Branchiura

(Fish lice)

Phylum Anthropoda

Subphylum Crustacea

Class Maxillopoda

Subclass Branchiura

Number of families 1

Thumbnail description
Branchiurans are ectoparasites of fishes, mainly living in freshwater habitats; they have flattened bodies comprised of five limb-bearing segments; the head has well-developed carapace lobes

Evolution and systematics

The subclass Branchiura comprises just four genera placed in a single family, the Argulidae, and about 175 species.

Physical characteristics

Branchiuran fish lice have flattened bodies, which have a low profile when attached to their hosts. The body comprises a head of five limb-bearing segments and a short trunk divided into a thoracic region, carrying four pairs of strong swimming legs and a short, unsegmented abdomen. The head has well-developed carapace lobes, which are posterior extensions of the dorsal head shield that mostly cover the legs on both sides of the body and may extend further to cover the abdomen. These carapace lobes contain the highly branched gut caecae and have two specialized areas ventrally, which are traditionally referred to as "respiratory areas," but appear to be involved in regulating the internal body fluids. Anteriorly, on the ventral surface of the head lie the short antennules and antennae. Both are provided with claws and are important organs of attachment to the host. The distal segments of the antennules are sensory and carry arrays of short setae.

Branchiurans have a tubular sucking mouth equipped with rasping mandibles located at the tip of the mouth tube. In Argulus there is a retractable poison stylet located just in front of the mouth. This stylet is absent in Chonopeltis and Dipteropeltis. The maxillules are developed into powerful muscular suckers in the adults of all genera, except Dolops, which retains long clawed maxillules into the adult phase. The maxillae are uniramous (one branched) limbs with spinous (spine-like) processes on the basal segments and small claws at the tip. The four pairs of thoracic swimming legs are biramous and directed laterally. The first and second legs commonly carry an additional process, the flagellum, originating near the base of the exopod. The third and fourth legs are usually modified in the male and are used for transferring sperm to the female during mating. The abdomen contains the paired testes in the male and, in the female, the paired seminal receptacles, where sperm are stored until needed to fertilize eggs. The abdomen terminates in paired abdominal lobes separated by the median anal cleft, in which lies the anus and the minute paired caudal rami.

Distribution

Argulus species occur in freshwater habitats on all continental land masses. Species of Dolops exhibit a southern distribution, occurring in freshwater in southern Africa, South America, and Australasia (Tasmania). Chonopeltis species occur only in African freshwaters, while the sole species of Dipteropeltis is restricted to South America.

Habitat

Branchiurans are ectoparasites of fishes, but are occasionally reported from the tadpoles of amphibians. They live mainly in freshwater habitats, both running and static water, and may occur at high density in artificial water bodies such as reservoirs, ornamental fishponds, and fish farms. A few species of Argulus infest estuarine and coastal marine fishes, but they do not occur in oceanic waters.

Behavior

Only the behavior of Argulus is well known; little is known of the other genera. After taking a meal, a mature female Argulus will leave its host and begin to lay eggs in rows on any hard, submerged surface. The eggs are cemented to the substrate and abandoned. These eggs hatch into free-swimming larvae equipped with setose (bristly) swimming antennae and mandibles and rudiments of the maxillules, maxillae, and first two pairs of swimming legs. These larvae function as a dispersal phase and molt into the second stage, in which strong claws have replaced the setae on the antenna and the setose palp of the mandible is lost. Branchiurans are parasitic from the second stage onwards, but appear to leave the host and then find a new host at intervals throughout development. Changes during the larval phase are gradual, mainly involving the development of the thoracic legs and reproductive organs, except for the maxillule, which undergoes a metamorphosis around stage five, changing from a long limb bearing a powerful distal claw to a powerful circular sucker. This is one of the most remarkable transformations known for any arthropod limb.

Feeding ecology and diet

Branchiurans attach to the skin of their fish host and feed on its blood and external tissues. They have rasping mandibles, which scrape tissues into the opening at the tip of the tubular sucking mouth. In Argulus, the poison stylet is used to inject a secretion into the host. The secretion may contain digestive enzymes to begin to break up host tissues before ingestion. Paired labial stylets, lying within the opening of the mouth tube, are also secretory and may produce secretions with a similar pre-digestive function. Host blood is also taken and is digested within paired, lobate gut caecae that lie within the carapace lobes.

Reproductive biology

The sexes are separate and, in most branchiurans, males transfer sperm directly to the females using a variety of structures on the third and fourth thoracic legs. In Dolops, however, sperm are transferred in chitinous packages called spermatophores.

Conservation status

No species are listed by the IUCN.

Significance to humans

Branchiurans are important pests in fish culture facilities, mainly in freshwater facilities, but occasionally in marine fish farms.

Species accounts

List of Species

Fish louse

Argulus foliaceus

order

Arguloida

family

Argulidae

taxonomy

Argulus foliaceus Linnaeus, 1758, Europe.

other common names

German: Karpfenläuse.

physical characteristics

Abdominal lobes broadly rounded at tip; anal cleft less than half the length of abdomen. First to third legs, with darkly pigmented patches near base. Male with triangular process on posterior surface of leg two, directed towards base. Body length to 0.39 in (10 mm) for female and 0.35 in (9 mm) for male.

distribution

Europe, through to central Asia and Siberia.

habitat

Ectoparasitic; attaching to wide variety of freshwater fishes.

behavior

On hatching, the larvae swim actively in the water column for about 2–3 days, after which their infectivity decreases.

feeding ecology and diet

Feeding externally on host, will take host epidermis and blood.

reproductive biology

Eggs are laid in strings, of between two and six rows, containing up to 400 eggs; hatch after 25 days (at 59°F [15°C]), and development time is very dependent on temperature. Eggs within a string tend to hatch within 4–6 days. First larval stage lasts about six days, and molts occur at intervals of about 4–6 days until maturity.

conservation status

Not listed by the IUCN.

significance to humans

May occur as epizootic infestation in fish hatcheries and other facilities. Can cause severe mortality in cultured fish stocks and can transmit viral diseases such as spring viraemia between fishes.

No common name

Argulus japonicus

order

Arguloida

family

Argulidae

taxonomy

Argulus japonicus Thiele, 1900, China.

other common names

None known.

physical characteristics

Abdominal lobes acutely rounded at tip; anal cleft more than half the length of abdomen. First to third legs, without any darkly pigmented patches near base. Male leg two with rounded processes at either end of posterior margin. Body length to 0.35 in (9 mm) for female and 0.31 in (8 mm) for male.

distribution

Originally described from China, this species has spread into Europe, North America, Australasia, and Africa by the movement of fish stocks, particularly of koi carp, in the ornamental fish trade.

habitat

Ectoparasitic; attaching to freshwater fishes, especially on koi carp, goldfish, and other ornamental fishes.

behavior

Newly hatched larvae swim strongly towards light. May live 3–5 days off the host.

feeding ecology and diet

Feeding externally on host, will take host epidermis and blood.

reproductive biology

Eggs laid in multiple rows, in batches of up to 200, hatching after 15–30 days, depending on temperature. Larval development is similar to Argulus foliaceus, but fewer larval stages are reported.

conservation status

Not listed by the IUCN.

significance to humans

A pest species, particularly of ornamental fishes, that has become established on several continents after accidental introduction.

No common name

Dolops ranarum

order

Arguloida

family

Argulidae

taxonomy

Dolops ranarum Stuhlman, 1891, Africa.

other common names

None known.

physical characteristics

Maxillules forming large paired claws. Lacking preoral poison spine and lacking hooks on the antennules. Body length 0.23–0.27 in (6–7 mm).

distribution

Sub-Saharan Africa.

habitat

Permanent freshwater bodies and rivers.

behavior

Larvae resemble miniature adults, swim actively, and appear to be infective immediately on hatching.

feeding ecology and diet

Ectoparasitic; attaching to wide range of freshwater fishes and, rarely, to amphibian tadpoles.

reproductive biology

Males transfer sperm in spherical spermatophores, which are placed on the female during mating. Eggs are laid in clusters and those on the periphery tend to hatch first. Development takes about 30 days at 73.4°F (23°C).

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Overstreet, R. M., I. Dyková, and W. E. Hawkins. "Branchiura." In Microscopic Anatomy of Invertebrates. Vol. IX, Crustacea, edited by F. W. Harrison. New York: J. Wiley and Sons, 1992.

Periodicals

Gresty, K. A., G. A. Boxshall, and K. Nagasawa. "The Fine Structure and Function of the Cephalic Appendages of the Branchiuran Parasite, Argulus japonicus Thiele." Philosophical Transactions of the Royal Society of London B339 (1993): 119–135.

Geoffrey Allan Boxshall, PhD