Mermaids: Pt.1- Eye Physiology

A friend of mine gave me the suggestion to do this week’s post when her daughter asked how mermaids can keep their eyes open under the water, and whether or not they lay eggs or have live offspring.

Mermaids, they’re mythological creatures of the sea, half human and half fish. They’re known to sing to sailors and entrance them, distracting them from their word and causing people to walk off the deck or run their ship aground. The first mermaid stories appeared in Assyria ca 1000BC. It’s said that the goddess Atargatis, mother of an Assyrian queen, loved a mortal and when she unintentionally killed him, she jumped into a lake. Ashamed, she tried to take the form of a fish, but the waters couldn’t conceal her beauty so she took the form of a mermaid, human above the waist, fish below.

This isn’t the only story. There are some from all around the world, at all periods of time. A sighting was even proposed in 2009 off the town of Kiryat Yam in Israel, offering a prize of one million dollars for proof after dozens of people had reported seeing a mermaid leaping out of the water like a dolphin.

This brings me to mermaid physiology. Can mermaids see under the water? If so, how? Human eyes aren’t designed to see clearly underwater as we can out of it. Yes, there are those who can get used to it, but for the most part, eyesight will be blurry, not to mention chlorine and salt water will make them sting. If mermaids are human from the waist up, wouldn’t their eyes be like ours? As something that has adapted to life underwater, there are certain things that would need to be different. Just like needing gills to breathe, mermaids would need eyes like a fish to see. If you think about it, it’s believed that a long time ago, humans evolved from fish, so why not have a branch of that evolution that stayed in the water? Just think about Neandertals and Homo Sapiens. They lived side by side. Mermaids could be a line that just hung around. If mermaids were a new evolution, I’d say there’d be more sightings and we wouldn’t necessarily see stories about them going back to 1000BC. Yes, there could be a mermaid mutation gene that is rare, but still, you’d hear about some babies being born with tails and whatnot.

(Before rotten food gets thrown my way, I know this is a bit far-fetched, but, the point is that this is all the use of imagination and evolution does weird things. Some lines branch off and continue, some stay in limbo, some haven’t been discovered yet. Fossil records are spotty. There’s so much we’re still discovering.)

Ok back to fish eyes. Fish don’t have true eyelids, not like humans who have them to prevent their eyes from drying out or protecting against dirt. A fish’s eyes, however, are always covered by water. Whereas human irises can contract or expand depending on light conditions, fish irises don’t because light never changes in intensity underwater. They don’t have need for such an adaptation. The biggest difference between the human and fish eye, occurs in the lens. With humans, ours are fairly flat or ‘dishlike’. In fish, however, it is spherical or ‘globular’. Human eyes are capable of changing the curvature of the lens in order to change the focus at varying distances (flatter for long-range and more curved for shorter). Although fish eyes have a rigid lens and the curvature can’t change, it can move toward or away from the retina like when you’re focusing a camera.

Fish that live in dimly lit regions usually are found to have larger eyes. Because mermaids would probably live deep in the sea where it’s not as light, they would also have larger eyes than humans. Next week, I’ll get into the second part, do mermaids lay eggs or have live birth? What do you think?

Check out this interesting article if you want to read more about mer-physics.