Nemertean worms (Phylum Nemertea) are among the least known invertebrates but this is changing, due, in part, to increased research attention in recent decades but also due to increased awareness and image capture by amateur naturalists, especially SCUBA divers. The worms range in size from a few millimeters to as much as 30 meters in length but even the longest worms rarely are more than 10 mm in diameter. They are exceedingly elastic and not constrained by hard parts such as a skeleton. This property, in addition to enabling them to hide by squeezing through the tiniest crevices, can be put to good use in swallowing prey that may be several times the diameter of the worm, as seen in the video below of a nemertean (on a reef off Cayman Islands) securing a fish dinner for itself.
The fish is a grunt (probably Haemulon chrysargyreum), which may be 10 times the worm’s normal diameter of about 5 mm. At the video’s start the worm already has engulfed the grunt’s head, much like a python swallowing a large meal. We also see wrapped around the fish two coils of the bluish-gray remainder of the worm (which I estimate may be a meter long). The grunt, evidently, isn’t aware of this subtle attack. Is this strategic predation on a grunt taking its daily nap, as I am told grunts do; did the worm neutralize the fish’s ability to sense what is happening; or did the worm luck into a recently dead fish? We don’t know (but the initial condition of the fish and eye suggest it was very recently alive). Over the course of 20 minutes (edited to 6 min here) the fish is completely engulfed and pushed backward into the worm’s gut. The condition and shape of the fish can be seen to change as the worm’s digestive juices take effect, with no response by the fish.
We know about 1300 named species of nemerteans but have actual observations of feeding for only a few tens of those; all those are carnivorous and most are predators. This particular feeding event is remarkable if attacking “sleeping” fish is this worm’s primary feeding strategy. Whereas many of the studied nemerteans seem to have strong prey preferences, the Antarctic 1-2 meter-long nemertean Parborlasia corrugatus is well-known to be a voracious omnivore – equally fond of live clams and carrion – and, as do starfish, it can evert its foregut to exodigest food even it can’t swallow; such as the seal carcass shown below, for instance. Worms of the order Hoplonemertea actually paralyze prey… but more on that some other time.
Regrettably, unlike the fish, the worm’s color is not a useful identification diagnostic. Despite its large size, the worm in the first video may represent an unnamed species, because we know of no similar large species from the region. Though some large nemerteans are well known in certain shore habitats, sightings by SCUBA divers are rare because the worms tend to remain hidden. Feeding observations are especially rare and informative. My thanks to Sander Kean (who recorded this fish-eating worm for 20 min!) and to the other “amateurs” who help us know more about this poorly known but obviously fascinating phylum!