Behavior and Interactions among Microorganisms and Invertebrates - Freshwater Ecology: Concepts and Environmental Applications - page 380

TABLE 18.2 Parasitic Diseases of Freshwater Fish

Taxonomic group
of fish parasite Example of diseases (causative agent)/ fish affected

Virus Lymphocystis/aquarium fish

Bacteria Furunculosis (Aeromonas salmonicida) /salmonids

Fin rot (several species of bacteria)/aquarium fish

Fungi Ichthyophonus hoferi /all known aquarium fish

Protozoa Whirling disease (Myxospora)/cold-water fishes

Ich or whitespot disease (Ichthyophthirius multifiliis) /many fish

Trematoda Hookworms/freshwater and marine fish

Hirudinea Leaches/freshwater species

Copepoda Fish lice/can attack many species

Vertebrata Lamprey eel (Petromyzon) /salmonids

snail as an intermediate host. One option for control of this disease is to
control the snail species that spread the infection. Molluscicides are gener-
ally used to do this, but resistant snails can occur. Understanding the ecol-
ogy of the snails may increase the effectiveness of control strategies. Future
control efforts include the development of a human vaccine (Butterworth,
1988) to be used in combination with biological control of the snails.
Blooms of phytoplankton may be heavily infected with fungal
pathogens (Van Donk, 1989). This is a potentially important controlling
factor in successional sequences. Such parasitic infections are more likely
to spread when populations are growing more rapidly and population den-
sities are high.
Actinomycetes (bacteria) prey on cyanobacterial cells. About half of
the strains of actinomycetes isolated from a lake sediment lysed Microcys-
tis cells (Yamamoto et al., 1998). Such predation may offer methods for
controlling algal blooms.
Microbial parasites are common on frogs (Smyth and Smyth, 1980).
Frogs are hosts for all major groups of animal parasites: Protozoa, Trema-
toda, Cestoda, Acanthocephala, and Nematoda. Some of these life cycles
are fairly complex. For example, the trematode Gorgoderina vitelliloba in-
fects the frog Rana temporaria . Adult insects or tadpoles ingest the parasitic
trematode initially and it enters the kidney. After 21 days, the trematode
flukes enter the bladder and deposit eggs, which are excreted into the wa-
ter. The trematode eggs hatch, and a small swimming form enters the gills
of the freshwater clam Pisidium . From here, they emerge as a cercaria form
(a small worm-like form) that is eaten by tadpoles or aquatic insect larvae.
The cercaria encyst in the body cavity of the tadpoles or the larvae. Adult
frogs then eat the infected tadpoles or adult aquatic insects, and the cycle
of infection begins again (Smyth and Smyth, 1980).

Other Exploitative Interactions
Other exploitative interactions rarely are appreciated as general
processes in aquatic communities, but they undoubtedly are important.