Monsters Mini-Post: On the Various Types of Eyes in the Animal Kingdom, or, The Eyes Have It

Alright, sure, let’s play with that pun on “the ayes have it.” What is it, exactly, that eyes have?

Well, at their most basic, the eyes of animals have three jobs, according to biologists:

1. the detection of light
2. the detection of shadows
3. the transmission of this information about light and dark to motor structures (because it’s one thing to be able to tell where a shadow’s coming from, quite another to be able to move away from it if you think it’s the shadow of a big, bad beast)

In the simplest kind of eye in the animal kingdom, found most often on the various tiny marine creatures we collectively call plankton, the motor structures that act on visual information aren’t muscle cells, like what you’d find in humans and other vertebrates, but cells lined with tiny hairs called cilia, which move the itty bitty organisms through their watery environments like the oars on a ship. The eye that provides these ciliated cells with info is similarly simple (alliteration away!); it’s made up of only two cells, one that receives the light, and another that processes shadows with the help of pigment.

Animal eyes range wildly in complexity, from the basic cup-shaped light and shadow detectors on the top sides of flatworms to the camera-like instruments of focus that we humans tend to roll every time we hear our bosses announce another brilliant idea for efficiency in the workplace.

There are sorts of stops in between, too. Squids and octopuses, for example, possess eyes that have lenses, as human eyes do, but lack the cells called cones that provide the human eye with the ability to perceive color. They can adjust their lenses to focus on objects near or far away but can only visualize those objects in terms of light and dark (though research suggests that octopuses have ways of reacting to color in their environments that have no parallels in human anatomy, which makes them really, really neat).

When it comes to human eyes, it’s been proposed that eyes have other functions besides the three listed above. Humans eyes not only receive information; they apparently also communicate it.

Unlike our relatives the great apes, humans have eyes that are partially white. This white part of our eyes is a structure called the sclera. The primate eye has a sclera, too, but it’s dark in color, frequently brown.

In 2006, anthropologists at the Max Planck Institute for Evolutionary Anthropology ran an experiment in which they had both great apes and human babies watch while researchers looked in one direction, then another, either by moving their heads or moving their eyes. The anthropologists found that the apes were more likely to follow the researchers’ gaze when the researchers moved their heads, while the human tots were more likely to follow when the researchers moved their eyes.

What this suggested is that our eyes evolved as a way of helping us cooperate on certain tasks. With our irises more visible against a white background, the researchers proposed, it would be easier to see where we were looking, so that others could see what caught our attention, too.

And let’s face it: as all the selfies on the Internet demonstrate, we spend a lot of time looking at human faces. In fact, it’s been suggested that eye contact is essential for creating a bond between a human infant and a caregiver, and that human babies spend twice as long staring into their caretakers’ eyes as primate newborns do. Is this behavior influenced by the evolution of the human eye? As always in science, research continues, but there’s a strong possibility that the eyes in this case really do have it.

On Water Sprouts, Tentacles, and Growth, or, Hail Hydra!

Ask just about any person who’s gone through puberty: hormonal changes can have some interesting effects on human growth.

They can have some interesting effects on plant growth, too. Like humans, plants have hormones running through their systems. True, plant hormones aren’t the same as the ones found in humans and other animals, and they act in completely different ways than animal hormones do. When it comes to trees, though, in some cases plant hormones have an effect similar to the one that testosterone and everything else in the human chemical cocktail had on most of the boys in my junior high class: they cause them to grow long, spindly limbs.

Like other plants, trees produce hormones in different parts of their bodies, such as the tips of their branches and deep down in their roots. When those parts get damaged, the distribution of hormones throughout the tree changes. Suddenly, the redistributed hormones cause thin, quick-growing branches to develop from previously dormant buds, called epicormic buds, along the branches and trunk.

The photo that inspired today’s post.

The shoots that arise from these slumbering buds are known commonly as water sprouts. When water sprouts grow along other branches, they grow vertically, shooting straight up toward the sky like hands of the newly risen dead bursting forth from the grave in a zombie flick. When these monsters shoots emerge from the trunk, the concept of vertical growth sometimes gets a little tentacle-y instead, as you can see in the photo that inspired this post’s illustration.

Aside from their occasional resemblance to Lovecraftian appendages, water sprouts are discouraged for other reasons. As some  have pointed out, water sprouts block air and light from moving among a tree’s branches, inhibit fruit growth, break easily, and divert energy from stronger branches.

Water sprouts also grow back easily, as pruning a water sprout creates a fresh wound that the tree ends up attempting to compensate for with new limbs and more leaves for carrying out photosynthesis. It’s not quite the same as the whole cut off a limb, two more will take its place dynamic that fans of Marvel Comics will appreciate, but it does make plant growth one more example of regeneration in nature, alongside some impressive mentions from the animal kingdom:

• Of course, the hydra, a relative of jellyfish, with tentacles that would make a tree with water sprouts feel envy if trees could feel such emotion. The hydra constantly generates new cells, meaning that if a hydra is cut in half, the cells from the creature’s main body column will differentiate. One half will grow a new head; the other will grow a new foot. And the cells retain a “memory” of which side of the body is up and which is down, even after being spun in a blender, which is better than I can do after a roller coaster ride.

• Octopuses, while I have water sprouts and tentacles on my mind (though an octopus’s appendages are referred to as arms, not tentacles). Male octopuses use one of their eight arms, called a hectocotylus, specifically to give packets of sperm to their female mates. It’s a gift that keeps on giving: males of such species as Abdopus aculeatus lose that arm during copulation and regrow a new one later.

• Salamanders, such as the axolotl. Thanks to the activity of certain genes, along with a little bit of electrical stimulation that’s believed to be carried along nerves, salamanders can regenerate a lot of their organs, including parts of their brains and spinal cords.

• Can’t forget about flatworms, a.k.a. planaria. As researcher Thomas Hunt Morgan discovered in the 1890s, cut one flatworm up into 279 pieces, and 279 more will come back to take its place. It’s fascinating any time the action of genes ultimately triumphs over the action of a knife.

Human beings exhibit a lot of different kinds of talent that other forms of life on this planet don’t. You’ll never hear about an octopus preparing dessert or a tree repairing a car’s transmission. When it comes to composing operas, I feel pretty confident in declaring that no other organism here has us beat.

When it comes to regenerating parts of our bodies that get damaged, however, compared to other creatures, we might seem rather unimpressive. Some parts of our bodies can regrow over time; the liver is a notable example. But brains, or even our fingers? We’re notably capable when it comes to reworking the world around us, notably less capable when it comes to repairing ourselves.

At least, we are right now.

Some researchers believe that the genes like the ones that stimulate regeneration in flatworms and salamanders could be activated in human beings, too. If so, human development could go well beyond the awkward growth spurts we experience around age 13. We could begin treating severe injuries in ways previously unimaginable.

Hail Hydra indeed.