ARGULUS - Fish Lice

Nice little creatures that resemble small crabs crawling all over your fish.  Fish lice are not insects, as their name suggests, nor are they related to the louse that infest humans or animals.

Argulus is a branchiuran crustacean, related more closely to shrimp and crabs. Like other crustaceans, argulids must molt or shed their shell periodically to grow and mature.

Argulus is a common parasite of ornamental fish, particularly koi and goldfish. Approximately 100 species are recognized, but most show little host specificity. The majority occur in freshwater environments, although marine species exist.

Fish lice are usually only a problem in outdoor ponds rather than aquariums, and are only brought into aquaria on wild or pond-raised fish. Argulids are extremely irritating and infested fish rub on submerged objects or bottom substrates in an attempt to dislodge the parasite from their skin – affected fish flash and jump, dive to the bottom of a tank and roll onto their side.  The fish will not eat, and may hide and lose their color due to all of the stress involved with a fish lice infestation.

The feeding activities of argulids are highly damaging to fish. The fish louse possesses a long stylet that pre-digests the fish tissue and musculature into food for itself.  Heavy infestations typically result in gradual loss of physical condition and weakening of the host to such an extent that it becomes susceptible to secondary bacterial infections, especially where the argulid injected the fish with it's stylet. Argulids are also capable of transmitting certain viruses, bacteria, and blood parasites as they feed.

The fish louse is visible to the naked eye, and has a broad, flat,oval-shaped body, with a shell and four pairs of swimming legs. They bear some resemblance to a small brown crab. Argulids attach by a pair of hooks and two large anterior suckers, but remain free-swimming throughout their life. The parasite may periodically release itself from one fish and swim to another, but cannot live for extended periods off of a fish host.

The adult female leaves the fish host to lay her eggs on submerged objects or plants. Eggs hatch and must find a host within a few days or they will die. After a series of molts the adult stage is reached and the cycle begins again. The entire life cycle takes 40 to 100 days depending on the water temperature.

Outbreaks can occur from spring through fall at moderate to warm water temperatures, but development is discontinued below 60 degrees Fahrenheit.  So in winter, your fish are safe from these parasites, but may awaken in springtime to something else.

Outdoor ponds can be quite a bit of work, and frustrating at times.  Do not forget that all of those birds drinking water out of the pond...may be bringing bacteria and parasites with them also.