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Common Freshwater Fish Parasites Pictorial Guide: Crustaceans

Deborah B. Pouder, Eric W. Curtis, andRoy P.E. Yanong

Introduction

This publication is one in a series of pictorial guides that is designed to assist in the identification of common freshwater fish parasites. 

The information provided in this guide is not intended to be a complete, detailed description of each parasite or parasite group and its characteristics but rather is intended to assist in the visual identification of some of the most common species or groups of parasites seen in freshwater fish. For further information on each parasite, refer to publications in the "Recommended Reading" and "Reference" sections below.

Guide Information

  • Target Tissue: provides the location on/in the fish where the parasite is most commonly found.
  • Characteristic: provides a brief description about the appearance of the parasite.
  • Size: provides the size or size range of the parasite. (1 µm = 0.001 mm = 0.0001 cm) (µm = micron or micrometer; mm = millimeter; cm = centimeter)
  • Movement: provides the type of movement, if any, of the parasite.
  • Note: provides a brief comment of interest about the parasite.

Crustaceans

Argulus ("Fish Louse")

two samples under microsope, right has two circular suckers labeled.
Figure 1. Argulus 

Target Tissues: Skin, fin

Appearance: Flattened, saucer shape; two suckers; two eyespots

Size: Juveniles approx. 1–3 mm in length; adults approx. 5–20 mm in length

Movement: Free-moving; may glide over fish when feeding

Note: Can be seen with unaided eye; common in koi and goldfish; sucks body fluids from host; egg-layer (requires multiple treatments)

Ergasilus

See caption.
Figure 2. Ergasilus on gills (left); egg sac and grasping antennae (right) 
Credit: Courtesy of Christin Densmore (left) 

Target Tissues: Primarily gills

Appearance: Conical, segmented body with grasping antennae

Size: Usually greater than 2 mm in length

Movement: Generally seen fixed (attached with grasping antennae) on fish

Note: Direct life cycle; often incidental on fish but modified antennae used to grasp fish can cause severe, localized damage and infection

Lernaea ("Anchor Worm")

See caption
Figure 3. Samples under microscope, Lernea on fin (left); arrows indicating anchor and egg sacs (right)  

Target Tissues: Skin, fin

Appearance: Long, narrow; "anchor" at one end imbeds into fish; pair of egg sacks often seen at end opposite of anchor

Size: Approx. 5–25 mm in length

Movement: Generally seen fixed (attached with anchor) on fish

Note: Only females are parasitic; can cause secondary bacterial infections due to wound from anchor; common in koi, goldfish, and golden shiners

Acknowledgements

The authors thank Christine Densmore for the photograph she contributed to this publication.

Recommended Reading

References

Hoffman, G. L. 1999. Parasites of North American Freshwater Fishes. Ithaca, NY: Cornell University Press. https://doi.org/10.7591/9781501735059 

Longshaw, M., and S.W. Feist. 2001. Parasitic Diseases. Pages 167–183 in W.H. Wildgoose, editor. BSAVA manual of ornamental fish, second edition. British Small Animal Veterinary Association, Gloucester, England. https://doi.org/10.22233/9781910443538.21 

Noga, E. J. 1996. Fish Disease: Diagnosis and Treatment. St. Louis, MO: Mosby-Yearbook, Inc.

Stoskopf, M. K. 1993. Fish Medicine. Philadelphia, PA: W. B. Saunders Company.

Woo, P. T. K., editor. 1995. Fish Diseases and Disorders, volume 1: protozoan and metazoan infections. CAB International, Wallingford, United Kingdom.