The ocean is a dangerous place, especially if you are a soft-bodied squid or octopus. Many predators in the bathypelagic zone, a dimly-lit section of the ocean that extends from 700 to 1000 meters below the surface, spot the silhouettes of their prey against the lighter background of the surface layers. Others, like the well-known anglerfish, use the searchlights on their heads.
To better avoid their predators, prey species have evolved a variety of defensive mechanisms. For instance, transparent and reflective animals can hide themselves effectively from the silhouette-spotting predators. However, as Duke postdoctoral researcher Sarah Zylinski notes, such defenses would crumple before the headlight fish.
Transparency is the default form of Japetella heathi, a short-armed, three-inch-long octopus, and Onychoteuthis banksii, a five-inch squid. Although certain organs, such as the eyes and guts of these cephalopods, cannot be made invisible, they are protected by their ability to reflect light. Both species dwell in the bathypelagic zone and are virtually invisible to the silhouette-spotters below.
For the headlight searchers, however, the cephalopods are extremely easy to spot. When hit with a flash of bluish light, the skin pigments, or chromatophores, of these cephalopods activate and immediately turn red.
Zylinski studied the transparent cephalopods during ship-board experiments over the Peru-Chile trench in 2010. When she shined blue-filtered LED light on the squid and octopus specimens, she found that their bodies rapidly changed from clear to opaque. When the light was removed, the specimens immediately reverted to their transparent states.
During a second research cruise in the Sea of Cortez in 2011, Zylinski tested the reflectivity of the octopodes. She found that the octopodes reflected twice as much light in their transparent state, compared to their opaque state. Zylinski then expanded her sample size and experimented with 15 to 20 different cephalopod species from the deep. To her surprise, only two of these species responded to the blue light.
Zylinski tested several variables that she believed would stimulate the transition behaviors. Although shallow-water cephalopods, such as squid, octopodes and cuttlefish changed their body patterns when a shadow or shape passed overhead, deeper-water animals did not exhibit this response. While the animals were observed tracking movement around them, only light made them switch on their pigments. Zylinski's work on cephalopods appears in this month's issue of the journal Current Biology.
In the future, Zylinski plans to investigate the link between transparency and habitat depth for the Japatella octopus. She notes that young animals that reside at shallow depths have fewer chromatophores and are, therefore, more reliant on transparency as a defense mechanism. This makes sense as there are no predators at shallow depths that use searchlights to hunt for prey.
In contrast to young octopodes found at shallow depths, mature adults can be found below 800 meters under the surface, a dark region of the ocean where bioluminescence is the dominant light source. They have a higher density of chromatophores, making them potentially more opaque than their juvenile counterparts.
In addition to transparency, the octopus has been known to use a variety of unique defense and survival mechanisms. These include regenerating lost arms, utilizing its flexible body to squeeze through tight spaces, and using its chromatophores to blend in with its background and warn other octopodes of danger. It also uses the unique and well-known defense mechanism of ejecting ink to confuse predators.
A few of the 300 known species of octopus have the unique ability to transform themselves to resemble fierce or highly toxic marine animals in order to intimidate their predators. The mimic octopus, Thaumoctopus mimicus, is the best-known and perhaps most skilled user of animal mimicry.
As with other animals, the defense mechanisms that the octopus can use are determined by its specific environment and the type of predators present.
