Ask someone to picture a sea sponge and they will usually imagine a soft, tan, rounded object with irregular holes, the kind sold for the bath. On a real reef, sponges are some of the most colourful and varied animals you will see. Some are as tall as a diver, some form organ-pipe clusters of purple tubes, others spread across rock like spilled paint. Many divers swim past them without realising they are animals at all.

Sponges are among my favourite subjects underwater, partly because they hold still. On a Key Largo weekend I photographed reefs covered in sea fans, brain corals and sponges in many colours, and adding a second, more powerful light to my rig made a visible difference to the colour and focus in those shots. This guide covers the main shapes you will see, what affects their colour, the animals that look like sponges but aren’t, and some tips for photographing them. Our sea sponge species page covers classification in more depth.

How to recognise a sponge underwater

Sponges come in so many forms that shape alone is not a reliable guide. Look for the openings.

Almost every sponge has two kinds of openings. The surface is covered with tiny pores, often too small to see clearly, that give it a velvety or fuzzy texture. Scattered over the sponge, or at the tips of tubes and the tops of barrels, are larger round holes, the oscula, where water leaves. A cluster of neat round holes on an otherwise irregular lump is a strong sign you are looking at a sponge. Sponges have no tentacles, no moving parts and no polyps; if something retracts when you wave a hand nearby, it is probably a coral, anemone or tube worm. Sponges also feel firm, rubbery or crusty rather than slimy or soft, but please don’t touch to check: some, such as the fire sponge, cause painful skin reactions, and touching damages the sponge. Our sponge diagram explains the parts.

Barrel and vase sponges

The biggest and most photographed sponges on Caribbean reefs are the barrel and vase sponges.

The giant barrel sponge, Xestospongia muta, is a huge reddish-brown or pinkish barrel with a rough, ridged outer wall and a wide central opening. Large ones can be taller than a diver, and some are thought to be centuries old, which is why it is called the redwood of the reef. On the walls of Cozumel, where the current brings constant food, barrel sponges grow especially large. Vase sponges are cup or bowl-shaped, often with thinner walls; the azure vase sponge, in pale blue to lavender, is one of the most beautiful sponges in the Caribbean. On St Vincent’s New Guinea Reef, terraces covered in vase sponges are one of the features of the dive, and frogfish often sit on sponges there, matching their colour so closely that the best clue is a “sponge” with a mouth.

Tube, rope and branching sponges

Many sponges grow as clusters of tubes, long ropes or tree-like branches.

Tube sponges form clusters of hollow cylinders, each with an osculum at the top. The stove-pipe sponge, Aplysina archeri, can grow long lavender or grey tubes more than a metre tall; see our page on the stove-pipe sponge. Yellow tube sponges and purple branching tube sponges are common on Caribbean walls. Rope sponges grow as long, thin, often drooping strands, sometimes in purple, red or orange, hanging from walls and overhangs. Branching sponges, such as the red tree sponge, grow like small bushes or antlers. These upright forms reach into the current to filter more water, and they often host small fish, shrimp and brittle stars among their branches.

Encrusting, ball and boring sponges

Some of the most common sponges are the least noticeable: thin crusts and lumps that cover rock, dead coral and wreck structures.

Encrusting sponges spread across hard surfaces as sheets only a few millimetres to a few centimetres thick, often in bright orange, red, yellow or purple. They are particularly obvious on shipwrecks and under ledges, where they compete with corals and other animals for space; divers exploring the James Bond wrecks off Nassau or the wrecks of Key West will see their hulls and frames covered in sponges. The orange icing sponge and the elephant ear sponge are good examples; see orange sea sponges. Ball sponges form rounded lumps. Boring sponges of the genus Cliona live inside coral rock and shells and show only as small yellow or orange patches or pore-covered spots on the surface; some, such as the brown variable boring sponge, spread over and slowly kill living coral.

Glass sponges and deep-sea forms

Beyond diving depths, sponges take on even stranger shapes.

Glass sponges, with skeletons of silica, often look like delicate cups, tubes, vases or lattices, white or pale and almost translucent in an ROV’s lights. The Venus flower basket is a curved glass tube with an intricate lattice wall. On the Antarctic shelf, glass sponges form forests of barrels and sacs; see Antarctic glass sponges. Some deep-sea carnivorous sponges look like harps, lollipops or feathery stalks; our page on carnivorous sponges shows their strange forms. Most people will only ever see these in photos and video from research expeditions, but they are among the most beautiful animals in the deep sea.

Why sponges are so colourful

Sponge colours come from pigments in the sponge’s own cells and from the microbes living inside them.

Many sponges contain carotenoid pigments, which give oranges, reds and yellows. Others host large populations of symbiotic cyanobacteria that can colour them green, blue or purple and may provide some food through photosynthesis. Colour may protect against ultraviolet light in shallow water, deter predators or simply be a by-product of chemistry, and the same species can vary in colour from place to place. Some sponges change colour when exposed to air or when they die, which is why the dried sponges sold in shops look nothing like living ones. See blue sea sponges and yellow sea sponges for examples.

Look-alikes: what isn’t a sponge

Several animals are easily mistaken for sponges, especially by new divers.

Tunicates, or sea squirts, often grow as colourful blobs or clusters with two siphons; they are actually chordates, distant relatives of vertebrates. Our pages on sea squirts and the painted tunicate explain the difference. Soft corals and gorgonians look plant-like but have tiny polyps with tentacles. Colonial anemones such as zoanthids form mats that can look like encrusting sponge until you see the polyps open. A quick rule: two neat siphons suggest a tunicate; polyps with tentacles suggest a coral or anemone; round oscula and no moving parts suggest a sponge.

How to photograph sponges

Sponges are ideal photographic subjects: big, colourful and perfectly still. The challenge is colour.

Water absorbs red light first, then orange and yellow, so at depth an orange sponge looks brown and a red one looks almost black. To restore colour you need artificial light from a strobe or video light, close to the subject, or careful white balance and colour correction afterwards. In my own photography the biggest improvement came from better lighting: adding a second, more powerful light on that Key Largo trip noticeably improved colour and sharpness, and I still colour-correct almost every shot. Get close to reduce the water between camera and sponge, shoot slightly upward to put open water behind barrel and tube sponges, and include a diver or fish for scale. Look for the animals that live on sponges too: frogfish, gobies, brittle stars, crabs and shrimp. Our underwater photography guide has more techniques.

Good buoyancy matters more with sponges than with almost any other subject. Photographers often settle close to a sponge to steady the camera, and a single fin kick or knee can break a tube sponge or crush a crust that took years to grow. Hover, brace with a finger on bare rock if you must, and back away slowly. If you are shooting on a wall, keep an eye on depth as you concentrate on framing; a big barrel sponge on a drop-off can draw you deeper than you meant to go.