Holtodrilus Truncatus – Parasites in Shrimp Keeping Hobby

Unfortunately, shrimp keeping hobby is not always sunshine and rainbows.  So, today I am going to continue my series of articles on dwarf shrimp diseases and parasites. This time it is a relatively new problem that came to use from the Asia aquaculture ponds – Holtodrilus truncatus.

Holtodrilus truncatus are small, worm-like obligate epibionts (organisms living on other organisms) of crustaceans, mainly crayfish and shrimp. These worms do not appear to prefer to anchor to the rostrum, instead of anchoring under the shrimp right between the pleopods.

Surprisingly, these organisms proved to be resistant to popular aquarium treatments but with one exception. Their salinity tolerance is very low, therefore, we can use salt baths to efficiently remove them.

Keep reading for more information on Holtodrilus truncatus and how to treat these new worms in our aquariums based on existing official studies, experiments, researches, and experience of aquarists. 

What Is Holtodrilus Truncatus? Ectosymbiosis

Holtodrilus truncatus (or shrimp worms) are leech-like, clitellate annelids belonging to the order Brachiobdellida that form an obligate, ectosymbiotic association primarily with shrimp and crayfishes.

Ectosymbiosis means that they live on the body of the host, including internal surfaces, for example, gills.

At the moment, science has no explanation of how this symbiosis works and what form it has.

Some scientists believe that it can be commensalism. This is a relationship between species in which one benefits and the other is unaffected. 

Other scientists presume that this relationship can be even beneficial for the host shrimp because Holtodrilus truncatus feed on organic matter on the shrimp gills, and thus clean them, which may indicate a certain degree of mutualism.

According to some other studies, it is more like parasitism. This is a relationship between two biological species where parasites are organisms that harm their symbiotic partners eventually. I will talk about it a little bit later.

In any case, their behavior is not entirely clear. There are many unknown factors that affect the symbiotic relationship between Holtodrilus truncatus on the host shrimp.

Description of Holtodrilus Truncatus

These are small worms. The body length of Holtodrilus truncatus usually ranges from 1.0 mm (0.04 inches) to 5.0 mm (0.2 inches).

This shrimp worm is characterized by its cylindrical and pinkish-transparent body form (with a large medial tooth and three pairs of smaller lateral teeth), and no trunk appendages.

The ‘head’ is always broader than the first segment of the body. The other end of the worm appears to be a sucker for attachment.

Some Facts About Holtodrilus Truncatus

Holtodrilus truncatus are opportunistic omnivores. They hunt for plankton and algae but also feed on detritus as well as host hemolymph.

Although they usually attach to the external surface of the antennule, the rostrum, the carapace, and the abdomen, these worms can get inside the carapace of the host as well.

According to the experiment, once removed from the host shrimp and without additional food, Holtodrilus truncatus could survive up to 46 days!

How to Identify Holtodrilus Truncatus in Aquarium

These worms behave like Scutariella japonica but are much bigger in size. In addition, Holtodrilus truncatus also show microhabitat preferences.

According to the study, they tend to attach to its host shrimp at several anatomical locations, for example:

• 44.3% occurrence in the pereiopods area,
• 22.1% in the rostrum area,
• 21.3% in the pleopods and abdomen area, and
• 12.3% in the gills. 

Read more about “Dwarf Shrimp External Anatomy” here.

The choice of this area cannot be accidental because it facilitates access to the gill chamber, carapace, and eggs of the shrimp.

When not attached to a shrimp, it moves like an inchworm, with the “head” moving from side to side, like, it is ‘smelling’ or detecting something. When decided, it then moves its sucker to its head location and begins again.

They can easily move from shrimp to shrimp. It seems like they can sense each other or a weak shrimp. So you can see them gathering together over time.

Reproductive of Holtodrilus Truncatus

These shrimp worms are hermaphrodites. After ‘mating’ they lay cocoons only inside in the shrimp’s gill chamber.

Cocoons are transparent and ovoid in shape. Their size barely reaches 0.5 – 0.7 mm (0.02 – 0.03 inches) in diameter. According to the study, each cocoon can contain up to 14 juvenile worms (developing embryos).

There is no information on how long the incubation period lasts.

Newly hatching from the cocoon, Holtodrilus truncatus immediately enter the gill chamber of the host shrimp. At this stage, they are almost completely transparent.

Are Holtodrilus Truncatus Dangerous to the Shrimp?

There have not been enough studies regarding this question.

As I have mentioned before, some scientists believe that these worms do not have a clear negative effect on the shrimp.

However, there is also a completely opposite opinion. In accordance with the observation on Neocaridina species, biologists detected damages in gills as well as the pleopodal and abdominal area. In addition, females more likely presented damages in both areas, suggesting that the activity of Holtodrilus truncatus might be higher in them than in males.

Almost 30 % of Neocaridina shrimp showed signs of damages as a result of Holtodrilus truncatus activity.

At first glance, the infected shrimp do not appear to be bothered by the worms, they can feed regularly: even the ones who had numerous worms. However, in the long-term, these organisms can affect shrimp’s wellbeing by causing distress. As a result, it may lead directly to shrimp weakness, loss of color, and even casualties.

In addition, another experiment showed that a single shrimp can host as many as 39 Holtodrilus truncatus. So, even if these worms are harmless to shrimp, but only until they multiply in large numbers and do not fall into the gills, that can also cause the suffocation of the shrimp.

Note: Infection in the gills of shrimp usually does not affect the growth of the shrimp. Because they do not derive nourishment directly from the host but are considered to have a synergistic effect during the period of stress.

To sum up, it would be dangerous to exclude that the occurrence of these worms on aquarium shrimp may have a detrimental effect on host health.

Name	Holtodrilus truncatus
Type	Epibionts (organisms living on other organisms)
Threat level	Low
Treatment difficulty	Easy
Treatment efficiency	100%
Treatment length	1 day

Are Holtodrilus Truncatus Contagious?

Yes, these shrimp worms always try to find new hosts. If left untreated, these worms may progressively affect the entire population of the shrimp colony.

For example, in the case of Neocaridina species, Holtodrilus truncatus started attaching to the host shrimp after 30 minutes. All worms attached within 3 hours after exposing themselves to their new host.

In addition, if shrimp dies, these worms immediately migrate to the healthy shrimp.

What Shrimp Species Does Holtodrilus Truncatus Affect?

Here comes the bad news. These worms do not care.

They have the ability to infect Neocaridina and Caridina species alike. Even more, there were multiply reports that crayfish species (for example, Procambarus clarkii) are also on the list.

What Causes Holtodrilus Truncatus?

I would like to start off by saying that Holtodrilus truncatus came to us from Korea, China, Japan, and Taiwan. Nowadays these worms have become one of the major problems in breeding dwarf shrimp in Asian aquaculture ponds.

It is a well-known fact that the growing market of aquarium shrimp has caused the establishment of new aquaculture farms modified to produce mass quantities of low-cost shrimp.

Regrettably, in most cases, those farms do not have proper maintenance. As a result, poor water conditions and dirt are the main reasons for the development of potentially undesirable organisms called epibionts.

Therefore, it is absolutely crucial to quarantine any shrimp we are going to put in our tank. Especially, if we know that those shrimp are from Asian aquaculture ponds.

For more information, read the article “How To Setup an Easy Quarantine Tank”.

Holtodrilus Truncatus and Other Parasites

Holtodrilus truncatus worms are not the only organisms that lay eggs inside the gill chamber of the shrimp. Eggs of Scutariella japonica could be observed there as well.

It is quite interesting but these organisms do not have any contact. There is no interaction no predation between them. They can happily co-inhabiting the same shrimp host.

How to Treat Holtodrilus Truncatus in a Shrimp Tank

Now we have come to the most important part, how we can get rid of them.

Remarkably but these shrimp worms are pretty resistant to popular aquarium treatments. There were reports that No-planaria, Planaria zero, Canine Dewormers, Panacur did not have an effect on Holtodrilus truncatus.

However, a simple salt bath showed amazing efficiency against them. According to the salinity tolerance experiment, Holtodrilus truncatus have a very low threshold of salinity tolerance. It is only between 0–0.5%. So, we can use it to our advantage.

Freshwater aquarium salt vs Holtodrilus Truncatus

Freshwater aquarium salt (link to check the price on Amazon) would be the safest choice. Although you can use table salt as well.

Note: In shrimp keeping hobby, there is an opinion that table salt is not completely safe because it contains potassium iodine. Personally, I have not found any proof that potassium iodine can harm the shrimp.

In addition, I would like to thank our fellow shrimp keeper Megan Jerbic, for testing and reporting the effectiveness of this method.

Salt Bath Method:

1. Take 1 tablespoon of salt and add it to 1 – 5 cups of aquarium water.
2. Dissolve the salt in the water by stirring it.
3. When it is completely dissolved, take the infected shrimp out of your tank and put it into the cup for about 30-60 seconds.
   Note: Due to the size of the worms it is easy to see all of them fall off the shrimp, and it happens in less than a minute. 
4. Then remove the shrimp and put it back in your aquarium.

The main downside of this method is that you have to catch every infected shrimp and it can be very problematic. Especially when you have a big aquarium and lots of shrimp.

Aquarium Salt Treatment Method:

1. Take 1 tablespoon of the freshwater aquarium salt per 5 gallons (20 liters) of water.
2. Increase dosage as needed.
3. Water changes each week about 20%

Be very careful, if you decide to treat the tank instead of giving the shrimp a salt bath. A small dosage will be ineffective, however, overdosing can be even a worse problem! You have to understand the risk.

Feedback from another fellow shrimp keeper Angela C Materials needed: 2 containers 1 for salt water, 1 for plain water, 1 net, a sink near by. Steps: Add 50-56 grams of aquarium salt to 800 grams of tank water (good enough pool for multiple shrimp at once) into one container. Swish the water around until fully dissolved. Prepare second container with some amount of tank water. Scoop shrimp out of tank with the net and into the salt bath for 1-1.5 min each. Rinse net in tap water. After the time is up, use the net to transfer to tank water container. Rinse net. Repeat the process until all shrimp are now in the tank water container. Rinse the net one more time, inspect the shrimp in the container and then net all the shrimp back into the aquarium. With the process above so far no evidence of the return of parasite and all my shrimp are thriving after processing.

How to Prevent Holtodrilus Truncatus in Aquariums

These worms mostly feed on detritus, plankton, and algae (green water). So, the best way is to improve the quality of water and do not overfeed.

It will reduce their nutrient base and without food, they die out. However, do not expect fast results, as I have mentioned earlier, these worms can survive without any external food source for several weeks.

You can also read the article “How Often and How Much to Feed Shrimp”.

In conclusion

Holtodrilus truncatus (shrimp worms) is potentially a new threat that came to use from Asian aquaculture ponds.

Historically, they are endemic (native to particular countries). However, considering the fact that in recent years, dwarf shrimp gained increasing popularity in the aquarium trade, it has become a leading pathway for the introduction of them in our tanks.

There are also two opposite opinions on how dangerous they can be:

1. People usually call them parasites because of not understanding their role or biology. Holtodrilus truncatus do not have a clear negative effect on the shrimp. On the contrary, they keep the shrimp clean from dangerous epibionts (like Vorticellids). 
2. According to the second opinion, these worms can affect shrimp wellbeing by causing distress which leads directly to shrimp weakness, loss of color, and even casualties. The good thing though is that we can easily remove them by using salt baths.

In any case, we need more data.

Related articles:

• Understanding Dwarf Shrimp Diseases and Parasites

References:

1. Microhabitat distribution and behaviour of Branchiobdellidan Holtodrilus truncatus found on the freshwater shrimp Neocaridina spp. from the Sugo River, Japan. Central European Journal of Biology. 9(1). 2014. 80-85 DOI: 10.2478/s11535-013-0184-3.
2. Niwa N., Presumption of invasion time and route of ectosymbiotic branchiobdellidans Holtodrilus truncatus attached to the freshwater shrimp Neocaridina spp. (Caridea, Atyide) in the Sugo River, Hyogo Prefecture, Cancer, 2011, 20, 29-31.
3. Niwa N., Ohtaka A., Accidental introduction of symbionts with imported freshwater shrimps, Proceedings of International Conference on Assessment and control of biological invasion risks, 21st Century COE Program Environmental Risk, Management for Bio Eco-System, Biodiversity Network Japan (NGO, IUCN member) Yokohama National University, 26-29 August 2004, 60.
4. Epibionts of ornamental freshwater shrimps bred in Taiwan. Animal Science No 57 (2), 2018: 133–142 (Ann. Warsaw Univ. of Life Sci. – SGGW, Anim. Sci. 57 (2), 2018). DOI 10.22630/AAS.2018.57.2.13.
5. First report of the branchiobdellidan Holtodrilus truncatus (Annelida: Clitellata) found on the freshwater atyid shrimp Neocaridina sp. from Korea. Article in Journal of Species Research. October 2016. DOI: 10.12651/JSR.2016.5.3.459.
6. Cohabitation of Ectosymbiotic Branchiobdellida (Annelida, Clitellata) and Scutarielloidae Platyhelminthes, Rhabditophora, Temnocephalida) on Atyid Shrimps in Taiwan. 台灣生物多樣性研究(TW J. of Biodivers.) 17 (3): 253-262 , 2015.
7. First report of freshwater atyid shrimp, Caridina formosae (Decapoda: Caridea) as a host of ectosymbiotic branchiobdellidan, Holtodrilus truncatus (Annelida, Citellata) Aquat. Ecosyst. 2020, 421, 33.
8. Distribution of Holtodrilus truncatus, a Branchiobdellidan Ectosymbiotic on Atyid Shrimps in the Kii Peninsula, Western Japan, with Reference to Salinity Tolerance and Host Preference. ZOOLOGICAL SCIENCE. 33(2):154-161. DOI: 2108/zs150049