Holtodrilus truncatus Worms in Freshwater Shrimp: Identification, Prevention & Treatment

Worm on your shrimp? Yeah, it happens. More than beginners might think! Many creatures make our ornamental dwarf shrimp their homes, but most don't warrant panic. If you suspect your shrimp have Holtodrilus truncatus, keep reading to find out what these worms are, how to confirm your diagnosis, and how to use simple salt to get rid of them.

What is Holtodrilus truncatus?

The short answer: Holtodrilus truncatus is a branchiobdellidan epibiont or parasite of atyid shrimp.

In English: Holtodrilus truncatus is a type of worm similar to a leech. It has evolved to exist attached to crustaceans, in this case favoring the family of shrimp known as the Atyidae (which happens to include all the shrimp we keep in our aquariums). The worm attaches itself to its host's exoskeleton, usually around the swimming legs or on the head, and likely benefits from the shrimp's mobility to obtain its diet of microorganisms. It supplements its meals with algae attached to the shrimp, as well as hemolymph, the shrimp version of blood.

As with many of the bugs found on our shrimp, researchers don't entirely agree yet whether H. truncatus is an epibiont, which doesn't harm its host, or a parasite. Its taste for hemolymph would indicate the latter, but on the other hand, the worm's foraging may also help to keep the shrimp's gills clean. What is certain is that its entire life cycle centers around its host. Eggs* are laid inside the shrimp's gill chamber, so all the baby worms have to do once they hatch is crawl out and attach on a fresh spot.

If the host molts or dies, no problem: Holtodrilus just migrates to a new one. This means that if you see one worm, you can assume they're all over the tank.

*Technically, they're cocoons that contain up to 14 developing worms. Ugh!

How bad are they?

Researchers are still politely battling out this question. Some are firmly in the epibiont camp, but others disagree; in one study, 30% of shrimp examined showed signs of damage as a result of Holtodrilus activity. Based on this, we consider this worm "problematic but not panic-worthy", especially because treatment isn't as difficult as with some other epibionts/parasites (lookin' at you, Cladogonium).

Because prevention is better than cure, let's first take a look at how our pet crustaceans get H. truncatus in the first place — and how you can keep worm-laden shrimp out of your tank.

Where does Holtodrilus truncatus come from?

Holtodrilus truncatus is naturally found in the same areas that our shrimp are: East Asia. This also happens to be where dwarf shrimp are bred on a massive scale in order to meet Western demand. The densely stocked ponds of Taiwan in particular are very strongly associated with all sorts of epibionts and parasites. Stressed shrimp kept in unclean water are susceptible to being infested, after all.

Many shrimp sellers import their stock from Asia. This isn't a problem in itself, but it can become one when they don't quarantine or preventively treat the shrimp before selling them on to you. That's why we so strongly recommend banishing all new additions to a quarantine container for the first 30 days after you receive them. Any hitchhikers will hopefully show themselves during this time, and you'll be able to treat them much more easily in quarantine than in a display tank.

A 2020 study of Holtodrilus in Caridina formosae, a species sometimes kept in aquariums, confirms: "(...) health issues of intensively farmed low-cost shrimps are often overlooked by the breeders. This has largely contributed to the presence of other, undesirable species, such as epibionts in the breeding ponds. In some cases, unchecked imported shrimps, become the source of the accidental introductions of epibionts in aquaria and seminatural pond farms as well as within natural water bodies." The authors note that 23% of the 120 shrimp they bought from Taiwan carried H. truncatus.

Unless you add infested shrimp to your aquarium, it's extremely unlikely for them to catch H. truncatus once they've settled in; it's something they arrive with, not something they develop in our homes. That doesn't mean you can let the water quality slip, though, especially if your shrimp do have Holtodrilus. Clean water is the best cure!

Symptoms & diagnosis

If you're reading this, it's probably because you already spotted one or multiple worms on your shrimp, or maybe their tiny white cocoons in the gill chambers. Most shrimp show no changes in appearance or behavior as a result of Holtodrilus truncatus unless they're very heavily infested (one study found 39 Holtodrilus on a single shrimp in the lab), but even at that point, you'd still notice the worms first!

Once you've concluded that "there is a worm", you're basically done. Still, although treatment for both is the same, you could take another minute to figure out whether it's really Holtodrilus truncatus, or another wormy hitchhiker known as Scutariella... or both.

Holtodrilus truncatus:

• Up to 0.2" inch when fully grown, which is quite large
• Prefers inhabiting the walking legs, but can also pop up on the head, abdomen, or gills
• Segmented body tapering from wide to narrow, with rounded tail end
• Eggs can be visible in the gill area (behind the head)
• Moves like an inchworm when not attached to a host
• Prefers female shrimp and infests them more heavily (likely because the eggs mean more nutrients)

Scutariella japonica:

• Up to 0.08" when fully grown, which is quite small
• Almost exclusively found on the shrimp's head, forming a sort of "crown" when a lot of worms are present
• Unsegmented body (flatworm) with a U-shaped tail end
• Eggs can be visible in the gill area
• Moves like a Planarian worm when not attached to a host
• No preference between male and female shrimp

Treatment for Holtodrilus truncatus

Although the jury is still out on how detrimental Holtodrilus is, we do think treatment is warranted. And luckily, that's not too much of a hassle, because these worms are very sensitive to salt. So are shrimp, but less so than the worms. You'll exploit that difference using short salt dips to get rid of the Holtodrilus while keeping your shrimp alive. A tank-wide anti-Holto regime will hopefully dispatch any stragglers.

Here's your step-by-step shrimp deworming plan:

• Make sure the water is (and stays) pristine and in the target range for the shrimp species you keep. Feed high-quality, nutritious shrimp foods and vacuum the substrate thoroughly every few days.
• Perform daily salt dips on each shrimp for a week (again, this is why you quarantine; this is more challenging if they're in a display tank). Find out how to do this in our guide to salt dips.
• Remove any shrimp molts immediately! They are the #1 Holtodrilus vector.

You should see the worms fall off the shrimp within seconds of being introduced into the salt bath. Don't stop the regime too quickly, though, as there may still be eggs (cocoons) around. And again, don't slack off on maintenance and water changes, either: clean water means starving worms, which means it'll be harder for them to regain a foothold after the salt dips. You may have to repeat a few times.

If the worms do keep coming back, or you can't catch all the shrimp, you can consider lightly salting the entire tank for a week or so. This does come with risks: fish don't tolerate salt well, nor do plants. It's not a good idea to fiddle with teaspoons and tablespoons, as salt coarseness varies — you could accidentally overdo it. Start with four grams per five gallons of tank water (remember to subtract water volume for substrate and equipment) and go as high as eight grams per five gallons. Raise the salinity slowly and stop if you notice adverse effects; your normal weekly water changes will bring it back down once treatment is done.