Flashing and flickering squid

“Humboldt squid can be very cannibalistic,” Stanford University graduate student and researcher Hannah Rosen told me via email, “and if they sense a weakness in another individual they will attack and eat it.”

I had asked her to speculate on the fate of one particular Humboldt squid. “Though I can’t say with any certainty what its fate was since the camera was ripped off, I’d say there is a good chance it was killed,” she replied.

Rosen and a team of researchers from Stanford University and National Geographic recently reported the results of a study in which they outfitted three Humboldt squid, large cephalopods that can grow up to four feet long (not including their arms and tentacles – scientists report squid sizes in terms of the length of the mantle, the torpedo-shaped part of the body above the head), with video cameras so they could spy on their underwater color-changing behavior without the interference of divers or submersible vehicles.

Two of the squid were lit only by natural light filtering down through the water; the recordings they gathered showed Humboldt squid exhibiting two types of dynamic color-changing behavior: ‘flashing,’ a whole-body, rhythmic and rapid change in color, and ‘flickering,’ a wave-like scattering of color across the skin that, the authors write, “mimic[s] reflections of down-welled light in the water column,” much like the play of light against the bottom of a pool. (Humboldt squid chromatophores, the small organs in their skin that they reveal to change their appearance, are a single, reddish-brown color; unlike some other species of squid which have chromatophores of many different colors, Humboldt squid are either white, when their chromatophores are hidden, or red, when they’re exposed.)

Flickering, the authors suggest, may be a form of camouflage for the squid, helping them to blend into their environment and perhaps avoid being eaten. Flashing occurred primarily when the squid were in groups, suggesting that it may be a form of communication; the video recordings captured a number of interactions between squid, including physical contact, possible mating attempts, and arm-splaying that appeared to be directed toward the squid wearing the camera.

One of the cameras also captured an aggressive episode of “numerous attacks” in which, the researchers write in their paper, “several other squid . . . tore the camera package off the camera-bearing squid shortly after it was released.”

That was the squid I asked Rosen about. The scientists had equipped that squid’s camera with a red LED light so they wouldn't have to rely on natural illumination, allowing them to observe nighttime behavior. The red light apparently had the unintended consequence of aggravating the surrounding squid, leading to the attacks. 

“We were hoping the red light would be out of their visual range,” Rosen said in an email, “but were obviously wrong.”

Rosen and her team didn’t give up there, though. Last year they tried infrared lighting, which did not lead to the same problems as the red LEDs; unfortunately, Rosen said, “it also didn’t provide enough light to really see anything that was happening around the squid.”

Moving forward, Rosen intends to continue studying Humboldt squid, their color-changing behavior, and how they control their chromatophores. In the meantime, she hopes that non-scientists embrace the importance of studying animals that experience the world in a completely different way than humans do.

“It’s easy to assume an animal is stupid just because of how it looks,” she said, “but just because an animal doesn’t act the same way we do, that doesn’t mean it isn’t smart, it might just have skills we can’t imagine because they aren’t something we would ever need.”

Researchers attached a video camera to a cloth sleeve slipped onto the mantle of each squid. The cameras were programmed to detach and float to the surface at a specified time. 

(Image by Joel Hollander)

Zebrafish cannibals

Cannibalism is not uncommon in the animal kingdom. Many different types of organisms occasionally prey on members of their own species – house mice, monarch butterfly larvae, wandering spiders, crows, and many kinds of fish, just to name a few. As Laurel Fox, the author of a scientific paper entitled “Cannibalism in natural populations,” (the source of the examples of cannibalistic species listed above) wrote in 1975, “[c]annibalism is not an aberrant behavior limited to confined or highly stressed populations, but is a normal response to many environmental factors.”

Fox details a number of reasons why cannibalism can occur among animal populations, including a lack of other sources of food, overcrowding, stress, and simple availability. “In many examples initiation and control of cannibalism has not been ascribed to any obvious factor,” Fox writes, “and in these cases cannibalism may be a response primarily to the presence of vulnerable individuals.”

In fact, the behavior is so common among zebrafish (a small, freshwater species popular in both living room fish tanks and research labs) that online guides dedicated to the care of pet zebrafish warn their readers to keep adults separate from eggs and larvae. It’s also common enough that when a group of researchers was searching for a predator of zebrafish larvae to use in a study, they settled on the adult form of the species.

Those researchers recently published the findings of their study – an investigation of the mechanisms behind the ability of larval zebrafish to evade predators – in The Journal of Experimental Biology. Based on earlier experiments, the scientists already knew that the larvae sense their predators by feeling the flow of water that the larger fish push before them as they move rather than by seeing them coming, but in order to successfully avoid being eaten, the larvae need to avoid their predators as well as sense their presence.

This, the researchers found, is exactly what zebrafish larvae do; based on the cues they got from the water flowing around a predator (a dead adult zebrafish that had been preserved with formalin, and was guided through the experimental tank with a robotic arm), the “larvae direct[ed] their escape away from the side of their body exposed to more rapid flow. This suggests that prey fish use a flow reflex that enables predator evasion by generating a directed maneuver at high speed.”

Zebrafish larvae are equipped with the ability to feel a predator coming, and the reflex to swim away from it, even when that predator is an adult zebrafish.

Zebrafish, like many other animals, sometimes exhibit cannibalism. 

(Image by Soulkeeper via Wikimedia Commons)

Sound pressure

In the autumn of 2001, ships employing compressed air guns were prospecting for gas and oil off the northwest coast of Spain. In the process of mapping the sea floor, they employed pulses of air that created explosively loud, high-intensity, low-frequency waves of sound. During September and October, five giant squid, cephalopods which can grow to be over 40 feet long, were found stranded along the coast, a much higher number than usual; and in 2003, amidst continued oil and gas exploration, four giant squid were found, dead or dying, on the coast or near the shore.

Examination of the animals and further scientific investigation suggested that the squid died as a result of physical damage to their statocysts, the structures they use to detect sound and orient their bodies in the water. (I read about these events here [pdf], here, and here.)

New research conducted on another cephalopod, the common cuttlefish, suggests that, in addition to being damaging at high intensities, lower intensity sound may be an important way that cephalopods sense the environment around them.

A team of scientists monitored the behavior of cuttlefish exposed to a series of sounds of different frequencies and intensities, each lasting for three seconds. The most extreme behaviors the researchers observed were “escape responses” – jetting quickly away, or inking. (Cuttlefish, like most cephalopods, can produce a cloud of ink that obscures them from a predator as they escape.) These behaviors were elicited by low-frequency, high-intensity tones, while lower intensity tones were associated with less extreme behaviors like body movements and changes in body coloring. (Cuttlefish are capable of altering both the color and the pattern of their body, in order to deter predators, attract mates, or blend into their surroundings.)

The researchers also assessed the extent to which cuttlefish become habituated to certain sounds – in other words, whether or not they will stop responding to a repeated tone. They found that “[a]fter several exposures and no imminent threat, the number of escape responses decreased, suggesting the cuttlefish were able to filter out the ‘irrelevant’ acoustic stimuli,” though the cuttlefish never completely stopped responding to the repeated sounds.

Sound may be an important indicator of their surroundings for cuttlefish – the scientists write that the “evasion responses suggest that the cuttlefish initially reacted to the [sound] stimulus as they would react to a predator or other form of danger, and that sound detection could be a mechanism for predator detection” in cuttlefish.

Sound can have disastrous effects on cephalopods – but in other circumstances, or at different intensities or durations, it may serve ecologically important functions like announcing the presence of a predator.

Common cuttlefish (Sepia officinalis) can grow to be up to two feet long

(Image by David Sim via Wikimedia Commons)

Seal meals

Ecology is the study of how living organisms interact with their environment, and with each other. One of the most commonly studied interactions among different creatures is the flow of ingested energy – in other words, mealtimes.

An investigation into an animal’s eating habits might involve many questions – not just, ‘what does it eat?’ But also, ‘how often?’ And ‘where?’ And ‘at what time of day (or night)?’ And ‘under what conditions might it not eat at all?’

Direct observations can answer some of these questions, but the problem becomes more complicated if all the action occurs underwater – for instance, if you’re talking about harbor seals (or other marine mammals). Harbor seals can dive hundreds of meters underwater to catch the fish and other seafood that make up their diet.

Underwater video footage is one way to spy on harbor seals as they hunt for their meals, but, as a team of researchers who recently reported an alternate method in The Journal of Experimental Biology points out, the presence of the required light source may influence the very behaviors videographers attempt to witness and record during deep dives.

By strapping an accelerometer – a device that measures changes in speed – to the head of a harbor seal using a small neoprene headband, the scientists were able to record a characteristic jerk of the seal’s head each time it captured a fish. The researchers were working with a single harbor seal in a controlled environment for the purpose of testing the accelerometer; however, they say the method has the potential for use in months-long studies in the wild, partly because the battery demands of the accelerometer are so low.

“Such long records of foraging behavior will help us to understand how free-ranging aquatic predators search for and acquire energy from their dynamic environment in time and space,” the scientists write. By answering questions like, ‘when and where do seals find their meals?’ the researchers will be able to investigate the rest of the food web, too.

Harbor seal swimming in shallow water on the California coast.

(Original image by Tewy via Wikimedia Commons)