Behavior Biological oceanography deep sea Ecology

Red Light, Green Light… Squid Light?

Burford, B. and Robison, B. H. (2020). Bioluminescent backlighting illuminates the complex visual signals of a social squid in the deep sea. PNAS, 2020.

An example of the bioluminescent signaling used by Humboldt squid. From Burford and Robison 2020.

If you’ve spent any time browsing the site, you have probably seen several posts on the deep sea and know that there is still a lot scientists haven’t discovered. Many of the animals that live in the deep sea are social animals, which is surprising as they live in total darkness. However, some of these deep sea dwelling organisms have developed ways of communicating with each other. Bioluminescence, or the process by which organisms produce their own light, is one such way that animals light up the darkness of the deep sea and communicate basic information to each other. This recent study by Burford and Robison shows how the Humboldt squid takes deep sea communication to the next level. 

From 2005 to 2012, Burford and Robison analyzed the footage of over 20 research dives conducted by the Monterey Bay Aquarium and Research Institute. The video below is an example of the kind of footage collected by a remotely operated vehicle (ROV) research dives and provides invaluable evidence of the complex behaviors and patterns the squid use to communicate.

Using an ROV, the researchers were able to observe how the squid communicated in total darkness. Based on the photos and video footage of the squids, they connected several light and color patterns with behaviors that required coordination, such as feeding and schooling. In order to better categorize the patterns, the team focused on the frequency and arrangement (or sequence) of colorful displays observed within the shoal. 

Frequency: The frequency of signals refers to how quickly or often the squid change their light and color patterns. The squid can display signals using flickering or flashing. Flashing occurs when the squid quickly switch between light and dark signals. Flickering occurs when the squid use a “dynamic mosaic” of colored signals, providing a complexity that goes beyond simple posturing or locomotion. 

Arrangement: The arrangement of signals refers to the sequence in which they occur. Much like human speech, the order of signals (like words) matters when trying to communicate with other individuals. This kind of “syntax” has been largely unobserved in deep sea creatures, so this was an exciting discovery for the researchers. It also provides evidence that other squid species may do something similar.

Figure 1. The left y-axis shows the common arrangement of signals as squid attempt to capture prey. The top shows common locomotion signals and the right y-axis shows the timeline of capturing prey. Combined, this chart gives an overview of the locomotion and chromatic signals that squid use when foraging. A larger red dot indicates a that the combination was observed more frequently than those with smaller red dots. See the full chart here.

In addition to the frequency and arrangement, the researchers also looked at the intensity of bioluminescence in the displays and made another exciting discovery. Based on the footage, it’s highly likely that the Humboldt squid use bioluminescence to light up their bodies internally, rather than project light outward like other bioluminescent species. This allows the squid to light up specific regions of their bodies, and thus create more complex arrangements for communication. The so-called “backlighting” of the patterns also increases visibility, which allows the squid to coordinate their schooling and feeding behaviors more easily. 

The researchers recommend undertaking more ROV research dives to better understand the complex signals the Humboldt squid use to communicate. They believe that other deep sea squid and cephalopods (like these) may use similar methods of bioluminescent communication. And while this episode from the Twilight Zone gives a pretty good visual representation of signaling at work, you can be sure that squid aren’t trying to take over the world.

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