Sponges are among the most widespread animals on the planet. Wherever there is water moving past a stable surface and enough food in it, there is probably a sponge. They are found in warm, shallow reefs and in the coldest, darkest parts of the ocean, in sunlit seagrass meadows and in pitch-black caves, and even in rivers and lakes far from the sea. Their basic way of life, pumping water and filtering out tiny food particles, works almost anywhere, and that has let them spread into nearly every aquatic habitat.

Some of my favourite sponge habitats are ones I’ve dived. On St Vincent’s New Guinea Reef, a terraced slope from roughly 9 to 45 metres is covered in sponges, and the island’s volcanic rock and sand favour sponges and encrusting life over the broad coral gardens of flat limestone islands. On the walls of Cozumel, the plankton-rich current feeds giant barrel sponges, purple tube sponges and orange elephant ear sponges. This guide covers those habitats and many more.

What sponges need

Whatever the habitat, sponges have three basic requirements.

First, most need a firm surface to attach to: rock, dead coral, shells, mangrove roots, wrecks or other hard objects. A few deep-sea sponges anchor in soft mud with tufts of long glassy spicules that work like roots. Second, they need water carrying food: bacteria, tiny plankton and dissolved organic matter. That is why sponges thrive where currents bring a steady supply, and why some grow larger in nutrient-rich water. Third, they need stable conditions without too much sediment, because silt clogs their pores and canals. Most sponges do not need light at all, which is why they can live in caves, under ledges and in the deep sea, although some shallow-water species host photosynthetic microbes. Our page on what sea sponges eat explains their diet.

Coral reefs and reef walls

Tropical reefs are where most people meet sponges, and on Caribbean reefs in particular sponges can rival corals in abundance.

Reef sponges grow on dead coral, rock and the bases of living corals, and on steep reef walls they can dominate, hanging from overhangs and reaching out into the current. Giant barrel sponges, tube sponges, vase sponges, rope sponges and colourful encrusting species are characteristic of Caribbean walls such as those of Cozumel, Cayman, Roatan and Turks and Caicos. In the Indo-Pacific, sponges are often less conspicuous on the reef surface but abundant in crevices, under ledges and in deeper zones. Sponges filter enormous volumes of reef water and play a central role in recycling nutrients; see coral reef sponges and sponges and coral.

Mangroves, seagrass and shallow lagoons

In warm, shallow coastal waters, sponges grow on the roots of mangroves and among seagrass beds.

The submerged prop roots of red mangroves in the Caribbean are often covered in sponges, tunicates and other filter feeders, forming colourful communities in water only a metre or two deep. Seagrass beds and hard-bottom lagoons, such as those of Florida Bay and the Florida Keys, support sponges including commercial bath sponge species. These shallow sponges can be very important to the ecosystem; when sponge die-offs struck Florida Bay after algal blooms in the 1990s, the loss of their filtering capacity was linked to further water-quality problems. Our page on natural bath sponges covers the species harvested from these habitats.

Caves, wrecks and temperate seas

Sponges thrive in dark, sheltered places and on artificial structures, and they are common in cooler seas too.

Underwater caves and overhangs suit sponges because they need no light and face less competition from algae and corals. Mediterranean sea caves are famous for their sponge communities, and in 1995 scientists described a carnivorous sponge living in a shallow cave near Marseille, a species previously known only from the deep sea; see carnivorous sponges. Shipwrecks quickly become sponge habitat; the James Bond film-set wrecks off Nassau and the wrecks of the Florida Keys are covered in sponges, sea fans and corals. In temperate waters, sponges grow on rocky reefs, among kelp and on piers and pilings, often in bright yellow, orange and red. Our page on Mediterranean sea sponges covers cave sponges in more detail.

Polar seas and the deep sea

Some of the largest and oldest sponge communities live where few people ever see them.

On the Antarctic continental shelf, glass sponges form dense sponge grounds in water below freezing, including giants such as the volcano sponge, which can reach about 2 metres; see Antarctic glass sponges. In the North Atlantic, dense deep-water sponge grounds, known in Norway and the Faroe Islands as ostebund or “cheese bottoms” for their appearance in fishing nets, cover large areas of the seafloor. Off British Columbia, living glass sponge reefs, discovered in the late 1980s, rise from the seafloor in a habitat once thought to have vanished with the Jurassic. Deeper still, sponges live on seamounts, the abyssal plains and in ocean trenches several kilometres down. Some deep-sea sponges are thought to live for thousands of years; see deep-sea sponges and deep sea sponge lifespan.

Freshwater sponges

A few hundred sponge species live in fresh water, in lakes, rivers, ponds and even canals.

Most belong to the family Spongillidae and are found on every continent except Antarctica. They usually grow as thin crusts or finger-like branches on rocks, logs and submerged plants, often green from symbiotic algae or pale brown or grey. Many survive cold winters or dry seasons as tiny, tough resting bodies called gemmules, which can withstand freezing and drying and grow into new sponges when conditions improve. Lake Baikal in Siberia, the world’s deepest lake, has its own family of endemic sponges, the Lubomirskiidae, including large branching green species that form “forests” on the lake bed. Freshwater sponges are often overlooked, but they are a sign of clean, well-oxygenated water. If you swim or paddle in a clear lake or slow river in summer, look closely at submerged branches and rocks: a greenish, slightly fuzzy coating with small round holes may well be a sponge. They are harmless, though some contain fine spicules that can irritate skin, so it is best to look rather than handle them.

How sponges spread to new places

An animal that never moves as an adult has still managed to colonise nearly every body of water. The secret is in its early life and its toughness.

Most sponges reproduce sexually by releasing sperm into the water, and many brood their embryos and release swimming larvae. The larvae drift and swim for hours to days before settling on a suitable surface, which lets sponges reach new rocks, wrecks and reefs. Sponges can also spread by fragmentation, when storms or predators break off pieces that drift away and reattach, and some produce buds that detach and settle nearby. Freshwater sponges have an extra trick: their gemmules can survive drying and freezing and may be carried between ponds and rivers on the feet or feathers of birds or in mud. Over millions of years, these modest means of dispersal have carried sponges into habitats from Antarctic shelves to mountain lakes. Our page on sea sponge reproduction and the life cycle of a sea sponge cover these stages in more detail.

Sponges in your aquarium

Sponges also find their way into one more habitat: home aquariums.

In reef aquariums, small sponges often arrive on live rock and grow in shaded places such as overflows, sumps and the undersides of rock, where they filter fine particles from the water. I have used live rock in my reef systems since 1996, and it is part of what makes a tank feel like a real piece of reef. Some reef keepers also keep colourful photosynthetic or filter-feeding sponges deliberately, though many sponges are hard to keep alive long-term in an aquarium because they need a steady supply of very fine food. Our page on sponges in reef tanks covers what to expect and how to care for them.