Parasitic Crustaceans of Fish: Anchor Worm and Fish Lice
Anchor worm, fish lice and gill copepods are crustaceans, not worms. Because only some stages live on the fish, treatment must be repeated across the life cycle.
Several of the most visible parasites of pond and aquarium fish are not worms at all but crustaceans, relatives of crabs and shrimp. The three a hobbyist is most likely to meet are anchor worm, fish lice and gill copepods. They share one practical feature that shapes every treatment: only certain life stages live on the fish, while eggs and larvae develop in the water.
The three you will meet
Anchor worm (Lernaea) is a parasitic copepod. The attached adult female loses her copepod form and becomes a rod- or sac-like structure, appearing as a barb-like attachment embedded in the skin or gills with egg sacs trailing into the water, and causing haemorrhage, anaemia and tissue destruction with frequent secondary infection. It is common in goldfish and other cyprinids.
Fish louse (Argulus) is a branchiuran with a dorsoventrally flattened body, two compound eyes and two prominent suckers, adapted for rapid movement over the skin. Rather than staying fixed, it attaches periodically to feed by inserting a piercing stylet, causing pinpoint haemorrhages, fin and scale loss, excess mucus, lethargy, erratic swimming and flashing. Gill copepods (Ergasilus) attach to and infest the gills.
Why single treatments fail
These parasites occupy specific stages of a complicated life cycle, so treatment must target multiple developmental phases. The Argulus cycle runs about 30 to 60 days depending on temperature, with eggs laid on hard surfaces and vegetation and larvae that must find a host within 2 to 3 days of hatching. The Lernaea cycle takes about 18 to 25 days at 25 to 30 C, and only adult females are attached while free-living nauplii develop in the water. A single dose kills only what is exposed at that moment, so treatment of the entire system, repeated at intervals, is necessary.
Treatment options and sourced doses
| Treatment | Dose (as published) | Source |
|---|---|---|
| Diflubenzuron (chitin synthesis inhibitor) | 0.03 mg/L once; retain treated water 28 days; 0.066 mg/L kills molting Lernaea stages | Merck; UF/IFAS FA185 |
| Potassium permanganate | Argulus: 10 mg/L for 30 min, or 1.3 mg/L twice over 3 days; Lernaea: 25 mg/L 30-min bath (kills larvae, adults may survive) | UF/IFAS FA184/FA185 |
| Salt | Lernaea (food fish): 4.8 g/L for up to ~30 days | UF/IFAS FA185 |
| Trichlorfon (organophosphate) | Argulus: 0.25-0.50 mg/L once a week for 4 treatments (now largely unavailable/restricted) | UF/IFAS FA184 |
Note that there are currently no FDA-approved drugs for Argulus, and diflubenzuron is a restricted-use pesticide. Large anchor worms can be removed individually from the fish with forceps, though this is often incomplete, and the wound should be cared for to limit secondary infection. For food fish, simply removing all fish for about 7 days breaks the Lernaea cycle, since larvae cannot survive without a host.