5 Curiosities About the Carp Louse

Written and verified by the biologist Georgelin Espinoza Medina

Last update: 18 November, 2023

The carp louse is a parasite that, as its name suggests, affects carp or Cyprinus carpio, in both its wild and ornamental forms. It’s also called freshwater fish louse, because it can harm other species (such as thornyheads or perch, among others), even certain amphibians. It is small, but it can be seen with the naked eye.

It belongs to the crustacean group. In particular, to the Arguloida order, the Argulidae family, and the Argulus genus. It’s of great economic importance because it parasitizes specimens of interest in fish farming. Read on to the end and find out some curious facts about this type of arthropod.

What does the carp louse look like?

This louse has a segmented body, and its morphology is ovoid and flattened. It has an oval head with 4 antennae, 4 maxillae, 2 mandibles, and a pair of compound eyes. In addition, it has a thorax with appendages for swimming and a short abdomen with two lobes. The appendages have long bristles.

The head is covered by a carapace, which in some types of argulids may extend into the thorax or abdomen.

The antennae have hooks to aid in attachment. Likewise, the second pair of maxillae has hooks at the distal end, although their appearance is more like appendages. The first pair of maxillae is specialized, like suction discs or cup-shaped suction cups to adhere to the host. It also has a hollow, retractile stylet that pierces the skin to suck its food, mucus, epithelial cells, and blood.

It’s small and generally doesn’t exceed 10 millimeters (0.4 inches). However, some specimens are larger. For example, Argulus foliaceus can reach 15 or 22 millimeters in length (0.6 to 1 inch). As for its coloration, we can say that it’s variable, although it’s usually transparent with greenish tones.

Curiosities about the carp louse

Freshwater fish lice cause considerable damage, particularly when there’s a large group of them. Here we’re going to bring you 5 curious and interesting aspects of these dreaded little crustaceans.

1. Males are smaller than females

Although the sexes in this genus are separated, that is, there are individuals of both types (dioecious), males are shorter than females. Females gain size, which increases their fertility. On the other hand, male lice sacrifice this characteristic in order to invest energy in the search for a mate. In this way, they guarantee reproduction.

In a study conducted in Uruguay, in 2017, a population of these lice was found in catfish. It was possible to determine the measurements between males and females. Thus, the male members had an average of 3.8 millimeters, with a range that went from 2 to 5.7. In the case of the females, they acquired higher figures, with an average of 4.1 and a maximum value of 6.4.

2. They are non-permanent parasites

Another interesting fact about these creatures is that they’re non-permanent parasites. In addition, the adults periodically leave the host fish to swim freely. On the one hand, the females leave them after mating in order to lay their eggs, while the males must do so in order to find a mate.

These lice can survive for some time away from their hosts. It’s estimated that this period of time in the species A. foliaceus is about 15 days. Although they can only be at their full capacity to search for a new host for half of that period.

3. Each female can lay up to 200 eggs

Carp lice copulate on the hosts. After this process, the female, with her thorax full of eggs, detaches to lay her eggs. Laying can take place on substrates such as rocks, aquatic vegetation, or flat surfaces.

Fertility is high, as females can lay up to 200 ovoid-shaped eggs, 0.3 to 0.6 millimeters long. They also cover them with a gelatinous mass that helps them stick to the substrate. In this way, they remain grouped and quite compact. The hatching time of the larvae varies with water temperature.

4. There’s a gland at the base of the stylet

The stylet of the carp louse is delicate and hollow. Apart from its power to pierce, it’s equipped with a gland at its base. This is quite small, but it’s able to secrete a liquid to help with digestion. It also has anticoagulant action, which means that it prevents the blood from clotting. This substance comes out of one of the two openings at the tip of this structure.

5. Carp lice cause a loss in body condition

Most parasites attach themselves to the integument. However, they also invade the gills or the inside of the mouth. Although they cause higher mortality in younger fish, damage to adults can be considerable, depending on the size of the fish and the number of parasites.

They cause loss of general body condition, decreased growth rate, weakness, and behavioral changes. They also cause damage to the integument, which may result in some alterations in the usual coloration of the fish. This also leads to ulcers. This damage to the skin is very dangerous, as each wound can serve as a gateway to other pathogens, complicating their condition.

As you can see, the carp louse is an insect that’s very well prepared for its life at the expense of others, with suckers, hooks, and a piercing stylet. In addition, it injects digestive and anticoagulant substances, which also harm the host. These weapons turn it into a bold and effective parasite that you certainly don’t want to have on your koi or any other fish.