Ask any long-term SPS keeper which coral pest has cost them the most prized colonies, and Phestilla comes up with remarkable consistency. Unlike AEFW (the other major SPS killer) which targets acropora, these nudibranchs target the entire Montipora genus — which for many reef keepers is half the SPS collection. The damage pattern is distinctive: colonies lose tissue in a slow, creeping pattern that moves from one edge across the colony, leaving bare white skeleton behind. By the time the pattern is obvious, the population is usually laying eggs on every Montipora in the tank. Survival of the collection depends on how quickly you recognise what’s happening.

Identification — what does a Montipora eating nudibranch look like?

Adult Phestilla subodiosa are small (5–10 mm), elongated, soft-bodied slugs with distinctive cerata along the back. Colour is pale cream to pinkish, perfectly matched to the host.

The body is slender and tapered with a pair of rhinophores on the head, oral tentacles, and the signature aeolid cerata — finger-like projections running along the dorsum. In Phestilla, cerata contain branches of the digestive gland carrying partially-digested Montipora tissue, which gives the cerata a faintly brown or pinkish tinge that further helps blend with the host. The overall impression is a pale pinkish blob moving slowly across a Montipora surface — almost invisible unless you’re specifically looking.

The most reliable field sign is not the slug itself but the pink spiral egg ribbons they lay on the dead skeleton at the base of a Montipora colony, or on the plug or rock adjacent to the coral. Egg ribbons are 3–8 mm across, spiral-coiled, and bright pink to salmon-coloured — noticeably easier to spot than the adult slug. If you see pink spiral masses near a Montipora that is losing tissue, you almost certainly have Phestilla.

The second reliable sign is the tissue loss pattern on the coral itself. Phestilla feed in a creeping wave, starting at one edge of a plating Montipora and moving across the surface leaving bare white skeleton behind. Unlike bleaching (which affects the whole colony or is patchy), Phestilla damage advances as a clean front, often from the edges or the base inward.

Life cycle and reproductive rate

Phestilla reproduce rapidly under reef-tank conditions. A single pair can seed an outbreak across every Montipora colony in weeks.

Egg laying. Adults lay spiral ribbons directly on dead coral skeleton, usually at the base of the host colony. Each ribbon contains 100–400 eggs. A single adult can lay multiple ribbons over its lifespan.

Hatching. Eggs hatch in 5–7 days into tiny veliger-stage larvae. Juveniles are small enough to drift in water currents across the tank, settling wherever they encounter Montipora tissue.

Growth to reproductive age. Juveniles reach adult size and reproductive maturity in 10–14 days under reef-tank temperatures. This means a single pair can produce a new breeding generation every three weeks.

Population explosion. Starting from a single introduction, the population can grow exponentially for 4–8 weeks before visible tissue loss becomes obvious. By the time you notice damage, you are dealing with multiple generations of slugs and egg ribbons laid across every Montipora surface in the tank.

Phestilla vs AEFW — distinguishing SPS pests

Montipora-eating nudibranchs and Acropora-eating flatworms (AEFW) are the two major SPS killers. They look similar but require different treatments.

How to remove Montipora eating nudibranchs

Treatment requires moving affected colonies to quarantine, repeated dipping over 3–4 weeks, and manual removal of egg ribbons between dips. No single step works alone.