Introduction
Among the cold-blooded vertebrates, fishes (Phylum: Chordata) are the most diverse group of aquatic organisms. For swimming and breathing, they have fins and gills, respectively. Fish of different genera have efficient body parts for convincing movement in the water and scales to protect themselves (Tedesco et al. 2017).
On top of that, fish is considered as an essential part of human meals and contributes significantly to the economies of many states globally (Andronova and Yakimovich 2019). Of the almost 28,900 species of fish that exist worldwide, 13,000 have been divided as freshwater species, comprising 170 families and 2,513 genera, and are found in rivers and lakes (which make up 1% of the world’s total water volume); the remaining 16,000 species are found in saline environments, which make up 70% of the planet (Leveque et al. 2008).
Parasitic fauna of fish
Various parasites can damage fish directly or indirectly, leading to a high mortality rate within this species. Fish infections are mostly caused by four primary types of parasites: nemathelminthes, acanthocephala, platyhelminthes (monogenean, digenean, cestodes), and protozoa (ciliates, flagellates, microsporidians, and myxozoans). Fish physiology contributes to the spread of deadly infections that cause large-scale deaths (Lerssutthichawal et al. 2015).
Protozoan parasites of fish
Protozoans are the significant parasites of fish kept under intensive fish culture. The major groups of protozoa infecting the fish are myxozoans, microsporidians, ciliates, and flagellates (Wang et al. 2019). Myxozoan parasites infect many groups of fish, including Cichlidae, Cyprinidae and Mugilidae (FAO 2015).
List of parasitic fauna harboring various fresh, Brackish and salt-water fish fauna around the world
Parasites of integument & skeletal system
| Sr. # | Parasite Name (Common name) | Site of Infection | Host | Disease | Source |
| 1 | Amyloodinium Ocellatum | Gills & Skin | All types of fishes | Amylodiniosis/Marine velvet disease | (Moreira 2017) |
| 2 | Trichodina | Gills & Skin | All | Trichidiniasis | (Nofal 2017) |
| 3 | Cryptocaryon irritans | Skin, Fins, Gills | Marine Fishes | Cryptocaryonosis/White spot disease | (Liu 2020) |
| 4 | Ichthyophthirius multifiliis | Skin | All | Ichthyophithiriasis/white spot disease / Ich | (Buchmann 2020) |
| 5 | Monogeneans | Skin & Gills | Goldfish, channel catfish, angelfish | Skin and gill monogean disease | (Neves 2020) |
| 6 | Isopods | Gills & Body surface | Epinephalus (Ep.) coidois, Ep. malabaricus | – | (Purivirojkul 2020) |
| 7 | Brooklynella hostilis | Skin & Gills | Damselfish, Amphiprioninae | Brooklynellosis/Clownfish Disease | (Anshary 2020) |
Parasites of eye
| 1 | Diplostomum spathaceum (Eye Fluke) | Eye | Fresh water and brackish water fishes | Diplostomiasis | (Vyhlídalová 2020) |
| 2 | Tetrahymena corlissi | Skin, Eye & Muscles | Fresh water fishes’ guppies | – | (Imai 2000) |
| 3 | Uronema Nigricans | Connective tissues (Skin, Fins & Nervous Tissues) | Bluefin tuna · Thunnus maccoyi | – | (Arévalo 2018) |
| 4 | Myxosoma (My.) cerebralis/ Myxobolus (M.) cerebralis | Head, Cartilage & backbone | Salmon and trout | Whirling disease | (Alexander 2020) |
Helminths ecto-parasites of fish
The majority of parasites that infect fish are called helminths. Both freshwater and marine fish are being infected by more than 30,000 different helminth species, some of which are known to cause serious fish diseases or to pose a significant risk to public health (Williams and Jones 1994). The three main types of helminth parasites are Platyhelminthes (flatworms: cestodes, monoge-neans, and digeneans), Nemathelminthes (roundworms: nematodes), and Acanthocephalan (thorny-headed worms) (Nguyen et al. 2020).
Trematodes
Trematodes are a class of helminths that are further divided into monogeneans and digeneans. Flatworms and flukes are other names for this group of organisms. The skin, fins, and gills of freshwater and brackishwater teleosts are the sites of infection (Antar and Gargouri 2018).
Monogeneans
Monogeneans are host-specific by nature (Saad-Fares 1992). They have sensory structures, a mouth with or without auxiliary suckers, unique glands, and clamps for attaching to the host at the anterior end. They are hermaphrodites. They mostly live on the host’s body surface, such as the nares, branchiostegal membranes, lip folds, scales, fins, and gills. They do not require the assistance of a middle host in their direct life cycle (Buchmann 2002).
Digeneans
With over 50 species and 15 families, trematodes are a highly varied category of fish parasites (Selbach 2020). They can be internal or exterior parasites that affect many organs, and they mostly parasitize freshwater fish. The heart, brain, eyes, and other sensitive organs are harmed by them (Kirk 2012).
Cestodes
Because of their multisegmented, ribbon-like bodies, cestodes are also known as tapeworms. Their host specificity is a defining property; cestodes often infect siluriform organisms. They may be broadly classified into two groups: the amphilinid, which includes segmented Pseudophyllideans and Proteocephalideans, and the monozoic forms, which include Caryophyllaeidae.
Nematodes
Nematodes are significant parasites of a number of fish species found in aquaculture and the wild. They cause severe illness that affects nearly all of their hosts’ organs and uses fish as intermediate or temporary hosts (Ali et al. 2014). In post-mortem settings, nematodes are comparatively more resilient and durable than platyhelminths due to their unique form and robust cuticles (FAO 2015). From 15 different types of fish found around the Karachi coast, 13 different species of nematodes—eight of which are new—were found in Pakistan.
Next Part : Some significant Ecto-parasites of fish