If you’re a fish, you don’t want to meet the tentacled snake, Erpeton tentaculatus, as this video shows. If it wasn’t an odd metaphor for an aquatic organism, I’d say it was lightning fast. Blink and you’ll miss it. The video, by Ken Catania from Vanderbilt University, shows the strike in increasingly slow motion. The images are quite beautiful, as long as you aren’t the fish.
Last year, Ken published an article in PNAS (open access) in which he described how the snake apparently both influences and predicts how the fish will turn. This escape response is known as the “C-turn”, and is coded by the “Mauthner neurons”, which we’ve discussed before on the Z-letter. The snake feints like a footballer, causing the fish to respond, then immediately strikes in the opposite direction.As Ken’s abstract puts it: “As a result, fish that were oriented parallel to the long axis of the snake’s head most often turned toward the approaching jaws, sometimes swimming directly into the snake’s mouth.”
Now Ken has just published an article in Journal of Experimental Biology (subscription needed to get past abstract) which explains exactly how the snake manages to detect the fish’s movement. The secret is in those odd tentacles coming out of the front of its head, which you can see in the video. You can see them quite clearly here:
The aquatic snake, is found in SE Asia and can grow to well over 1 metre in length. Since it was first described in 1800, these “tentacles” have been ascribed various functions, including as lures to convince hapless and hungry fish to come closer. Covered in scales as shown in this false colour electron microscope from Ken’s article, the tentacles are in fact mechanoreceptors.
The JEB article shows that the neurons that are found in the tentacles project deep into the snake’s brain, intimately related to the snake’s representation of its visual world. Although snakes will attack video images of fish, showing they do not need mechanical stimulation, they can also successfully feed in the dark, showing they can kill without seeing their prey.
Finally, in a great piece of electrophysiology, Ken and his coworkers recorded from the tentacle neurons to show they responded to mechanical stimulation, and from the snake’s brain to show how both visual and mechanical stimuli overlap.
As shown above, the tentacles are effectively a branch of the trigeminal nerve, which in most vertebrates detects sensation in the face and is involved in eating. In the tentacled snake, the sensory aspects have been developed to an incredibly high level. Two questions remain: first, how is the detected movement transformed into the snake’s ballistic attack and second, what happens if the snake receives contradictory information from its visual and mechanosensory neurons? Which is more important – sight or touch?