Monogenean Parasites of Fish 1 Peggy Reed, Ruth Francis-Floyd, Ruthellen Klinger, and Denise Petty2

FA28 Monogenean Parasites of Fish 1 Peggy Reed, Ruth Francis-Floyd, RuthEllen Klinger, and Denise Petty2 Introduction of water, may increase the density of parasites on wild fish and consequently result in disease. In addition, the release Monogeneans are a class of parasitic flatworms that are of monogenean-infested fishes to the natural environment commonly found on fishes and lower aquatic invertebrates. can have potentially devastating effects. One example is the Most monogeneans are browsers that move about freely on movement of resistant Atlantic salmon Salmo salar from the fish’s body surface feeding on mucus and epithelial cells Sweden that is suspected to be the source of Gyrodactylus of the skin and gills; however, a few adult monogeneans will salaris that caused heavy losses of susceptible salmon in remain permanently attached to a single site on the host. Norwegian rivers. Another example is the introduction of Some monogenean species invade the rectal cavity, ureter, monogenean-infested stellate sturgeon Acipenser stellatus body cavity, and even the blood vascular system. Between from the Caspian Sea into Lake Aral that decimated the 4,000 and 5,000 species of monogeneans have been ship sturgeon Acipenser nudiventris population, which was described. They are found on fishes in fresh and salt water not resistant to the monogenean. and in a wide range of water temperatures. Morbidity and mortality epidemics caused by excessive Classification and Identification of parasite loads are not uncommon in captive fishes and Monogeneans have also occurred in wild fishes. Captive fishes are usually held in more crowded conditions than fishes in the natural Though the terms “monogenetic trematodes” and “flukes” environment. This practice allows hatching monogeneans are often used to describe this group of parasites, both are to easily find host fish. In addition, stressors found in incorrect because monogeneans are not trematodes or the captive environment such as aggressive behavior by flukes. In fact, they are distinct from the other parasitic tankmates, poor nutrition, and poor water quality may flatworms, which include turbellarians, tapeworms, and impact the ability of the fish’s immune system to respond to trematodes (the true flukes). Trematodes and tapeworms the presence of the parasites. Although monogeneans are with rare exceptions only live internally in their host, and commonly found on wild fish, they seldom cause disease turbellarians occasionally parasitize the skin of marine or death in free-ranging populations because under natural fishes. At their posterior end monogeneans have a haptor, a conditions they are usually not present in high numbers on specialized “holdfast organ” that has hooks or clamps that individual fish. However, any change that results in crowd- enable them to attach to their host (Table 1). Turbellarians, ing of wild fish, which could include drought and diversion tapeworms, and trematodes do not have a haptor. The life cycle of monogeneans also differs from the life cycle of 1. This document is FA28, one of a series of the Fisheries and Aquatic Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date June, 1996. Reviewed May 2009. Revised June 2012. Visit the EDIS website at http://edis.ifas.ufl.edu. 2. Peggy Reed, director, All Florida Veterinary Laboratory, Alachua, FL; Ruth Francis-Floyd, professor, Department of Fisheries and Aquatic Sciences and Department of Large Animal Clinical Sciences (College of Veterinary Medicine); RuthEllen Klinger, biological scientist, Department of Large Animal Clinical Sciences (College of Veterinary Medicine); and Denise Petty, clinical assistant professor and extension veterinarian, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611. The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A&M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Thomas A. Obreza, Interim Dean tapeworms and trematodes. Monogeneans have a direct life and the number of pairs of anchors (hamuli), transverse cycle, which means they go directly from host to host (fish bars, and marginal hooks on their haptors. These features to fish). Tapeworms and trematodes have an indirect life can only be seen with a compound microscope and are cycle that often requires multiple hosts (different types of summarized in Table 2 and diagramed in Figure 2. The animals). most common groups of monogeneans on marine fishes are the capsalids and ancyrocephalids. The most common Monogeneans can be divided into two major groups, the groups of monogeneans on freshwater fishes are the gyro- monopisthocotyleans, which have hook-like organs on dactylids and the ancyrocephalids, which differ markedly their haptors to attach to their host, and the polyopistho- in their reproductive strategies as well as their preferred cotyleans, which use clamp-like structures for attachment. attachment sites on host fish. The polyopisthocotyleans are not as commonly diagnosed as the monopisthocotyleans, so they will not be covered in this article. The haptor is usually shaped like a disc. One to three pairs of hook-like structures called anchors or hamuli are commonly located in the center of the haptor. In many monogenean species, the anchors are used to attach to the host by penetration of the host’s skin. Anchors are often supported by ventral bars, transverse bars or accessory sclerites, which provide stabilization and/or attachment. Very small hooklets called marginal hooks are located near or on the periphery of the haptor and are the primary method of attachment for some monogeneans. Suction created using the haptor itself is another way some mono- Figure 2. Diagram of anchors of gyrodactylids (left) and ancyrocephalids (right). geneans “stick” to their host. Some species have adhesive glands on the haptor, and also on the head. The majority of gyrodactylids are viviparous (produce live There are 13 families of monogeneans; of these, four are young) and are generally found on the bodies and fins of frequently diagnosed on aquacultured fishes. These four fish, though they occasionally may occur on the gills. Adult families are Gyrodactylidae, Dactylogyridae, Ancyroce- parasites carry fully developed embryos (identical to the phalidae, and Capsalidae (Figure 1), and parasites of these adult), which in turn carry the young of the next genera- families are commonly called gyrodactylids, dactylogyrids, tion. Each individual parasite represents several genera- ancyrocephalids, and capsalids, respectively. Distinguishing tions, which is why they are often referred to as Russian characteristics include the presence or absence of eye spots, nesting dolls. This reproductive strategy allows populations of gyrodactylids to multiply very quickly, especially in a closed system. The few gyrodactylids that are egg-layers are usually found on members of the South American catfish families, Loricariidae (armored catfishes such as the “pleco”) and Pimelodidae (long-whiskered catfishes such as pictus cats). These gyrodactylids use an adhesive material to “glue” their eggs to the skin of the catfish. Gyrodactylids may be found on freshwater, marine, and brackish water fishes. Gyrodactylids have a pair of anchors with both dorsal and ventral bars and 16 marginal hooks, and do not have eye spots. Attachment to the fish is made with the marginal hooks; the anchors are used as a spring-like device to assist attachment with the marginal hooks. An embryo with its pair of anchors may frequently be seen inside an adult Figure 1. Diagram of four common families of monogeneans (not gyrodactylid (Figure 3). drawn to scale). 2 occasionally seen in association with gill tissue during microscopic examination. Ancyrocephalids are closely related to the dactylogyrids. Unlike the dactylogyrids, they are not as selective about the host species of fish. They are usually found on gills of freshwater, marine, and brackish fishes. However, some genera (Neodiplectanotrema and Paradiplectanotrema) were reported in the esophagus of marine fishes, and one genus, Enterogyrus, lives in the stomach of African and Asian cichlid species. Ancyrocephalids have two pairs of eye spots, and two pairs of anchors (Figure 5). Each pair of anchors has a transverse bar. Marginal hooks usually number 12–14, but may be absent in some species. Some species produce an adhesive Figure 3. A gyrodactylid from the skin of a goldfish Carassius auratus. substance to assist in attachment to the fish host. Ancyro- The arrows point to the anchors of embryos. Note the lack of eyespots cephalids are oviparous, and eggs are occasionally seen in (left, 100x magnification; right, 40x magnification). mucus when examining gill tissue with a microscope. Dactylogyrids usually prefer the gills as a feeding and attachment site (Figure 4) and are primarily found on freshwater fishes belonging to the cyprinid family (danios, rasboras, barbs, goldfish,

Load more