BIOL630 TERM PROJECT
SENSORY PROCESSES
When interacting and understanding the world around them, the octopus makes use of certain sensory processes. The first one to discuss is sight. Octopuses have two eyes that can vary in size depending on the species of octopus. The Common Octopus, O. vulgaris, have eyes that are 20 mm in diameter with different pupil conformations, as seen in Figure 1. For comparison’s sake, a human eyeball is 24 mm in diameter on average (Hanke and Kelber, 2020). Within the retina of the octopus’ eyes are photoreceptors that are densely packed together. Octopus also only have one type of photoreceptor – rhabdomeric, which have microvilli to increase the surface area – which points towards the direction of the incoming light (Hill, 2018). Researchers found in 2014 that O. vulgaris only has one visual pigment in the rhabdomeric photoreceptors, which is an “R-type opsin” (Cronin and Porter, 2014).
Figure 1. Three views of an O. vulgaris pupil. Picture A was taken in bright light and is "constricted". Picture B was taken in normal light, and is in a neutral position. Picture C was taken in darkness, with the pupil completely dilated. (Courtesy of Hanke and Kelber, 2020.)
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Check out this picture! The top photo is normal, and the bottom was taken with a wide-angle lens. This is an example of chromatic aberration, a hypothesized way that octopuses are able to see color.
Courtesy of Wikipedia.​
Because of the possession of only one visual pigment, octopuses are considered color-blind. However, more recent studies have indicated that the pupil shape and something called chromatic aberration may expand what an octopus can see. Chromatic aberration occurs when light rays passing through a lens focus at different points, and it’s a common problem for photographers who use wide-lens cameras (Source: Nikon). Researchers have speculated that because cephalopods have evolved the wider pupils as seen in figure 1, colors can still be “determined”, despite being technically colorblind (Stubbs and Stubbs, 2016).
As for the other standard senses, octopuses do things a little bit differently. Although it is still hotly debated, sensory physiologists have hypothesized that the statocyst, a structure filled with sensitive hairs, is used by octopuses to detect sounds between 400Hz and 1000Hz (Walker, 2009). Touching back on the topic of the suckers that line the 8 arms, more recent research has indicated that octopus are able to touch and taste through contact with the suckers. This “chemotactile” method of sensing is a unique adaptation, since the sucker rim epithelium cells are similar to receptor cells that can be found in other organisms (Van Giesen et al, 2020).
Octopuses also are specialized to sense light in an area besides their eyes – their skin! Flat cells containing pigment called chromatophores are scattered all over the skin of octopuses. Around these chromatophores are dozens of muscles arranged radially. Relaxing the muscles around the chromatophores contracts the pigment cell as small as it can go. Contracting the muscles expands the pigment cell, making the color visible in a wider area (Hill, 2018). The video shows a great example of an octopus paralarvae, where you can clearly see the chromatophores expanding and contracting!