Because sponges look so simple, people sometimes assume they are colonies of single cells, like a mat of bacteria, or something between a plant and an animal. Neither is true. Sponges are genuine multicellular animals, with cells that depend on each other, specialise in different tasks and develop from an embryo. What makes them unusual is how loosely those cells are organised. A sponge has no stomach, no muscles, no nerves and no organs of any kind. Instead it runs on a small set of cell types that cooperate, move around and change jobs as needed.

From the reef keeper’s side, this flexibility is one reason sponges are so resilient. Small sponges that arrive on live rock can be broken, shaded or partly exposed and still regrow from what is left, which is a direct result of how their cells work. Our page on sponges in reef tanks covers what that means in an aquarium.

What multicellular means

A multicellular organism is one made of many cells that stick together, communicate, divide the work between them and cannot easily live apart.

Many single-celled organisms form loose groups or colonies, but each cell can usually survive and reproduce on its own. In a true multicellular organism, cells are specialised and interdependent: some feed, some protect, some reproduce, and the whole develops from a single fertilised egg. Sponges meet all of those criteria. Their cells are held together by adhesion molecules and a collagen matrix, they communicate through chemical signals, they specialise into distinct types, and a sponge develops from an embryo into a larva and then an adult. Our page is a sea sponge an animal or a plant? explains why that makes them animals.

The main sponge cell types

Sponges get by with a relatively small number of cell types, each with a clear role.

Choanocytes, or collar cells, line the internal chambers or central cavity. Each has a whip-like flagellum that beats to drive water through the sponge and a collar of microvilli that traps bacteria and food particles. Pinacocytes are flat cells that cover the outside of the sponge and line its canals, forming a protective skin. Porocytes are tube-shaped cells that form pores in simple sponges and can open or close them. In the jelly-like middle layer, the mesohyl, live several kinds of mobile cells. Archaeocytes (also called amoebocytes) crawl around digesting and distributing food, removing waste and, crucially, turning into other cell types when needed. Sclerocytes secrete spicules, the mineral needles of the skeleton, and spongocytes secrete spongin fibres. Collencytes and lophocytes produce collagen. Myocytes, contractile cells, can narrow the oscula. And sex cells, sperm and eggs, develop from choanocytes or archaeocytes. Our labeled sponge diagram shows where each sits.

Why sponges have no true tissues

In most animals, cells of the same type are organised into tissues, sheets or masses of cells tightly joined and working together, sitting on a basement membrane. Sponges mostly do not have that level of organisation.

A true tissue, such as the lining of your gut, has cells joined by specialised junctions into a sealed layer resting on a basement membrane, a thin sheet of protein that anchors the cells. In most sponges, cells are more loosely attached, the outer pinacocyte layer is not sealed in the same way, and there is no basement membrane. Cells also move around, leaving one position and taking up another. That is why sponges are often described as being at the cellular level of organisation, rather than the tissue level seen in cnidarians or the organ level in more complex animals.

There is an interesting exception. Sponges of the small class Homoscleromorpha have a true epithelium with a basement membrane and cell junctions similar to those of other animals, which is why they are sometimes described as the only sponges with genuine tissues. Our page on Homoscleromorpha explains this unusual group. Glass sponges are unusual in another way: much of their body is a syncytium, a continuous mass of cytoplasm with many nuclei rather than separate cells.

Cells that change jobs

One of the most remarkable things about sponge cells is how flexible they are.

Archaeocytes behave like stem cells, able to become choanocytes, pinacocytes, sclerocytes or reproductive cells. Choanocytes can also transform into other cell types, including sperm and egg cells, and in some sponges they can turn back into archaeocytes. This flexibility lets sponges rebuild damaged areas, reorganise their canals as they grow, and regenerate from small fragments. Sponge farmers rely on it when they cut sponges into pieces and grow each piece into a new sponge, and coral farmers like me rely on a similar ability in corals when we fragment colonies. Our page on sea sponge reproduction covers how sponges use this ability to reproduce asexually.

The cell-sorting experiment

A famous experiment from more than a century ago showed just how independent, and yet cooperative, sponge cells can be.

In 1907 the American biologist H. V. Wilson pressed living sponges through fine silk cloth, breaking them into individual cells and tiny clumps. Left in seawater, the cells crawled together, sorted themselves out and reorganised into small, functioning sponges, with canals, chambers and an outer skin. When cells from two different species were mixed, they tended to sort themselves by species, recognising their own kind. These experiments became classics in the study of how cells recognise each other and how tissues form, and later research identified the cell-surface molecules involved. Few animals can rebuild themselves from a suspension of cells, and the experiment is still repeated in teaching labs today; it is one of the strongest pieces of evidence that sponge cells are both specialised and able to act as a coordinated whole.

How sponges manage without organs

Without a heart, lungs, gut or kidneys, how does a sponge keep millions of cells alive? The answer is water.

Every cell in a sponge is close to a moving stream of seawater. The canal system brings food and oxygen to the choanocytes and carries away carbon dioxide and waste, while archaeocytes shuttle nutrients deeper into the mesohyl. In effect, the water current does the work that a circulatory system, a respiratory system and an excretory system do in other animals; our page do sponges have a circulatory system? explains how. Digestion happens inside individual cells rather than in a stomach. Coordination happens through chemical signals and through the contraction of cells around the oscula, rather than through nerves; see does a sea sponge have a brain? This arrangement scales remarkably well: sponges range from a few millimetres to giant barrel sponges about 2 metres across, and some are thought to live for centuries or longer.

The trade-off is speed. Sponges cannot move, chase food or escape predators, and their responses are slow. Instead they rely on chemistry and structure for defence, producing toxic or distasteful compounds and spiky skeletons that deter most predators. Many of those compounds have turned out to be useful to people, including the sponge chemicals that led to the leukaemia drug cytarabine. For an animal often called simple, a sponge is a very effective piece of biological engineering, and it has kept the same basic design working for more than half a billion years.

Sponges and the origin of animals

Sponges are one of the best windows we have into how multicellular animals first evolved.

Choanocytes look remarkably like choanoflagellates, single-celled organisms that are the closest living relatives of animals; some choanoflagellates even form small colonies, rosettes of cells that look like a miniature version of a sponge’s collar-cell chamber. That resemblance once suggested that the first animals were something like a sponge, a colony of collar cells that became specialised. When the genome of the Australian reef sponge Amphimedon queenslandica was published in 2010, it showed that sponges already have many of the genes used in other animals for cell adhesion, cell signalling, programmed cell death and development, suggesting that the genetic toolkit for multicellularity was in place very early in animal evolution. Whether sponges or comb jellies were the first animal lineage to branch off is still debated; see sponge fossils for the evidence from the rocks.