A number of different animal parasites are found in fishes. The following different groups of parasites are reported from both marine and freshwater fishes. They are protozoa, Trematodes (monogenetic and digenetic), Cestodes, Nematodes, parasites are mentioned below with diagrams here:
Protozoa – Protozoans are characterized by their microscopic size and by the fact that they are unicellular (plate I; A, B). There are many species of protozoans, both free living and parasitic. Their ability to multiply on or within their hosts names them in many instances very dangerous to fish. Protozoa belonging to the Cilliophora are equipped with cilia (short, fine cytoplasmic outgrowth), or structure derived from cilia by secondary modifications, or both. All Sarcomastigophora so far recorded (in Southeast Asia) belong to the subphylum Mastigophora i.e. they are all equipped with flagella as are always few in numbers. Finally, members of phylum, Myxozoa, during their vegetative stages have neither cilia nor flagella, but resemble amoebae in their mode of progression. There most characteristic and taxonomically important feature is spores.
Trematoda– monogenea: monogeneans are platyhelminths platyhelminths parasitic primarily on fishes, including Agnatha, the Chondrichthyes, on specific sites on the host such as the head and flank, the fins, the surface of nasal, epithelium or on the branchial arches. Exceptionally one or two are endoparasitic (Acolpenteron, kirtskyia occur in urinary ducts and bladder). A typical monogenean has a bilaterally symmetrical, dorsoventrally flattened body (plate I; C). The most characteristic structure is the opisthaptor, an attachment organ situated at the posterior extremity. It is armed with chitinoid structure important for attachment. The anterior end of the body carries a sucker like structure associated mouth opening. Monogeneans are hermaphrodite. Most monogeneans have direct life cycle, I,e., they use only one host. Most are oviparous. The viviparous Gyrodactylidae gives birth to live worms with already well developed reproductive system. The young worms tend to become attached immediately on birth.
Trematoda – Digenea: Digeneans are predominately endoparasites. They possess dorsoventrally flattened unsegmented body. Digeneans are commonly equipped with two attachment organs, the oral sucker and the ventral sucker or the acetabulum (plate I; D). An anterior sucker divorced from the oral opening, known as a rhynchus, is occasionally present. Digeneans are also hermaphrodites. The life cycle of Digenea involve more than one hosts, and their adult stage parasitic in vertebrates. The digenetic trematodes, almost without exception, pass through a change of host, involving a vertebrates and an invertebrate (a mollusc), and this is always coupled with an alternation of generation (hence the name “Digenea” = “generated in two ways”). Fish may also be infested by the metacercarial larval stage.
Cestodes: They never have an alimentary canal or any body cavity. Most are equipped with one attachment, or hold fast organ, known , unsegmented neck without sharply limited boundaries, passing into along body, whose ribbon like appearance prompted the common name of tapeworm (Plate II; B-E). The body is usually depressed. Its broad, flat dorsal and ventral surfaces are called surficia. The strobila the most cestodes are segmented, consisting of many compartments, the proglottids. Each proglottid is a separate unit, consisting one set each male and female reproductive organ. The reproductive organ does not mature at the same time, so that each segment is first male and latter female. The strobilla is produced by distant from it are the oldest. Consequently, proglottids nearest the neck are usually male and those furthest away are female. The life involves either one or more two intermediate hosts, mainly various invertebrates, but sometimes small vertebrates also.
Acanthocephala: the acanthocephalans are endoparasitic worms of slender cylindrical or slightly flattened from and hollow constriction. The diagnostic feature of the worm is the organ of attachment consisting of an invaginable proboscis that forms the anterior end (plate II; A). The proboscis is armed with rows of recurved hooks, when the common name of spiny-headed worms applied to this parasite. Mouth, anus and digestive tube are completely absent, and there is also on circulatory system. The males are provided with copulatory apparatus, and the terminal part of the female apparatus is also somewhat complicated. The adult parasite infests the digestive tract of various vertebrates, mainly fish, birds, and mammals and occur through out the world in marine, freshwater and terrestrial hosts. Pallisentis ophioccephali is a very common acanthocephalid worm infests the intestine of Channa striatus in Bangladesh.
Nematode: The body of Nematoda, the roundworms or threadworms, is cylindrical, but it may be fusiform or filliform; the body is not segmented (plate III; A, B). The length of the body lies mostly between ½ mm to 1m. In nematodes, sexes are mostly separate (males generally smaller than females). They usually appear colourless or white or yellowish; colouration is generally due to the colour of the intestinal content, especially when it is filled with blood. The body is enveloped by a firm but flexible cuticle. During the larval development, the integument is normally shed four times. The development of the nematodes proceeds schematically in the sequence of the embryonic development: larva I (L1 –first skin molt, larva I (L2)-second skin molt, Larva III (L3)-third molt, Larva IV (L4) fourth skin molt adult. There are species that do not change hosts and also those that have two-host or three-host cycle. The alimentary canal normally consists of a muscular midgut lined inside by cuticle; the oral aperture at the anterior end is surrounded by six labia or lips. Almost all species of nematodes of marine fish are diocious; these are mostly representatives of the suborders Ascaridata, Cucullanata, Camallanata etc.
Leech: Leeches are bilaterally symmetrical worms with more or less uniformly metameric bodies (plate III; C). They are mainly fresh water, though some are marine and others inhabit damp terrestrial habitat. Only about a quarter of leech species are parasitic, their parasitism being a temporary intermittent character. Although leeches are very quite widely in their morphology, they are always large enough to be observed in naked eye, oval and lanceolate in outline usually more or less dorsoventrally flattened and provided with anterior or posterior suckers. In some species the body is clearly divided into two parts; anterior small and narrow, posterior larger and boarder, often flattened. The hirudinean body is always transversely striated, the grooves dividing it into very definite annuli.
The most anatomical feature of leeches is their alimentary canal, adapted for their large and frequent blood meals. From Southeast Asia two genera Piscicola and placobdella involve in cultured fish disease. Both marine and freshwater leeches are to transmit haemoflagellates of the genera Trypanosoma and Cryptobia. Most of these leech adults in water fowls, but some mature in freshwater fishes.
Crustacea: Body externally segmented and divisible into head, thorax and abdomen. Body cuticle forms the exoskeleton; thorax segmented and carries appendages, a pair in a segment. Appendages are jointed. Parasitic copepods are of the following types:
Copepod worms like- On paired compound eyes, genital opening on last thoracic segment (plate III; E). Examples- Coligus, Lernaea, Ergasilus.
May be louse like- body flat, carapace disclike, compound eyes, parasitic with mouth suctorial, genital opening on fifth body segment. Example –Argulus (plate VI; A).
Isopodid parasite – The body is dorsoventrally compressed. There is carapace. The thoracic limbs usually are all alike (plateVI; D). Abdomen stout, and segmented. Example – Bopyrus is parasitic in the branchial chamber of prawn.
Glochidia: Larval stages of freshwater bivalve mollusks of the super families Unionoidea and Muteloidea are parasitic on fins and gills of fish. The larvae of Unionidae – known as glochidium – have calcareous bivalve shells often with little hooks on their inner edges (plate III; D). Once the larvae come into contact with a suitable host fish, they clamp on to the gills, fins or skin, where they become surrounded by host tissue (epithelial cells). This time they undergo metamorphosis to from juvenile mollusks, which then detach themselves from the fish.