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Munnid Isopods — What Are They?

For Christmas, I was surprised by my family with a lovely fish tank. I have always been fond of tropical fish, and having my own tank with some friendly fish swimming about was a really great way to relieve stress.

When I passed a local aquarium, I couldn’t help but buy a few more pretty fish, which were scooped into clear bags with water. I adjusted the temperature and mixed up the water to ensure the fish would be healthy and happy in their new home.

Imagine my surprise when I looked at the front panel of my tank the next day and saw what I could only describe as a white louse on the inside. Where had this little creature come from, and what was it?

I had no clue, and when my one fish quickly swam closer and devoured the little white bug, I was even more astonished. I was curious, and I headed to the local aquarium to get some answers.

Turns out, this little wiggly white creature was a munnid isopod.


Munnid Isopods

Munnidae are isopods that live in water. They belong to the crustacean family. They feed on algae and rotting plant material that covers the sides of fish tanks and reefs. They rasp and graze on the decaying organic material. They’re pale in color and resemble woodlice, measuring only 1/16th inch (2 mm).


Uncovering Munnid Isopods

Munnid isopods are all part of the isopod family. This means they are intrinsically crustaceans.

While some isopods live on land as land-based crustaceans, munnid isopods are usually found in a watery habitat like other aquatic isopods. They require submersion into the water to survive.

Like their land-based family members, munnid isopods are specially designed to eat the algae and mold that forms in watery areas.

In a fish tank, these isopods would be found against the glass panels where algae accumulate and they also thrive along the waterline where they are slightly safer from being eaten by a fish.

Completing the ecosystem, munnidae file away at reefs and the environment where diatoms and algae accumulate, thereby becoming nature’s little vacuum cleaners.


Appearance and Sexing of Munnidae

With their white segmented bodies, munnidae are quite different from other isopods in that they have more separated segments where their legs are attached, while other isopods are more unified in their segmental appearance.

Their heads are smooth and have no setae or spiney extrusions, except for the males.

The munnid males are the same size as the females, with the main distinguishing characteristic being the long pincer-like extrusions.

The females are often seen with eggs that show through the translucent egg pouch at the bottom of their dorsal shell.

Like other isopods, munnidae hatch from their eggs as miniature versions of adults, and they live in the marsupial egg sack of the female until they are large enough to take care of themselves.


Ecosystem Function of Munnidae

Isopod munnidae are great at rasping away at the edges of coral reefs and keeping a fish tank clean.

While they are not commercially available, they can accidentally be introduced to your fish tank with the purchase of new fish or aquatic plants as they are so small that they may not be seen upon the first inspection.

The good news is that munnidae are actually very beneficial to any aquatic system. These microcrustaceans help clean the aquatic system, feeding on the microalgae that the larger isopods may not have been able to clean up.

Their presence in your fish tank can ensure the glass and coral reefs remain cleaner for longer.

Since isopods are quite prolific breeders, they also multiply relatively quickly. Their increased numbers merely provide an additional food source for your fish who will happily gobble up a couple of munnidae when they get the chance.


How to Encourage Munnidae Population Growth

I discovered that you can’t buy munnidae isopods commercially, and having found some in my fish tank quite by accident, I was quite eager to ensure they stuck around since they are so useful.

This made me wonder how I could ensure there was a continued munnid population growth when my fish were clearly developing a taste for these tiny white bodies.


Don’t Clean the Tank

Remembering that isopods are your tank’s cleaning crew, I realized that I had to stop cleaning my fish tank every week. Instead, I began to look at other fish and crustaceans in addition to the isopods that could clean up my tank’s sides and polish the reef.

What I saw as being a dirty tank was a real buffet for the munnidae in my tank, so I packed away my gravel vacuum cleaner and let nature create a perfect ecosystem.


Ensure Cover for Evading Fish

Since fish enjoy snacking on the nearest isopod or munnidae they can find, I had to ensure there were plenty of hiding spaces in my tank for these micro-crustaceans to hide.

I invested in some coral reef sections that offered little caves where these isopods could hide, and as a bonus, my fish tank began to look like a real reef.

Another means to ensure the munnidae survived being eaten was to plant more aquatic plants that they could hide among and evade the fish.


Plant Appropriate Aquatic Plants for Feeding

Isopods need rotting or decaying vegetation or algae to feed on, and there are a few aquatic plants that come highly recommended for this purpose. I invested in some Java moss and water wisteria.

When little green algae began to form, I didn’t instantly clear this out, letting the fish and isopods snack away to contain these plants.


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Frequently Asked Questions about Munnid Isopods


What do aquatic isopods eat?

The herbivorous isopods like munnidae eat decaying plant matter, algae, and microorganisms like diomes that coat the reefs and structures in your fish tank.


How long do aquatic isopods live?

Depending on the species of isopod and their care, isopods can live for several months up to two years.


Are isopods parasites?

While most munnidae are herbivores, there are some isopods that can be predatory and even parasitic. Your fish tank can quickly turn into a slaughterhouse if you have the carnivorous isopod variety in your tank as they will attack and latch onto your fish, killing them slowly.


The Final Munnid

Now that I had some facts about the little visitor in my tank, I could begin to plan how to make her feel at home.

I knew I was correct in telling her gender when I saw the pale eggs in her marsupial pouch. Within a few weeks, I’ve got a whole new batch of little wiggly micro-crustaceans.

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