Color-changing octopuses: How do they do it??

Christina Forbes is a postdoctoral researcher in the School of Molecular Sciences at Arizona State University. She specializes in organic chemistry and biochemistry.

Octopuses are incredibly fascinating creatures: intelligent and clever, strange organ configuration (to me at least), and some really exotic forms of sensory perception.

Octopuses are part of the cephalopod family, with their cousins, squid, cuttlefish, and nautiluses. Fossil impressions of octopuses show evidence that these creatures have been around more than 200 million years, and so they have had plenty of time to evolve some really unique adaptations!

No octopus to see here!
(Flikr 808_Diver)

Some species of octopus have incredible abilities that make them masters of camouflage, where their skin can change color and texture. Check out this short TEDtalk if you want to see some examples of this amazing ability! So how does an octopus do this?

The skin of an octopus has several layers of cells that work together during color changes. One layer of skin contains chromatophores, which are tiny fluid-filled sacs that contain different pigments of colors. If the muscles around the sac squeeze it shut, then it hides the pigment. If the sac expands, then the color shows through. Opening and closing these different sacs of color can give an octopus a wide range of colors for its skin color (a little like a digital pixel display). Another layer of skin contains iridophores, which work to reflect surrounding light, especially green and blue colors. The base layer of skin contains leucophores, which is like a white background for all the other colors. Some octopuses have photophores, which give them special bioluminescent ability.

A close-up of squid skin, where the wide-open chromatophores reveal brownish colors
(Creative Commons, via wikipedia)

More deeply (as my neighbor asked me): does an octopus think about it and "try" to look like its surroundings, or does an octopus innately change color and texture without thinking about it?

Without being able to directly ask an octopus about whether its camouflaging ability is conscious or sub-conscious, some experts have tried to study this. One study found that an octopus tends to choose a particular object to imitate (a rock or piece of coral) rather than its entire surroundings, suggesting there is some thought behind camouflage. This suggests that at least some octopuses are using their vision to match its surroundings.

On the other hand, in addition to seeing light with their eyes, octopus skin could perceive light or color as well. Octopus skin cells contain opsin proteins, which are special sensors that allows their skin to detect color and light. That's not to say that octopuses can "see" with their skin, but their skin can pick up enough information about their surroundings so that they can change shape and color to mimic their environment without using their brains.
 
So maybe camouflaging is a little more innate than conscious!

So can octopus skin change color without the octopus thinking about it? Actually, yes, sort of: Skin that has been removed from an octopus (ouch!) can change color on its own, but its much slower than usual, suggesting that some neural input from the brain assists in color changing and camouflaging.

As if all this wasn't crazy enough, some species of octopus are color-blind. Or are they? A recent paper suggested that while an octopus might only have one photoreceptor (only see in black-and-white), their unique pupil shape (U-shaped) could allow them to interpret color though chromatic abberation. However, this work is only one possible model that remains to be proven. 

If only we could just ask an octopus.

Octopuses have been found to build and carry shelter materials. They can also open doors and screw-topped containers. Don't ever underestimate an octopuses ability.
(Creative commons, via pxhere)

Other awesome references:

https://www.nationalgeographic.com/science/phenomena/2014/08/18/adaptive-colour-changing-sheet-inspired-by-octopus-skin/

https://ocean.si.edu/ocean-life/invertebrates/how-octopuses-and-squids-change-color

https://www.nature.com/scitable/topicpage/cephalopod-camouflage-cells-and-organs-of-the-144048968

What drives ocean currents?

Welcome to the second installment of the Science on Main ocean-related blog posts! This week's topic comes from a part of the field called physical oceanography. This piece of the broader field of oceanography focuses on understanding the large-scale forces that drive ocean events, including currents, the topic of this week's post.

For most, especially anyone who has seen the movie Finding Nemo (Disney-Pixar, 2003), the fact that the ocean has currents is old news. Ocean currents are powerful features of their environments, continually pushing large volumes of water from one location to another, potentially thousands of miles away. One of the most famous ocean currents is the Gulf Stream, which is part of a larger circulation current called a gyre. There are five major gyres in the oceans: the north Atlantic, south Atlantic, north Pacific, south Pacific, and Indian.

Figure 1: The deflection of air currents due to the
Coriolis Effect

In the northern hemisphere, the gyres circulate in a clockwise direction. In the southern hemisphere, they circulate in a counter-clockwise direction. This is due to the Coriolis Effect. The Coriolis Effect is the name for the deflection of air currents in the atmosphere due to the rotation of the earth underneath it. If Earth didn't rotate, the air would naturally flow in a straight line from the high-pressure areas around the poles to the low-pressure equatorial area. Air deflects in opposite directions in the northern and southern hemispheres because of the direction of earth's rotation (Figure 1). So what does this have to do with ocean currents?

You may be surprised to learn that it is actually the wind that drives surface ocean currents! Each of the oceanic gyres has one eastern and one western boundary current, for a total of 10 major currents in the modern day oceans. The eastern boundary current is usually wider and travels slower, while the western boundary current is narrower and travels quickly. This is because the winds that drive surface currents are deflected out, the start to lose energy. Since they spiral west to east, the air currents are stronger on the western sides. The air pushes and shears the surface of the ocean, which causes the water to follow along. This is part of why hurricanes with high winds can produce huge waves and strong ocean currents.

Figure 2: The Gulf Stream. Warm colors represent warm water,
carried by the current to England and northern Europe.

One of the most famous currents is the Gulf Stream, which carries warm, nutrient-rich water from the Caribbean, up the east coast of the United States, and then up and over to England, Iceland, and the Norwegian Sea. Despite England's high latitude, it has a relatively mild climate, and it is primarily due to the Gulf Stream, and the water it carries with it.

The Gulf Stream is only a surface current, however. There are also currents that bring nutrients from deep in the ocean to the surface, which are used to feed phytoplankton and other small animals. As phytoplankton may be responsible for up to 80% of all organic matter on Earth, it's important to keep them fed! These deep currents also bring oxygen with them, which every animal needs to survive. Water that has no oxygen in it are called hypoxic, and cannot support life. Without these deep currents, life in the oceans could not survive.

One example of phytoplankton, a diatom called
Amphiprora. From Dawn Moran, WHOI.

Deep ocean currents carry water along the bottom of the ocean, collecting nutrients and oxygen, before they are brought to the surface. This changes some of the water's properties within the current, including its density. When the density of the water on the bottom of the ocean is less than the density of the water above it, the ocean begins to overturn. There are specific locations in the oceans where upwelling occurs, when less dense water rises to the surface, and locations where the opposite, downwelling, happens. This density-driven circulation is necessary to keep the surface water healthy for all the plants and animals that rely on it.

It is important to study the oceans because they are responsible for moderating climate, driving weather, and maintaining the global food-web. Currents are an important piece of the puzzle of how the oceans work, as they transport materials from one place to another, but there are still many questions that remain. If you studied the ocean, what question would you want to work on? What do you think would happen if all ocean currents suddenly disappeared?

Sources:
https://oceanservice.noaa.gov/education/kits/currents/05currents1.html
https://oceanexplorer.noaa.gov/facts/currents.html
https://www.nationalgeographic.org/encyclopedia/coriolis-effect/
http://www.edc.uri.edu/restoration/html/gallery/plants/phyto.htm
https://www.whoi.edu/main/topic/phytoplankton