Eliminating a Resistant Acropora-Parasitic Copepod with High-Dose Milbemycin Oxime: A Case Study

We wanted to document our experience treating what appears to be an uncommon and potentially undescribed Acropora-parasitic copepod. Very little information is available on similar parasites, and standard red bug treatment protocols were ineffective in our case.

The parasite

These were not the classic Acropora bugs commonly reported in reef aquariums, such as red bugs (Tegastes and Schedomolgus), black bugs (Parategastes), or copepods belonging to Prionomolgus or Anchimolgus.

Observed characteristics:

• Visible, although only just, to the naked eye as tiny white to light-grey specks.
• Translucent white or grey under the microscope, with harpacticoid-like morphology.
• Observed only on Acropora colonies, including A. kenti, A. loripes, and A. millepora. They were not observed on other coral genera.
• Found primarily on the smooth coenosteum between Acropora polyps.
• Concentrated around colony bases during daylight but ventured farther across the colonies after dark.
• Actively crawled across coral tissue and appeared more active at night.
• Attached firmly to the coral when disturbed and could not easily be dislodged with a jet of water.
• Infestations were associated with noticeably reduced polyp extension, pigment loss, bleaching, and, in some cases, colony mortality.

 

The copepods were visible to the naked eye as tiny white specks moving across the coral surface.

 

Based on published descriptions, the closest morphological match we could find was Alteuthellopsis, or a closely related harpacticoid copepod. However, this remains a tentative identification.

Specimens have since been sent to a taxonomic specialist for detailed morphological examination and formal identification.

We also sequenced DNA from the specimens. The resulting sequences did not closely match any known species represented in our internal database or current public databases. This suggests that the species may not yet be represented in available sequence databases and raises the possibility that it could be undescribed. However, formal morphological and taxonomic analysis will be required before any conclusion can be made.

The affected aquarium

The affected aquarium was an SPS-dominant mixed reef system maintained at relatively low nutrient:

• Nitrate: approximately 5 mg/L
• Phosphate: approximately 0.05 mg/L

Microbiome testing indicated a healthy and balanced bacterial community. No other parasites or pathogens were detected using the testing methods applied.

 

Medication

Milbemycin oxime.  The veterinary tablet formulation used also contained praziquantel and inactive tablet ingredients. Milbemycin oxime was considered the primary compound responsible for the observed effects on the copepods.

 

Initial treatment options

Dipping affected corals with BioScrub coral dip successfully eliminated the copepods from treated frags. However, coral dipping provided only temporary relief. Because copepods may remain elsewhere in the aquarium, treated frags could become reinfested after being returned to the system.

Coral dips cannot be used as a whole-system treatment. A commonly reported whole-tank treatment for parasitic Acropora copepods is milbemycin oxime, historically sold under the brand name Interceptor.

Important: Milbemycin oxime is highly toxic to crustaceans. Shrimp, crabs, lobsters, and any other valuable crustaceans should be considered at high risk and should be removed before the treatment. Amphipods, copepods, and other small crustaceans left in the aquarium will be severely affected or eliminated.

 

Treatment 1: Standard red bug concentration

We began with a concentration commonly recommended for treating classic red bugs:

• Milbemycin oxime: 0.015 mg/L
• Equivalent concentration: 15 µg/L
• Exposure time: 12 hours
  

During treatment

• Activated carbon was removed.
• The UV steriliser was turned off.
• Strong aeration was maintained.
• The skimmer cup was removed, allowing the skimmer to provide aeration while overflowing back into the aquarium.
• Acropora colonies were regularly blasted with a turkey baster to help dislodge paralysed copepods.

After 12 hours, a large water change was performed and activated carbon was added to reduce the remaining medication concentration.

Result

• Hermit crabs that could not be removed died.
• Amphipods and free-living grazing copepods were no longer visible on the aquarium glass or in the refugium. As expected, these beneficial crustaceans were affected by the treatment.
• The parasitic copepods remained fully active and appeared unharmed.
• No obvious signs of stress or harm were observed in the other aquarium inhabitants.
• Increased skimmate production

 

Treatment 2: Increased concentration

The display aquarium was then treated at approximately 2.6 times the standard concentration:

• Milbemycin oxime: 0.040 mg/L
• Equivalent concentration: 40 µg/L
• Exposure time: 12 hours

Result

The parasitic copepods remained alive and mobile. They continued to cling firmly to the coral when blasted with water.

It became clear that these copepods were considerably more tolerant of milbemycin oxime exposure than typical Tegastes red bugs.

 

Controlled small scale experiment

Before increasing the display-tank concentration again, we performed a controlled small-scale trial. The objectives were to determine whether the copepods were sensitive to milbemycin oxime and to identify the minimal effective dose.

Experimental conditions

• 2,000 mL of aquarium water per beaker
• Controlled temperature
• Gentle aeration for oxygenation and water movement
• Three infested Acropora frags per condition
• Each concentration tested in duplicate using two separate beakers

Milbemycin oxime concentrations

• 1×: 15 µg/L, the standard red bug concentration
• 5×: 75 µg/L
• 10×: 150 µg/L
• 15×: 225 µg/L

 

Results after 12 hours

1× concentration: 15 µg/L

All copepods remained motile and firmly attached to the corals. This was consistent with the result observed during the initial display-tank treatment.

5× concentration: 75 µg/L

Approximately 40% of the copepods showed signs of paralysis and could be blasted from the coral. The remaining copepods appeared largely unaffected.

10× concentration: 150 µg/L

All observed copepods became non-motile and could easily be blasted from the coral, indicating severe paralysis. Many became trapped in coral mucus after being dislodged.

15× concentration: 225 µg/L

The result was similar to the 10× concentration. All observed copepods became non-motile and were easily dislodged.

 

After treatment, paralysed copepods were unable to cling to the coral and became trapped in mucus after being blasted from the coral surface.

The Acropora frags survived all tested concentrations. No coral mortality was observed during the 12-hour exposure period or after the frags were returned to the display aquarium. This does not exclude the possibility of sublethal or delayed effects that were not apparent during the observation period.

 

Copepod recovery assessment

Milbemycin oxime causes neuromuscular disruption in susceptible arthropods, resulting in paralysis and potentially death. If the concentration is too low, partially paralysed copepods may recover after the medication is removed.

Even a small number of surviving copepods could continue their lifecycle and reproduce, resulting in treatment failure.

Recovery testing was therefore conducted on copepods collected from the 10× and 15× treatments. The paralysed copepods were rinsed several times in clean seawater to reduce residual medication and were then observed for 24 hours in clean seawater.

10× concentration recovery test

Approximately 99% of the copepods remained motionless.

A small number showed occasional twitching of the legs or antennae, but no crawling, attachment, or normal movement was observed. This was consistent with irreversible paralysis or mortality under the conditions tested.

15× concentration recovery test

All copepods remained motionless throughout the 24-hour recovery period. No normal movement or attachment behaviour was observed.

 

Whole-tank treatment

Based on the small scale dose trial, the minimum effective dose was 10x dose (150 µg/L) Milbemycin oxime concentration, and was used to treat the display aquarium.

There remained a risk that the increased concentration could harm non-target invertebrates. We reviewed the known mode of action and available treatment reports before proceeding, but off-target effects could not be ruled out.

During treatment:

• Activated carbon removed.
• UV steriliser turned off.
• Strong aeration maintained by removing skimmer cup and allowing the skimmer to overflow into the tank.
• Acropora colonies regularly blasted with a turkey baster to help dislodge affected copepods.

Following treatment

• A large water change was performed.
• Fresh activated carbon was added.

 

Results

Seven days after treatment:

• No visible adult copepods on any inspected colonies.
• Polyp extension had noticeably improved.
• No obvious adverse effects were observed in the corals or fish.

To maximise the chance of eradication, a second treatment at the same concentration (0.150 mg/L; 150 µg/L) was performed one week later.

The second treatment was intended to target newly hatched juveniles, hidden individuals, or copepods that may have escaped the first treatment.

 

Livestock effects

Animals observed to be affected

• Hermit crabs: Rapid mortality.
• Amphipods, free-living copepods, and other small crustaceans: Populations were severely affected or eliminated, as expected.
• Red sea star (Fromia indica): Developed slow, progressive loss of movement after the high-dose treatment and eventually became non-responsive. The precise mechanism was not determined, but the timing was consistent with a possible treatment-related adverse effect.

Animals observed to tolerate the high-dose treatment without obvious adverse effects

• SPS corals
• LPS corals
• Soft corals
• Mushroom corals
• Zoanthids
• Fish
• Snails (Turbo, Trochus, keyhole limpet)
• Tuxedo urchin (Mespilia globulus)
• Black sea cucumber (Holothuria leucospilota)

Aquarium pests that survived the high-dose treatment

• Vermetid snails
• Spirorbid worms

These observations come from a single aquarium and should not be interpreted as evidence that these concentrations will be safe for every species or system.

Conclusions

Based on this case:

• These Acropora-associated copepods appear substantially more resistant to milbemycin oxime than classic Tegastes red bugs.
• Standard Acro-bug treatment concentrations (15–40 µg/L) were ineffective.
• Performing a small-scale dose-response trial before treating the display aquarium was extremely valuable. It helped identify an effective concentration while reducing unnecessary risk to the aquarium.
• The lowest tested concentration that produced complete loss of motility and  eliminate the visible population was 150 µg/L.
• High-dose treatment still caused serious off-target effects in crustaceans and was potentially harmful to at least one sea star species.

We'd be very interested to hear from anyone who has encountered similar Acropora copepods, particularly those with microscope images, treatment observations, or preserved specimens.

Please feel free to contact us for further information.

 

Disclaimer

This article is intended to share observations and results from a single aquarium trial. It should not be considered a treatment guide or recommendation.

Every aquarium is different, and veterinary medications can cause serious effects in non-target organisms, including beneficial and valuable invertebrates. The treatment described here involved an experimental concentration substantially higher than commonly reported red bug treatments and resulted in mortality or severe adverse effects in some aquarium animals.

Anyone considering the use of a veterinary medication in an aquarium should first seek advice from an aquatic veterinarian or another suitably qualified aquatic animal health professional.

The information presented here is provided for educational and scientific discussion purposes only.