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.
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.
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.