Pest sponges — boring sponge

Identification

Most of the animal hides inside the substrate. Surface signs are small round holes up to about 5 mm across in shell, skeleton or rock, with yellow to orange sponge tissue or wart-like papillae (inhalant bumps) peeking from the openings — sulfur-yellow in Cliona celata, orange-red in relatives such as Cliona delitrix. Broken or sectioned material shows a honeycomb of etched chambers. In its later massive phase the sponge erupts out of the substrate as a yellow-orange lobed mass that can exceed a meter across in the wild. In tanks, check clam shells, the undersides of rock and old coral bases.

Often confused with

Harmless sponges, especially pineapple sponges (white, barrel-shaped, in sumps and shaded spots, short-lived) and ordinary encrusting sponges that sit on the surface — the warning sign for a borer is tissue appearing inside holes and pits in shell or skeleton with only papillae showing, plus progressive hollowing. Also confused with coral-boring spionid worms, which leave paired tiny tubes and mud chimneys on the surface rather than sponge papillae inside etched chambers.

What it targets

Calcium carbonate itself: live rock, coral skeletons (including the bases of living colonies), Tridacna and other clam shells, oyster and snail shells and barnacle plates. The host mollusc is not eaten — its shell is weakened until it cracks, gapes or dies, and heavily bored reef framework crumbles.

How it spreads

Arrives hidden inside live rock, on clam shells and in the skeleton under coral colonies — the tunnels are already there before anything shows on the surface. Spreads by free-swimming larvae that settle on new carbonate, by local growth through connected tunnels, and by fragments when bored rock or shell is broken up in the tank.

Control (safest first)

Quarantine and inspection first: reject or hold rock, clam shells and coral bases showing papillae in pits or honeycombed skeleton. WetWebMedia's stance, citing Reef Invertebrates, is that a true boring sponge has no place in a long-term reef aquarium — prevention beats every cure below. Manual removal on removable items: scrub the shell or surface hard with a toothbrush or wire brush in a separate container of tank water, changing the water as it fouls (per WetWebMedia advice for a clam shell case), then rinse before return. Expect regrowth from tissue left inside the tunnels — a single scrubbing rarely finishes a borer established in rock. Air exposure and drying: many sponges tolerate air poorly, and keepers use brief air exposure plus wiping on clam and hermit shells as a knockdown; for loose infested rock, extended air-drying (a week or more, per WetWebMedia advice on a yellow encrusting sponge takeover) before reuse is the reliable version. Never air-expose a live clam longer than the few minutes needed to scrub its shell. Chip or cut away infested material: chisel out bored rock sections, cut coral well above bored skeleton, and discard heavily tunneled pieces and empty shells. Sealing small surface patches with epoxy can entomb shallow papillae but fails where tunnels run deep. Salt and brine tricks come from shellfish aquaculture, not reef practice: a six-minute saturated brine dip eliminated Cliona celata from cultured oysters without harming the oysters in the published study, and giant-clam hatcheries scrub shells and use a chlorine solution on shells (10 mL bleach per liter of fresh water in that guideline, applied to the shell only). Do NOT dose bleach or brine into a reef display — these are external shell treatments for aquaculture stock, flagged here so nobody scales them into a tank treatment. Predators are impractical in a reef tank: sea urchins (Arbacia) and sponge-eating nudibranchs such as Doriopsilla albopunctata (reported on Cliona californiana) graze surface sponge, but they cannot reach tissue sealed inside tunnels, and most spongivore fish are coral-risky.

Natural predators

• Urchins
• White-spotted doris nudibranch

Field notes

A WetWebMedia keeper bought a first crocea clam partly for the bright orange patches with holes across its shell, then read that a boring sponge could kill the clam and asked whether it was contagious and removable; the crew confirmed it can spread and advised thorough scrubbing of the shell in a pan of water outside the tank, with frequent water changes as debris came off. Another WetWebMedia case shows the ID trap: a keeper found light beige sponge plastered flush into every depression of a mushroom-covered rock in quarantine, smooth as putty, and worried it was a borer; the crew judged a true boring sponge very unlikely from that surface look — most flush beige sponges in quarantine turn out to be ordinary encrusters, and the honeycomb has to be inside the material before the pest label fits. Reef2Reef's sponge threads carry the frustration from the encrusting end of the family: a keeper who first thought a spreading sponge was cool was soon picking it off a hermit's shell and every new rock, with other posters advising a pick or scraper, a wipe and air-drying the shell, and warning that severed fragments are how it colonizes the next surface.