storm-broken fragments reattach and regrow within days, and sponge farmers have long used that to grow bath sponges from cuttings. Some species bud off small daughter sponges. Freshwater sponges and a few marine ones make gemmules: tough capsules of spongin, often armoured with spicules, packed with nutrient-rich archaeocytes. Gemmules survive freezing and drying, and in spring they hatch into new sponges. Lifespans vary enormously. Temperate sponges usually live only a few years. Some tropical and deep-water species may pass 200 years. Glass sponges with growth rates of fractions of a millimetre per year can be thousands of years old.

Glass sponges, ancient reefs and a 3-metre spicule

Glass sponges (class Hexactinellida) build skeletons of pure silica and hold the records for age and for the strangest architecture of any sponge.

Monorhaphis chuni anchors itself to the deep western Pacific seafloor with a single giant spicule that can reach about 3 m long. Because the spicule grows in layers like tree rings, Klaus Peter Jochum and colleagues could date one specimen to about 11,000 years. Other Monorhaphis collected from 1,100 to 2,100 m were estimated at 6,000 to 18,000 years old, growing at about 0.14 mm per thousand years. Their layered silica also records the chemistry of the deep ocean through the end of the last ice age, which makes them natural climate archives.

The Venus’ flower basket (Euplectella aspergillum) lives below about 500 m in the western Pacific, especially around the Philippines. Its lattice of fused silica rods is so well engineered that engineers study it for lightweight structures. An aluminium tube of the same dimensions with the material spread evenly would be about a hundred times less stiff. The tuft of anchoring fibres at its base, each 5 to 20 cm long and as thin as a hair, transmits light much like commercial optical fibre, as a 2003 Nature paper showed, but is far tougher because the glass is layered with protein. A breeding pair of small shrimp often settles inside the basket while young, grows too large to squeeze out, and spends its life there cleaning the sponge. That is why the dried skeletons have traditionally been given as wedding gifts in Japan.

Living glass sponge reefs were thought to be extinct until 1987–88, when Canadian geologists mapping the seafloor of the Queen Charlotte Basin found four of them in 165 to 240 m of water off northern British Columbia. Together they cover more than 700 km². Radiocarbon dating shows they have been growing for about 8,500 to 9,000 years. The fragile silica framework is easily smashed by bottom trawl gear, and trawl damage has been documented on several reefs. Canada designated the Hecate Strait and Queen Charlotte Sound Glass Sponge Reefs Marine Protected Area in February 2017, and further reefs in the Strait of Georgia and Howe Sound are under fishing closures.

Sponges on the reef: builders, borers and recyclers

On tropical reefs, sponges often rival corals in diversity, and they shape the reef in ways most snorkellers never notice.

The “sponge loop”, described by Jasper de Goeij and colleagues in Science in 2013, explains part of the old puzzle of how coral reefs thrive in nutrient-poor water. Corals and algae leak large amounts of dissolved organic matter that most animals can’t use. Sponges absorb it, then shed their own cells at a remarkable rate as particulate detritus, which snails, worms and other detritus feeders eat. In effect, sponges turn invisible dissolved carbon back into food. Not all sponges are benign to reefs, though. Boring sponges in the family Clionaidae dissolve channels into coral skeletons and shells with chemicals released by their archaeocytes, and they are a major cause of bioerosion. The Caribbean chicken-liver sponge, Chondrilla nucula, releases toxins that kill coral polyps so it can overgrow them.

Sponges also get sick. Caribbean Aplysina sponges suffer from Aplysina red band syndrome, rust-coloured bands caused by a cyanobacterium that can ring whole branches. It affects around 10 percent of A. cauliformis on some Bahamian reefs. Commercial sponge beds have been hit hard by disease and algal blooms: a red tide in 1947 wiped out the Gulf of Mexico sponge grounds worked from Tarpon Springs, and many divers switched to shrimping. By the 1950s, overfishing had pushed the natural bath-sponge industry to near collapse, and synthetic foam took over most of the market.

Sponge divers and sponge drugs

Two very different industries grew out of sponges: hard-hat diving for bath sponges and the search for new medicines.

Natural sponges were harvested in the Mediterranean for thousands of years. In Florida, John K. Cheyney set up the first sponge business at Tarpon Springs in the 1880s, using hook boats. In 1905 John Cocoris brought in Greek divers, first from Aegina and Hydra and then from the Dodecanese islands of Kalymnos, Symi and Halki. Their helmet diving made Tarpon Springs one of Florida’s leading maritime industries. It also explains why the town still has the highest proportion of Greek Americans of any US city. The industry revived in the 1980s, when disease devastated Mediterranean sponge beds.

The drug story began in the early 1950s, when chemist Werner Bergmann isolated unusual nucleosides built on the sugar arabinose from the Caribbean sponge now called Tectitethya crypta. Those molecules showed that a modified sugar could jam DNA synthesis. That insight led to cytarabine, first synthesised in 1959 and approved by the US FDA in June 1969, which is still a mainstay of leukaemia treatment and on the WHO List of Essential Medicines. It also fed into antiviral nucleosides. A second success came from halichondrin B, isolated from the Japanese sponge Halichondria okadai. Its fully synthetic analogue, eribulin, was approved by the FDA on 15 November 2010 for metastatic breast cancer and in 2016 for liposarcoma. Because many sponge compounds are actually made by their microbial partners, researchers now culture the microbes or synthesise the molecules rather than harvesting wild sponges.