Acropora-eating flatworm guide

Common name: Acropora-eating flatworm
Scientific name: Prosthiostomum acroporae
Category: Aquarium pest and hitchhiker
Group: Coral-eating polyclad flatworms
Environment: Marine reef aquarium pest associated with Acropora
Origin: Known from infested Acropora in reef aquaria; natural geographic range remains incompletely resolved
Care level: Pest management
Adult size: About 4.0–6.1 mm at sexual maturity depending on temperature; dependable maximum adult size is unknown
Expected lifespan: Management timeline is temperature-dependent: eggs averaged about 9–26 days to hatch from 30–21 °C, hatchlings survived up to 9 days without Acropora, and minimum egg-to-egg generation time was about 38 days at 27 °C and 64 days at 24 °C.
Minimum housing: Not a display organism — isolate affected Acropora in a separate stable treatment or quarantine system
Temperature: Reproduction documented from 24–30 °C
pH: Match the quarantined Acropora system, normally about pH 8.0–8.4; pH change is not a safe treatment method.
Hardness / minerals: Unknown — maintain normal stable reef alkalinity, calcium and magnesium for the coral host
Salinity: Marine reef salinity; exact biological limits unknown
Lighting: Match the needs of the quarantined Acropora host
Placement / zone: On Acropora tissue and skeleton; eggs commonly occur on exposed dead skeleton
Filtration: Use a completely separate, mature quarantine filtration system with no shared water, tools or wet media. Protect biological filtration from any treatment and prevent flatworms, eggs or fragments passing through returns.
Aeration: Maintain strong gas exchange suitable for the host Acropora, especially during handling or any validated treatment. Never assume a treatment is coral-safe because the flatworms tolerate it.
Maintenance: Inspect every Acropora surface, exposed skeleton, base and mount on a repeated schedule; physically contain removed worms, egg masses and fragments; clean dedicated equipment; and continue surveillance beyond one apparent clean inspection.

Identity and search names

Species-level pest profile for Prosthiostomum acroporae. Feeding scars, tissue loss and flatworm-shaped outlines are warning signs, but confirmed diagnosis requires finding the worm or egg masses and excluding other Acropora pests and husbandry damage.

Search names: AEFW, Amakusaplana acroporae

Environment and housing

Treat every affected colony, fragment, plug, rack and shared tool as potentially contaminated. Use a separate stable coral quarantine system.

Social structure: Parasitic or predatory coral pest

Feeding and nutrition

Feeding ecology: Feeds on living Acropora tissue

Feeding routine: Not applicable — this organism must not be intentionally maintained or fed

Breeding, reproduction or spread

Egg clusters are deposited on bare skeleton. At 27 °C eggs hatch in about 11–12 days, sexual maturity occurs around 35 days and the minimum generation time is about 38 days; development is slower at 24 °C.

Parental care: None

Detailed care and app-ready planning

Identity scope: Species-level pest profile for Prosthiostomum acroporae. Feeding scars, tissue loss and flatworm-shaped outlines are warning signs, but confirmed diagnosis requires finding the worm or egg masses and excluding other Acropora pests and husbandry damage.
Adult size and space: This is not a display animal. Management space means a separate stable coral quarantine system large enough to isolate every exposed Acropora and to inspect the whole colony, base and mount without transferring water or debris.
Social structure: An infestation is a reproducing population across host corals, egg masses, loose fragments and connected wet equipment. One visible worm does not define the population size.
Swimming zone or placement: Adults remain cryptic on Acropora tissue and skeleton; egg clusters are commonly placed on bare skeleton. Hatchlings can crawl and swim between nearby colonies or connected systems.
Feeding ecology: Obligate Acropora predator that consumes coral tissue and leaves characteristic pale feeding scars while often remaining camouflaged against the host.
Feeding plan: Not applicable as husbandry. Remove the pest from captive Acropora systems and support the coral host; never deliberately feed, culture or distribute it outside authorised research.
Water stability: Keep the host coral in stable reef conditions during quarantine. Temperature changes alter pest development but should not be used to push Acropora outside safe limits.
Habitat structure: Use a bare, inspectable quarantine arrangement with removable racks and no porous shared décor. Keep each fragment contained and eliminate uninspectable transfer paths.
Compatibility plan: Treat all Acropora, attached bases, frag plugs and wet equipment from an exposed system as potentially contaminated. Do not move them to clean coral systems until a complete repeated-inspection program is passed.
Breeding or reproduction plan: Prevent reproduction and dispersal. Adults lay clusters containing multiple egg capsules on bare skeleton; warmer water shortens development, so repeat timing must be based on actual system temperature and continued until newly hatched worms cannot mature and lay again.
Quarantine: Quarantine all incoming Acropora separately with dedicated equipment. Inspect under strong light and magnification, including underside, base and dead skeleton; dips can dislodge mobile stages but may not kill or expose every egg mass.
Welfare checks: Monitor host tissue, colour, polyp extension, feeding scars, exposed skeleton, egg masses and stress from handling or treatment. Stop any intervention that is causing greater coral damage than the pest-control benefit.
Biosecurity and release: Never release flatworms, eggs, coral fragments, aquarium water, treatment water, filter sludge or wet equipment. Seal and lawfully dispose of removed material so it cannot reach drains, waterways, aquaculture or another aquarium.
Main risk: A false clean result after one dip or visual check allows hidden eggs or juveniles to restart the infestation and spread through shared systems.
Adult projected load: Projected load is infestation pressure, not aquarium stocking load. More host surface, warmer water and connected systems allow faster population growth, greater coral tissue loss and more contaminated equipment.
Filtration: Use a completely separate, mature quarantine filtration system with no shared water, tools or wet media. Protect biological filtration from any treatment and prevent flatworms, eggs or fragments passing through returns.
Aeration: Maintain strong gas exchange suitable for the host Acropora, especially during handling or any validated treatment. Never assume a treatment is coral-safe because the flatworms tolerate it.
Maintenance: Inspect every Acropora surface, exposed skeleton, base and mount on a repeated schedule; physically contain removed worms, egg masses and fragments; clean dedicated equipment; and continue surveillance beyond one apparent clean inspection.
Overridable warnings: Non-overridable: never return exposed Acropora to a clean system solely because one dip or one inspection found no worms. A specialist may alter the inspection or treatment method, but not the requirements for egg control, repeated surveillance, coral safety, equipment separation and no release.
Hosts and impact: Confirmed host association is Acropora. Feeding scars, pale bite patches, tissue loss, reduced colour and eventual colony death can occur.
Detection: Inspect branch undersides, bases, exposed skeleton and plugs under strong white light. Turkey-baster flow or a coral-safe diagnostic dip may reveal mobile flatworms; inspect separately for eggs.
Life cycle: At 27 °C eggs hatch in about 11–12 days, sexual maturity occurs at about 35 days and minimum generation time is about 38 days. At 24 °C the minimum generation time is about 64 days.
Containment: Immediately isolate affected Acropora and stop sharing water, tools, racks, plugs and hands between systems without disinfection.
Physical removal: Remove mobile flatworms with coral-safe dips or directed flushing, scrape or remove egg-bearing dead skeleton where this can be done without unacceptable coral injury, and discard contaminated plugs when practical.
Repeat schedule: Repeat inspection and removal frequently enough to catch hatchlings before reproduction. The exact schedule must match temperature, treatment method and coral tolerance; continue beyond one full life cycle.
Treatment limits: No single universally safe treatment eradicates all life stages. Many dips affect mobile worms but not eggs. Do not dose a display aquarium with unverified chemicals.
Coral support: Maintain stable temperature, salinity, alkalinity, calcium, magnesium, oxygen and flow. Reduce additional stress while preserving enough light and circulation for the host.
Equipment hygiene: Use dedicated tools where possible. Clean, disinfect, rinse and fully dry equipment before reuse in a clean system.
Disposal and release: Seal removed pests, egg-bearing material and contaminated disposable items before disposal. Never release aquarium organisms or water outdoors or into drains.

Compatibility, welfare and risk

Compatibility: Incompatible with Acropora; do not transfer affected corals, water, plugs or tools to clean systems.

Welfare: Protect the coral host: minimise handling stress, maintain stable chemistry and oxygen, and stop treatment if tissue damage is worse than the pest damage.

Main warning: Adults are cryptic and camouflaged, while egg clusters can survive treatments that remove mobile flatworms. A single dip is not an eradication plan.

Biosecurity and release

Quarantine all Acropora and wet equipment. Do not sell, trade or move affected stock. Disinfect tools and dispose of pests and contaminated material so none can reach drains or waterways.

Acropora-eating flatworm