Module 6 – Fungi and Fungal Diseases of Aquatic Organisms

Vocabulary flashcards covering key terms and concepts from the lecture on fungi and fungal diseases of aquatic organisms.

Fungi

Heterotrophic, chlorophyll-lacking organisms that may be filamentous and multicellular or unicellular; can be saprophytic or parasitic.

Hyphae

Individual fungal filaments that make up the body of a fungus.

Mycelium

A network or mass of interconnected hyphae forming the main body of a fungus.

Septate hyphae

Hyphae divided by cross-walls (septa) into distinct cell-like compartments.

Non-septate (coenocytic) hyphae

Hyphae without cross-walls, resulting in a continuous multinucleate filament.

Straminipilous fungi (Oomycetes)

‘Lower’ zoosporic fungi with cellulose cell walls, wide hyaline non-septate hyphae; important aquatic pathogens.

Deuteromycetes

‘Imperfect’ fungi that reproduce asexually by conidia and possess chitinous cell walls.

Ascomycetes

‘Sac’ or ‘cup’ fungi that form asci (cup-shaped fruiting bodies) and have chitin in their cell walls.

Zoospore

Motile spore produced by many aquatic fungi, enabling dispersal in water.

Saprolegniasis

Water-mold infection producing white-to-brown cotton-wool growths on fish eggs, skin, gills, and eyes.

Saprolegnia spp.

Common oomycete genus causing saprolegniasis in fish and eggs.

Aphanomyces spp.

Oomycete genus that can cause saprolegniasis and includes A. invadans, the agent of EUS.

Achlya spp.

Oomycete genus implicated in water-mold infections of fish.

Pythium spp.

Oomycete fungi occasionally infecting aquatic animals, contributing to saprolegniasis complexes.

Dichtyuchus spp.

Less common oomycete species associated with saprolegniasis-like infections.

Clinical signs of saprolegniasis

Lethargy, loss of equilibrium, cottony growths, necrotic epidermis, and inflammatory lesions.

Treatment for saprolegniasis

Salt baths (22 g L⁻¹ / 30 min or 30 g L⁻¹ / 10 min; 1-3 g L⁻¹ indefinite) or formalin 0.4-0.5 mL L⁻¹ for 1 h.

Epizootic Ulcerative Syndrome (EUS)

Seasonal ulcerative disease of freshwater and estuarine fish characterized by severe dermal ulcers and high mortality.

Aphanomyces invadans

Primary oomycete pathogen causing EUS; grows optimally at 20-30 °C and not at 37 °C.

Associated agents of EUS

Rhabdovirus (65 × 175 nm) and Aeromonas hydrophila bacteria often accompany A. invadans infections.

Environmental triggers for EUS

Low water temperatures (18-22 °C) and heavy rainfall events that stress fish populations.

Prevention/Control of EUS

Exclude infected fish, eradicate diseased stocks, disinfect facilities with lime or salt, reduce stocking density.

Aflatoxicosis

Mycotoxicosis in fish or shrimp caused by ingestion of feed contaminated with aflatoxins.

Aspergillus flavus

Fungus that, along with A. parasiticus, produces aflatoxins in improperly stored feeds.

Aflatoxin B1 (AFB1)

Most common, toxic, and studied aflatoxin contaminating grains and finished feeds.

Fish signs of aflatoxicosis

Yellowish body, opaque eyes, fin rot, pale gills, abnormal swimming, enlarged yellow liver with tumors, high mortality.

Shrimp signs of aflatoxicosis

Yellow-to-red body discoloration, brownish-red feces, soft shell, retarded growth, melanized hepatopancreatic tubules.

Prevention of aflatoxicosis

Use fresh, properly stored feed (<2 months), keep stores cool/ventilated, regularly test for aflatoxins.

Larval mycosis

Oomycete infection of crustacean eggs/larvae causing whitish appearance, weakness, and rapid mortality.

Lagenidium spp.

Marine oomycete genus commonly responsible for larval mycosis in hatcheries.

Haliphthoros spp.

Oomycete pathogens of marine larvae, producing non-septate, branched hyphae with discharge tubes.

Sirolpidium spp.

Oomycete genus causing mycosis; forms hyphae that may develop terminal vesicles for zoospore release.

Oomycete infection cycle (larvae)

Hyphae grow in host, produce zoospores released via discharge tubes or vesicles, zoospores infect new hosts.

Impact of larval mycosis

Infected eggs fail to hatch; hyphae replace host tissues; up to 100 % mortality within 2-3 days.

Control of larval mycosis

Reduce density, remove debris/dead larvae, maintain sanitation, disinfect eggs, treat water with trifluralin or formalin.

Trifluralin

Chemical (Treflan®) used at 0.1-0.2 ppm for 24 h (hatchery) or 5 ppm for 1 h (spawners) to control larval mycosis.

Formalin bath (fungicide)

Treatment at 1-10 ppm (larval mycosis) or 0.4-0.5 mL L⁻¹ for 1 h (saprolegniasis) to kill fungal pathogens.

Granulomatous response

Host tissue reaction with granuloma formation seen in EUS due to invasive oomycete infection.

Cellulose fungal cell wall

Structural polysaccharide composing the walls of oomycetes (e.g., Saprolegnia, Aphanomyces).

Chitin fungal cell wall

Structural polymer characteristic of most true fungi (Deuteromycetes, Ascomycetes).

Fungi

Heterotrophic, chlorophyll-lacking organisms that may be filamentous and multicellular or unicellular; can be saprophytic or parasitic.

Hyphae

Individual fungal filaments that make up the body of a fungus.

Mycelium

A network or mass of interconnected hyphae forming the main body of a fungus.

Septate hyphae

Hyphae divided by cross-walls (septa) into distinct cell-like compartments.

Non-septate (coenocytic) hyphae

Hyphae without cross-walls, resulting in a continuous multinucleate filament.

Straminipilous fungi (Oomycetes)

‘Lower’ zoosporic fungi with cellulose cell walls, wide hyaline non-septate hyphae; important aquatic pathogens.

Deuteromycetes

‘Imperfect’ fungi that reproduce asexually by conidia and possess chitinous cell walls.

Ascomycetes

‘Sac’ or ‘cup’ fungi that form asci (cup-shaped fruiting bodies) and have chitin in their cell walls.

Zoospore

Motile spore produced by many aquatic fungi, enabling dispersal in water.

Saprolegniasis

Water-mold infection producing white-to-brown cotton-wool growths on fish eggs, skin, gills, and eyes.

Saprolegnia spp.

Common oomycete genus causing saprolegniasis in fish and eggs.

Aphanomyces spp.

Oomycete genus that can cause saprolegniasis and includes A. invadans, the agent of EUS.

Achlya spp.

Oomycete genus implicated in water-mold infections of fish.

Pythium spp.

Oomycete fungi occasionally infecting aquatic animals, contributing to saprolegniasis complexes.

Dichtyuchus spp.

Less common oomycete species associated with saprolegniasis-like infections.

Clinical signs of saprolegniasis

Lethargy, loss of equilibrium, cottony growths, necrotic epidermis, and inflammatory lesions.

Treatment for saprolegniasis

Salt baths (22 g L^{-1} / 30 min or 30 g L^{-1} / 10 min; 1-3 g L^{-1} indefinite) or formalin 0.4-0.5 mL L^{-1} for 1 h.

Epizootic Ulcerative Syndrome (EUS)

Seasonal ulcerative disease of freshwater and estuarine fish characterized by severe dermal ulcers and high mortality.

Aphanomyces invadans

Primary oomycete pathogen causing EUS; grows optimally at 20-30 ext{ } ^{ extrm{o}} ext{C} and not at 37 ext{ } ^{ extrm{o}} ext{C}.

Associated agents of EUS

Rhabdovirus (65 imes 175 nm) and Aeromonas hydrophila bacteria often accompany A. invadans infections.

Environmental triggers for EUS

Low water temperatures (18-22 ext{ } ^{ extrm{o}} ext{C}) and heavy rainfall events that stress fish populations.

Prevention/Control of EUS

Exclude infected fish, eradicate diseased stocks, disinfect facilities with lime or salt, reduce stocking density.

Aflatoxicosis

Mycotoxicosis in fish or shrimp caused by ingestion of feed contaminated with aflatoxins.

Aspergillus flavus

Fungus that, along with A. parasiticus, produces aflatoxins in improperly stored feeds.

Aflatoxin B1 (AFB1)

Most common, toxic, and studied aflatoxin contaminating grains and finished feeds.

Fish signs of aflatoxicosis

Yellowish body, opaque eyes, fin rot, pale gills, abnormal swimming, enlarged yellow liver with tumors, high mortality.

Shrimp signs of aflatoxicosis

Yellow-to-red body discoloration, brownish-red feces, soft shell, retarded growth, melanized hepatopancreatic tubules.

Prevention of aflatoxicosis

Use fresh, properly stored feed (<2 months), keep stores cool/ventilated, regularly test for aflatoxins.

Larval mycosis

Oomycete infection of crustacean eggs/larvae causing whitish appearance, weakness, and rapid mortality.

Lagenidium spp.

Marine oomycete genus commonly responsible for larval mycosis in hatcheries.

Haliphthoros spp.

Oomycete pathogens of marine larvae, producing non-septate, branched hyphae with discharge tubes.

Sirolpidium spp.

Oomycete genus causing mycosis; forms hyphae that may develop terminal vesicles for zoospore release.

Oomycete infection cycle (larvae)

Hyphae grow in host, produce zoospores released via discharge tubes or vesicles, zoospores infect new hosts.

Impact of larval mycosis

Infected eggs fail to hatch; hyphae replace host tissues; up to 100 ext{ } % mortality within 2-3 days.

Control of larval mycosis

Reduce density, remove debris/dead larvae, maintain sanitation, disinfect eggs, treat water with trifluralin or formalin.

Trifluralin

Chemical (Treflan®) used at 0.1-0.2 ppm for 24 h (hatchery) or 5 ppm for 1 h (spawners) to control larval mycosis.

Formalin bath (fungicide)

Treatment at 1-10 ppm (larval mycosis) or 0.4-0.5 mL L^{-1} for 1 h (saprolegniasis) to kill fungal pathogens.

Granulomatous response

Host tissue reaction with granuloma formation seen in EUS due to invasive oomycete infection.

Cellulose fungal cell wall

Structural polysaccharide composing the walls of oomycetes (e.g., Saprolegnia, Aphanomyces).

Chitin fungal cell wall

Structural polymer characteristic of most true fungi (Deuteromycetes, Ascomycetes).

What is the optimal temperature range for the growth of Aphanomyces invadans?

It grows optimally at 20-30 ext{ } ^{ extrm{o}} ext{C} and not at 37 ext{ } ^{ extrm{o}} ext{C}.

What is the typical mortality rate seen in cases of larval mycosis?

Larval mycosis can cause up to 100 ext{ } % mortality within 2-3 days in infected crustacean eggs and larvae.

Which two Aspergillus species are most commonly associated with producing aflatoxins in contaminated feeds?

Aspergillus flavus and A. parasiticus.

Why are Deuteromycetes known as 'imperfect' fungi?

They are called 'imperfect' because their sexual reproductive stage has not been observed, and they primarily reproduce asexually by conidia.

What is the primary function of a zoospore produced by many aquatic fungi?

To enable motility and dispersal of the fungus in water.