Unsavory neighbours.

That go completely unnoticed until it is too late. The polyclad worms are a type of flatworm that normally comes with bad intentions.

This odd name refers to its characteristic internal network of gastro-vesicular cavities (poly-clad) connected to its mouth, allowing them to distribute throughout their body the food they are eating. This explains the amazing ability to camouflage themselves since the more tissue they ingest from their victim, the more they resemble to it.

Taxonomically speaking, the order of Polyclad worms is divided into two sub-orders: the cotylea and the acotylea, depending on the presence or absence of a suction organ. They all have a common characteristic: they are very specialized, and perhaps that is the reason why also very efficient predators. The most usual and main source of concern in our aquariums are called AEFW, acronym of “Acropora Eating Flat Worm” (amakusaplana acroporae), although it is not uncommon to find another small scoundrel, specialized in emptying the shells of our dear snails.

Of all sizes.

The polyclad range from barely visible up to as large as a hand while the smallest are often the worst ones precisely because of the difficulty involved in locating them. An example of this is the amakusaplana acroporae (AEFW) that rarely exceeds 12 mm in length and can be as small as a pinhead. Their small size and ability to mimicry helps them to be almost undetectable until it is too late and they have colonized all the aquarium.

gusanos polyclad

In keeping with this cute animal which has as its sole diet acropora's tissue, it has been found that its transparent skin allows us to view the contents of the vesicles spread all over her body, whom when filling their guts with the tissue that they eat, they acquire the same coral color so it is almost impossible to see them. There are however two quite effective ways to find out if an acropora is infected:

  • Take the coral out of the water and let it dry for a few minutes. The worm is unable to secrete the same coral slime, thus yielding a change in brightness that betrays it. It is a nocturnal parasite, in daytime you have to look under the branches and especially at their junctions.
  • Prepare a “disinfecting dip” with Betadine, Lugol, Coral-RX, Coral Protect, Levamisol or any of the many existing products. The acropora is dipped in for a few minutes (it depends on the product used but it is usually enough for two or three minutes) and vigorously shaken in the water. If there is any AEFW, it will fall to the bottom and die after some time.

If a single flatworm is detected it is important to set the coral in quarantine and repeat the dip every week to break their breeding cycle. The reason is that this polyclad becomes fertile soon after being born (normally when it reaches 4 or 5 mm in length) and will place eggs (very small, amber colour) in the dead area closest to the acropora's tissue.

AEFW eggs

These eggs perfectly resist any disinfectant solution and can hatch releasing hundreds of tiny, hungry scoundrels, so you should search carefully and remove them manually with patience. I must insist on the importance of quarantine and keep on disinfecting because with a single egg that we miss they will soon invade the aquarium again. In this picture you can see eggs of AEFW mature and about to hatch.

Highly specialized predators and parasites.

We saw in the previous example that there are Polyclad worms that act as very active parasites in certain species of coral. The AEFW, as indicated by its acronym only feed on acropora's tissue. And if this wasn't enough, they have their preferences and it is not common to see them in all species of acropora; they feel fondness for acropora millepora, valida, yongei, formosa or latistella to give some examples and it is more difficult to find them in the acropora “deepwater” as for example echinata, carduus o jacquelinae.

But not all of them are parasites, there are also predators, and they are more common than we would like. An example of this are Polyclad worms specialized in eating gastropods.

gusano polyclad

With an insatiable appetite.

Some Polyclad worms specialized in eating snails can reach a considerable size and decimate the population of the beneficial gastropods in a few days. Apparently they are able to smell the trace left by wandering snails and pursue them along the aquarium until they reach their prey. When the snail is reached little can be done because the worm inserts its proboscis and will begin to absorb their tissue in a long and cruel outcome.

They will mainly attack snails which do not have a callum that give them protection (the “lid” that have some species which allows them to take refuge in its shell and seal it hermetically) therefore if we begin to see empty shells trochus, strombus and nassarius we have reasons to suspect that some Polyclad worms is devouring them.

The good news is that these Polyclad worms are not so prolific (unless there are many snails that can sustain a large population) and will starve as soon as they finish with the last aquarium snail.

However, if we want to remove them from the aquarium, the only option is to make a funnel trap (an inverted funnel inside a plastic bottle for example) and place a bait which could be a mussel or a clam (they use to like it)

There are many species of polyclad worms and each one has its own characteristics, while a common one: best away from the aquarium. In this article I've done a brief description of the two most common polyclad flatworms and the best methods to combat them.

Perhaps some readers feel some restlessness or concern, if so, I consider the objective fulfilled. With these rascals you can not relax and early detection is very important. I know of several aquariums which have had to be dismantled just because they did not solve the problem in time, and I hope that does not happen to you.