Re: Why are plankton, fish and some cephlapods attracted to light at night?

Hi Lucy,

One of the largest migrations on the earth takes place every day in the oceans. It is called diel vertical migration (DVM) and refers to the movement up and down in the water column by millions of zooplankters and fish. Most of these animals are small, ranging in size from a few millimeters to a few centimeters, and travel 100's of meters (many times their body size) twice a day -- it really is an amazing feat.

Here's a brief description of how DVM works:

Phytoplankton are the primary producers of the open oceans; as photosynthetic organisms, they are restricted to the surface waters where enough sunlight penetrates for them to do their photosynthetic magic. Many zooplankters (copepods, many other crustaceans, larvae of various invertebrates and fish) feed directly on phytoplankton. However, feeding at the surface during the day would make these planktivores visible to their predators, so they (the planktivores) spend the daylight hours in deeper waters and come up to feed on the phytoplankton under the cover of the nighttime darkness. Of course, the predators (fish, larger invertebrates such as chaetognaths, crustaceans, and squid) follow their prey to the surface. An orgy of feeding occurs every night, and towards dawn the survivors return to deeper waters and hide until the next night.

I think that you are correct in guessing that fish move into lights to catch their prey. Fish are visual predators, and it seems that any additional light in waters that would otherwise be dark would help them locate their prey. Squids are also visual predators, as well highly intelligent (probably more so than fish). Other predators, such as chaetognaths, don't use vision to hunt, but have other sophisticated sensory systems to locate prey. Still, they make the nightly trip to the surface waters along with the other hunters because that's where the food is.

Obviously, becoming someone else's dinner is something that all these critters want to avoid, and even some of the phytoplankton have defense mechanisms against herbivory. But finding one's own food is usually the number one priority -- if you can't eat, you die. So yes, I'd say that the need to find food overcomes the fear of being preyed upon.

Thanks for the question. I hope I've answered it for you.

Allison J. Gong
Mad Scientist