Sponge Spicules Under the Microscope

Sponges have no bones, but many are held together by thousands of tiny mineral pieces called spicules. Under the microscope they look like glass needles, stars, anchors, hooks and spiked balls. They are also one of the easiest “hard part” specimens to prepare at home: the soft tissue dissolves in household bleach, and the spicules are left behind.

What sponge spicules are

Spicules are small skeletal elements made inside a sponge’s body. Depending on the sponge, they may be the whole skeleton or work alongside a fibrous protein called spongin. The bath sponges once harvested commercially have spongin skeletons with few or no spicules, so a natural bath sponge is often a poor specimen.

What are spicules formed by?

Spicules are made by specialised cells called sclerocytes. These cells lay down mineral, either silica or calcium carbonate depending on the type of sponge, around an organic core or within a membrane-bound space. They build the spicule into a shape characteristic of the species. Because those shapes are so consistent, spicules are one of the main features used to identify sponges.

Two kinds of mineral: silica and calcium carbonate

Siliceous spicules are made of hydrated, glass-like (amorphous) silica. They occur in most demosponges, which include most freshwater sponges, and in the glass sponges (hexactinellids). They look clear and glassy and often have a fine central canal running along their length.
Calcareous spicules are made of calcium carbonate and occur in the calcareous sponges (class Calcarea). These are marine and often small and pale.

Two simple tests can help tell them apart:

Acid test. Add a drop of vinegar or another dilute acid at the edge of the coverslip. Calcareous spicules slowly dissolve, often with tiny bubbles. Siliceous spicules are not affected by weak household acids.
Polarized light. Between crossed polarizers, calcite spicules are strongly birefringent and light up brightly against the dark background. Amorphous silica stays dark or nearly so, because it doesn’t strongly rotate polarized light. Two pieces of polarizing film, one under the slide and one over the eyepiece, are enough for a rough version of this test.

Megascleres and microscleres

Sponge spicules are often divided by size:

Megascleres are the larger structural spicules. They often measure from tens of micrometres to well over a millimetre, and you can see them at 40x to 100x.
Microscleres are much smaller spicules, often only a few to a few tens of micrometres. They are usually scattered through the tissue rather than forming the main framework.

So when people ask “what are micro spicules?”, the answer is usually microscleres. You’ll need 400x to see their shapes properly.

Common spicule shapes

Spicule names describe the number of axes and the shape of the ends:

Monaxons have a single axis. They include oxeas (pointed at both ends), styles (pointed at one end, rounded at the other), strongyles (rounded at both ends) and tylostyles (with a knob at one end).
Triaxons have three axes. Glass sponge spicules are often six-rayed (hexactines), built on three axes at right angles.
Tetraxons have four rays, sometimes forming tripod-like triaenes.
Microsclere forms include stars (asters), C- and S-shaped sigmas, and anchor-like chelae.

Freshwater sponges (family Spongillidae) often produce gemmules, small resistant bodies that survive winter or drought. Their protective coat is lined with distinctive small spicules called gemmoscleres, which are important for identification.

Where to get spicules

Freshwater sponges grow as green, brown or yellowish crusts on submerged wood, stones and plants in clean lakes, slow rivers and canals. A small fragment is enough. Collect responsibly and only where permitted.
Beach-cast marine sponges or small fragments from aquarium rock (with the owner’s permission).
Marine and lake sediment. Spicules are common in many sediments, alongside diatoms and other microfossils.
Prepared slides from biological suppliers, often labelled “sponge spicules” or “Grantia spicules” (a calcareous sponge).
Some skincare products are marketed as containing sponge spicules. Their contents vary, so treat this as a curiosity rather than a reliable source.

How to prepare a spicule slide

The household bleach method dissolves the soft tissue and leaves the spicules behind.

Cut a small piece of sponge, a few millimetres across, and put it in a small glass container such as a test tube or a shot glass.
Cover it with household bleach (sodium hypochlorite). Work in a well-ventilated place, wear gloves and eye protection, and never mix bleach with acids or ammonia-based cleaners.
Wait. The tissue usually breaks down over anything from a few minutes to several hours, depending on size. You’ll see a fine whitish sediment collect at the bottom.
Let the spicules settle, then carefully pour or pipette off most of the bleach.
Rinse several times. Add clean water, let the spicules settle again (a few minutes to an hour), and pour off the water. Repeat several times until no bleach smell remains. Distilled water is best for the final rinse.
Transfer a drop of the sediment to a slide with a pipette.
View wet under a coverslip, or let it dry and make a permanent mount (see below).

Bleach doesn’t dissolve calcium carbonate, so this method works for both calcareous and siliceous spicules. Avoid acids at any stage if you want to keep calcareous spicules.

Viewing tips

Start at 40x to 100x to find megascleres, then go to 400x for microscleres and fine surface details such as spines.
Use contrast. Clear glass spicules in water can be faint. Close the condenser diaphragm partway, or try darkfield, which makes them sparkle against black.
Try polarized light to tell calcite from silica, and for attractive photos.
Choose your mountant thoughtfully. Spicules show up less clearly in a mountant whose refractive index is close to their own. Silica’s refractive index is roughly 1.45, so a mountant with a noticeably different index gives more contrast. For general methods, see our guide to permanent microscope slides, mounting media and sealing.
Measure them. An eyepiece reticle calibrated with a stage micrometer lets you record spicule lengths, which matters for identification.

What are two functions of spicules?

The two functions most often given are:

Structural support. Spicules stiffen the body and help the sponge keep its shape and its water channels open.
Defence. Sharp spicules are thought to make sponges less attractive to many predators.

Other roles have been proposed in particular groups. In some glass sponges, spicules may conduct light, and some anchoring spicules root sponges into soft sediment.

Spicules in other animals, including nematodes

The word “spicule” simply means a small, pointed, needle-like body, and it appears elsewhere in biology:

Soft corals, sea fans and sea pens contain calcium carbonate spicules (often called sclerites), frequently colourful and knobbly.
Sea cucumbers have microscopic ossicles in their skin, shaped like wheels, anchors, plates and tables. These are also used for identification.
Nematodes: in many male roundworms, spicules are a pair of hardened, cuticle-covered structures near the tail. They are used during mating to hold open the female’s reproductive opening and help transfer sperm. They aren’t mineral and have no connection to sponge spicules beyond the name. Their shape is often used to identify nematode species.

Safety

Siliceous spicules are tiny glass needles. Don’t rub them on your skin, and wash your hands after handling them. Keep them away from eyes and avoid creating dust from dried samples.
Handle bleach with gloves and eye protection in a ventilated area, and dispose of rinse water down the drain with plenty of water.
Label your tubes, since a settled spicule sample looks a lot like plain water.

If spicule hunting gets you interested in other “hidden” structures in ordinary materials, see our guide to what everyday things look like under a microscope.