Porifera — the Sponges
The first branch on the tree of animals, and the strangest. A sponge has no mouth, no gut, no nerves and no muscles — and it is still an animal, pumping the sea through its own body and filtering out almost everything in it. Its name means "pore-bearer", and that is the whole idea.
In 1907 a biologist named H. V. Wilson took a living sponge and squeezed it through a fine silk cloth, so that it came out the other side as a cloudy soup of separate cells. He left the soup in a dish of seawater. What do you think he found a few days later?
New sponges. The loose cells crawled towards each other, stuck together in clumps, sorted themselves out into the right layers, and rebuilt small, working sponges. Try that with any other animal and you have only soup.
It works because a sponge is built so simply: a loose collection of partially specialised cells, each one able to survive and do its job with little help from the others. That is why sponges were once thought to be colonies of single cells rather than animals — and why your booklet says that, broken into fragments, a sponge can grow back into a whole new one.
Every animal from here to the end of the course is built more tightly than this. The sponge is where the animal story begins.
The First Branch on the Tree
A sponge is a benthic, asymmetrical filter-feeder built from a few kinds of specialised cells. It pumps water in through pores, strains out food with collar cells, and sends the water out through a larger opening — and that one current does almost everything the sponge needs.
Where it sits. On the tree from Topic 28, the sponges branch off at the very first branch point: multicellular animals, but without true tissues. Every other phylum in this course lies beyond the next branch point.
Where they live. Mainly marine — biggest and most colourful in the tropics, but found in most ocean habitats — with a limited number of freshwater species. They have no nervous system and no muscles, so the adults cannot move about. They grow attached to rocks: they are benthic, living on or near the seabed, fixed to a substrate.
How they grow. Sponges are asymmetrical and grow continuously, spreading as carpets or rising as tubes, vases, fingers and lumps, often in very bright colours. Because the shape depends so much on where a sponge grows, it is not a reliable way to identify one — specialists identify a sponge from the cells and spicules in its body wall.
"Tissue level" or "no true tissues"? Your sources disagree — here is what to do
CAPS says sponges have a "very simple tissue level of organisation, with three body layers", and your booklet and study deck repeat it. Topic 28, like most modern biology books, says sponges have no true tissues and sit at the cellular level of organisation. Both are describing the same animal; they draw the line in different places.
What is agreed: a sponge has three layers of cells (below), and its cells are specialised. What is debated: whether those layers count as true tissues. They lack most of what makes a tissue in other animals — the cells are loosely joined, can change from one type into another, and wander through the jelly — which is exactly why Wilson's cells could crawl back together.
📌 For your exam: describe the three layers as CAPS does, and say "a very simple tissue level of organisation". But know that these three layers are not the ectoderm, mesoderm and endoderm of a triploblastic animal. A sponge is not triploblastic — it has no germ layers at all.
A Living Pump
Cut a simple vase-shaped sponge in half and you find a hollow body with a wall full of holes. Water flows one way only: in through the many small pores, into the central cavity, and out through one large opening at the top.
↑ left: the whole sponge. Right: one collar cell, magnified — thousands of them line the inside and drive the flow.
The pump. Each collar cell has a whip-like flagellum that beats constantly. Thousands of them together drive a steady current: water is drawn in through the pores and pushed out through the larger exhalent opening, the osculum. Any current flowing past the sponge helps, by drawing water out of the osculum. The filter. Water passes through the ring of fine projections around the flagellum — the collar — and food particles stick to it. The cell engulfs them into a food vacuole and digests them inside the cell.
Sponges are so good at this that they remove almost all suspended particles — plankton, bacteria, organic specks — from the water they pump. In a classic textbook measurement, a small tube sponge just 10 cm tall and 1 cm across pumped about 22 litres of water a day. A reef covered in sponges filters its own water many times over: sponges are some of the most effective ecological filters in the sea.
Three Layers and Their Cells
A slice through the body wall, from the sea outside (left) to the central cavity (right). These are the exact letters of your booklet's labelling question. Name each letter before you tap it.
| Layer | What it does | Cells in it | Also called |
|---|---|---|---|
| Outer dermal layer | a skin that protects the inner two layers | flat dermal cells | pinacoderm (pinacocytes) |
| Mesenchyme | jelly between the two cell layers; houses the working cells and the skeleton | amoeboid cells, pore cells, spicule-producing cells | mesohyl (amoebocytes, porocytes, sclerocytes) |
| Inner collar-cell layer | drives the current and catches the food | collar cells | choanoderm (choanocytes) |
Your study deck uses the second set of names — pinacocytes, amoebocytes, choanocytes, ostia, osculum, spongocoel. They mean the same things. Use the booklet's words in the booklet's exam; recognise the others when you meet them.
Spicules — Skeleton, Armour and Fingerprint
Spicules are flexible or sharp, rigid fibres made by spicule cells. They form a wire-like lattice in the mesenchyme — a scaffold on which the inner and outer layers hang. When a sponge dies, the soft cells rot away and only this lattice remains. They come in three materials. Tap each one.
A lattice to hang on
The mesenchyme with its spicules provides the framework around which the inner and outer layers form. It gives the sponge both structural support and flexibility in moving water.
Spiky and bright
The dermis and spicules protect the sponge. Calcium carbonate and silica spicules have sharp tips that poke out beyond the dermal cells, making the sponge spiky to small grazers and very unpleasant to eat. Many are brightly coloured as a warning.
A fingerprint
Shape is only a rough guide to a sponge's species. Specialists confirm it from the spicules, because calcium carbonate and silica spicules often have shapes specific to one species. CAPS: shape and skeletal material are used to classify Porifera.
The Sponge's Life Patterns
The nine life patterns from Topic 28, solved the sponge way. This is the table your booklet asks you to build.
Reproduction — two ways
Gametes and a swimming larva
Gametes — sex cells with half the adult's chromosome number — form in the mesenchyme. Eggs are fertilised and then released. The multicellular larvae swim for a few days, then settle and crawl to a favourable substrate, where they attach for life. Some sponges are hermaphroditic, producing both eggs and sperm. That swimming larva is the only stage of a sponge's life that moves about — and the only way a sponge can reach a new rock.
Fragmentation and budding
Fragmentation: the sponge breaks into smaller parts, or splits down the middle, and each piece grows into a new individual — a storm that rips a sponge apart can multiply it. Budding: smaller individuals grow from the base of the adult and then break away. Both make genetically identical copies.
Sponges on Our Shore — and Why They Matter
Local connection — look under the boulders
Sponges are easy to walk past. At low tide in a Cape Recife or Sardinia Bay rock pool, roll a boulder over gently and look at its shaded underside: the thin orange, yellow, green or grey crusts dotted with tiny holes are encrusting sponges — like the breadcrumb sponge photographed in a Western Cape rock pool in your booklet. Look closely and you can sometimes see the larger oscula. Below the tide, on Algoa Bay's reefs and in the deep, they grow into fingers, fans, vases and great lumps, alongside the vagabond sponge the booklet shows from De Hoop. Then roll the boulder back exactly as you found it — the sponges and everything else under it need the shade.
The practical sheet turns this into a proper rock-pool study, as CAPS asks — including watching which way the water flows.
Why sponges must be conserved
They clean the water. Filtering almost everything out of what they pump, sponges keep water clear and recycle nutrients for the whole reef. They are homes. Their canals and cavities shelter shrimps, worms, brittle stars and small fish. They are slow. Many grow very slowly, and some deep-sea glass sponges are thought to be centuries or even thousands of years old — a trawl net can destroy in seconds a sponge garden that took longer than human history to grow. They are medicine chests. Sponges defend themselves chemically as well as with spicules, and those chemicals have given us medicines: a cancer drug used since the 1960s, cytarabine, was developed from compounds first found in a Caribbean sponge, and another, eribulin, from a Japanese one.
In the Lab — the CAPS Investigations
CAPS asks for "photographs, slide show or videos", a "study [of] live examples (aquarium or rock-pool study)", and — the one to be careful with — "isolate spicules for study under microscope, by dissolving sponge fragments in nitric or sulphuric acid". Strong acids are school-laboratory chemicals, never kitchen ones; the practical sheet supplies those results instead, and asks what the acid would really do.
Four investigations · one on the shore, every one with supplied data
- 🌊 shore The rock-pool sponge study — find, describe and photograph sponges under boulders, and use a drop of food colouring to see which way the water flows. Described sponges supplied.
- 🔴 lab 🟢 desk Spicules — the CAPS acid method, made safe — supplied spicule measurements and test results: identify the material, and work out which spicules would survive nitric acid.
- 🟢 desk How much can a sponge filter? — from an osculum's size and the speed of its jet, calculate the flow, and how long a sponge-covered rock pool takes to be filtered.
- 🟢 desk Label it and tabulate it — the booklet's body-wall labelling question, a drawn collar cell, and the six-heading adaptations table.
→ Open the Topic 29 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. The first twelve are the booklet's examinable definitions, in its own words; the last three are terms from your study deck worth knowing.
A cell capable of moving like an amoeba and/or engulfing food by means of phagocytosis.
Attached to or living in close proximity to the seabed.
A form of asexual reproduction in which smaller individuals grow from the base of the adult and then break away.
Allowing outward flow of the respiratory current from the organism — like a sponge's osculum.
Whip-like structures used by microorganisms to swim; such cells can be referred to as flagellated cells. (Correct spelling: flagella; one flagellum.)
A form of asexual reproduction in which parts of the adult animal break into smaller parts or split down the middle into two or more pieces, each of which grows to be a new individual.
Sex cells with half the chromosome number of the adult, which fuse to become a zygote.
Able to produce both male and female gametes.
A jelly-like layer between the outer dermal and inner collar cell layers of a sponge, containing scattered cells and spicules.
Flexible or sharp, rigid fibres that provide structural support and protection for sponges. Made of calcium carbonate or silica.
The main component of the skeletal framework of certain sponges — a flexible protein fibre; the natural bath sponge.
The surface to which a sessile organism attaches itself.
A cell with a flagellum surrounded by a collar of fine projections; it drives the current and traps food. Also called a choanocyte.
The large exhalent opening through which water leaves a sponge. Plural: oscula. The small inlet pores are ostia.
The central cavity of a sponge, between the pores and the osculum. Not a gut — no digestion happens here.
General terms — and one that clashes
Dermal to do with the skin · elevated raised · lattice a structure with a criss-cross pattern · prevalent common to a particular place · silica a glass-like substance.
⚠️ Dermis. The booklet's general term defines the dermis as "the lower or inner layer of the two main layers of cells that make up the skin" — which is the dermis of vertebrate skin. But in its sponge text it uses "dermis" for the sponge's outer layer. In a sponge question, "dermis" and "dermal layer" mean the outer layer. And spongin: the booklet calls it a kind of spicule, but it is a flexible protein fibre, not a mineral spicule — the reason bath sponges are soft.
Quick Check
Question 1
Well done!
That is the first branch. Next: the second — animals with true tissues, stinging cells and radial symmetry.