ECTO-PARASITES OF FISH

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.

Some significant ecto-parasites of fish

 Cryptocaryon irritans

White spot disease in marine fish kept in captivity in the wild is caused by Cryptocaryon (C.) irritans (Yin et al. 2018). This parasitic ciliated protozoan can withstand temperatures between 15°C and 30°C (59°F and 86°F). Numerous fish species have been documented to contract Cryptocaryon infections, and several strains of the virus have been identified from throughout the globe.

Trichodinas

One of the frequent parasites that live in freshwater and marine environments on fish, whether they are cultivated or wild, is Trichodina. According to Martins and Ghiraldelli (2008), these parasites are also referred to as ciliated protozoans and ecto-commensals. There have been reports of over 300 species from various settings worldwide thus far (Tang and Zhao 2012). Due to direct transmission, this ciliated parasite may infect the host in a short amount of time. It mostly infects fish housed under subpar circumstances.

Ichthyophthirius multifiliis

In warm, humid regions, freshwater fish harbor ciliated protozoa known as Ichthyophthirius (I.) multifiliis, which are obligatory parasites. Its outbreaks occur in the 15–25°C temperature range (Noga 2014). It results in severe mortality and financial ruin for different food fish used for aquaculture. According to Mallik et al. (2015), it also has an impact on a number of aquaculture species, such as snow trout, rainbow trout, channel catfish, and grass carp.

Brooklynella hostilis

The cause of Brooklynellosis, often known as slime-blotch or clownfish illness, is Brooklynella (B.) hostilis. Most teleosts in a marine aquarium are infected with B. hostilis. It is a kidney-shaped creature with bands of cilia that range in length from 60 to 80 µm. B. hostilis is a member of the monotypic Brooklynella genus, which has just one species, in the order Hartmannulidae.It requires dead skin to reproduce, damages gills severely, and replicates by binary fission (Fioravanti and Florio 2017). Abnormal breathing and discolouration are common signs. Additionally, it may result in gill congestion and cast aside skin.

Trypanoplasma

 Trypanosomes and Trypanoplasma are frequent parasites of freshwater and marine fish in the kinetoplast genus. Fish from freshwater and occasionally marine environments in Europe, Asia, and North America have similar blood flagellates. Pathogenic blood parasites called Trypanoplasma (T.) borreli and T. bullocki infect marine flatfish in the Atlantic United States and cyprinids in Europe, respectively (Carrington 2017).

Myxobolus brainicum

A parasite of the Salmonidae family, which includes trout and salmon, is called Myxobolus (M.) cerebralis (Hedrick and El-Matbouli 2002). Whirling illness is a myxosporean-caused sickness. It was initially reported in Germany in relation to rainbow trout, and it was then found in South Africa, the United States, and Europe.

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