I've been doing some research and looking through a lot of the photos and videos that people are posting of nemertean worms and it occured to me that the majority of the animals I'm seeing are not really nemerteans, but likely the flatworm Rhyncodemus sylvaticus. This species is widespread in the continental USA and has been found in Canada as well. It also predates on small insects so it more/less fills the same niche as the nemertean that shows up most often in the hobby, Argonemertes dendyi.
How to distinguish between the 2:
R. sylvaticus and A. dendyi are both variable in colour ranging from a pale yellowish to a brownish grey. They also reach approximately the same size - ~2 inches tops when stretched, generally around an inch when resting. Their appearance and movements, however, are quite different. The animal that has the pointed end and the "finger waving" movement is in fact R. sylvaticus. These worms lack the trademark nemertean proboscis and instead hunt their arthropod prey by simply "pouncing" on them, capturing them with an expanded cephalic hood and then immobilizing them with mucous, draining the fluids out and leaving a perfectly dry exoskeleton. Here is a photograph and a video of Rhynchodemus sylvaticus:
Rhynchodemus sylvaticus
Several members have/had the nemertean Argonemertes dendyi which has also been introduced to various tropical/subtropical areas including Hawaii and possibly Florida. These worms have a blunt rounded head and are generally more stocky looking than the flatworm. They move in a smooth gliding motion and hunt by firing the proboscis at the prey item and envenomating it. Feeding methods of the nemertean also leaves a dry exoskeleton. An easy way to identify A. dendyi (or rather to confirm that your mystery worm is in fact a nemertean) is to lightly prod the posterior end of the worm and see if it fires its proboscis to try and escape as shown in the video below.
Argonemertes dendyi
While I'm not sure if this information is too relevant (I'm not sure if there are any significant differences that could aid in eradication of one or the other for those who want them gone), I figured at the very least it could potentially help others identify their little monsters.
How to distinguish between the 2:
R. sylvaticus and A. dendyi are both variable in colour ranging from a pale yellowish to a brownish grey. They also reach approximately the same size - ~2 inches tops when stretched, generally around an inch when resting. Their appearance and movements, however, are quite different. The animal that has the pointed end and the "finger waving" movement is in fact R. sylvaticus. These worms lack the trademark nemertean proboscis and instead hunt their arthropod prey by simply "pouncing" on them, capturing them with an expanded cephalic hood and then immobilizing them with mucous, draining the fluids out and leaving a perfectly dry exoskeleton. Here is a photograph and a video of Rhynchodemus sylvaticus:
Rhynchodemus sylvaticus
Several members have/had the nemertean Argonemertes dendyi which has also been introduced to various tropical/subtropical areas including Hawaii and possibly Florida. These worms have a blunt rounded head and are generally more stocky looking than the flatworm. They move in a smooth gliding motion and hunt by firing the proboscis at the prey item and envenomating it. Feeding methods of the nemertean also leaves a dry exoskeleton. An easy way to identify A. dendyi (or rather to confirm that your mystery worm is in fact a nemertean) is to lightly prod the posterior end of the worm and see if it fires its proboscis to try and escape as shown in the video below.
Argonemertes dendyi
While I'm not sure if this information is too relevant (I'm not sure if there are any significant differences that could aid in eradication of one or the other for those who want them gone), I figured at the very least it could potentially help others identify their little monsters.