"what's on my fish" ?

Costia (Ichthyobodo),

Ichthyobodo, also known as costia, are flagellate protozoan parasites. They spread easily from one fish to another and can result in diseases as well as a host of secondary infections and illnesses.

Even a clean, well-maintained pond can become plagued by costia via being introduced by newly acquired pond fish.

As such, you must be monitoring your fish and water quality daily. If any symptoms occur, you must act quickly to treat and eliminate the parasites before too much damage is done to your fish.

What Is Costia?

Costia are incredibly small flagellate parasites, meaning that they have appendages to help them swim about and more easily find and latch onto hosts.

At only 5 to 20 micrometres in size, they are smaller than the width of a human hair (about 50 micrometres), which makes them incredibly hard to detect and diagnose. They’re hardy creatures and are found

around the world, able to parasitize both marine and freshwater fishes as well as amphibians. Two Ichthyobodo species are commonly reported among farm and hobby fishes:

Ichthyobodo pyriformis and Ichthyobodo necator.

Cod, salmon, halibut, koi, goldfish, and many others are commonly preyed upon as Ichthyobodo species are generalist and can adapt to just about any aquatic niche.

In nature, oftentimes they are introduced to ecosystems via fish restocking efforts.

Trout, salmon, and other such game fish are often overstocked in fish farms, resulting in parasites like costia, infections, and disease being able to establish and spread easily among fish

How to Identify Costia in Ponds Their appearance can be described as a body that is distinctly bean-shaped or closely resembling a comma. Depending on the life stage, two long, thin, hair-like flagella may be visible on the backside of the body.

When not attached, they may appear to have more of a disc-like shape and move about in a bumpy sort of twirling fashion as they use their flagella to navigate. Most often, they’re introduced to garden and ornamental ponds via new fish.

For this reason, any newly acquired fish should be quarantined and observed for any signs of illness for at least several days before adding them to your pond. Due to their tiny size, the best way to identify costia is by using a microscope. Juvenile fish are the most likely to be affected by Ichthyobodo species due to their underdeveloped immune systems.

Costia Life Cycle

Costia are single-celled organisms and reproduce asexually 


via cell division. This means that their numbers can explode very quickly, as they do not require a mate – they simply split into two. They can parasitize fish almost immediately after division, so there is no larval or development stage. Water temperatures between 50 and 77° F (10 to 25° C) result in the most rapid reproduction rate.

This depends somewhat on species, but Ichthyobodo pyriformis and Ichthyobodo necator (the two primary species that are known to infect ornamental fishes)

seem unable to reproduce when temperatures exceed 86° F (30° C). Below 46° F (8° C), costia will fall into a dormant cyst-like state on fish until the waters warm up. Division occurs for about 10 to 12 hours.

If they are unable to feed on a host for two hours or more, oftentimes these parasites will die, with most species certainly dying after 24 hours without feeding if temperatures are above 59° F (15° C).

The lower the temperature, the more slowed costia become, thus increasing the amount of time they can go without feeding. Ichthyobodo attaches via a disk-like structure that cuts into the skin and then extend microtubules into the fish’s cells that can harvest the cell’s contents.

Multiple costia can attach to a single cell, and they can attach anywhere on the body.

What Are Symptoms of Costia?

Flashing Fish

infected with costia may flash, flick, and scrape themselves against various surfaces to help alleviate discomfort from the parasites.

Excess Mucus

As with most parasitic infections, fish are likely to produce more epidermal mucus to try to fight off costia that have attached themselves. This mucous may be greyish blue.

Trouble Breathing

Fish that suffer from costia that have attached themselves to the gills are likely to display respiratory distress, most notably laboured breathing.


In the case of severe infestations, fish can become lethargic due to the extreme loss of energy and body resources.


Fish that are acutely infected may suffer from a depressed appetite, resulting in serious and noticeable weight loss.


If quite a few Ichthyobodo is parasitizing a fish, it may develop red sores and lesions. These invite bacteria and viruses that lead to secondary illnesses and diseases.

Clamped fins

Costia establishing themselves on or behind fins may result in fish clamping their fins close to their body to lessen discomfort from the parasites.


Since costia can be difficult to detect due to their minute size, sometimes an outbreak goes unnoticed.

Occasionally, fish won’t have any obvious symptoms and pond owners will have no idea until fish begins to die for no apparent reason. The best way to be sure is to take water and skin/scale samples and assess them under a microscope.