One Ocean: Finding Symmetry All Over Our Ocean

As a new month approaches, I began checking my calendar for the various meetings and calls scheduled in February, as one does, only to have February 2, 2020, create a huge light-bulb moment.

The reason for this is that 02-02-2020 is a palindrome. For those who are new to the term, a palindrome is a word, phrase or sequence that reads the same backward as it does forward (popular examples being madam or racecar or the ever-famous “A Man, A Plan, A Canal – Panama!”). Palindromes are about words, but being an ocean-lover, this had me wondering—what sort of palindrome-like symmetry could we find beneath our wonderful waves? For animals, symmetry can be determined by drawing an imaginary line (or lines) on the body where the pieces created look the same. For example, imagine a line going down the middle of the human body lengthwise where the two halves are a mirror image of one another. That’s what’s known as bilateral symmetry. It turns out there are four main different types of symmetry found in animals: bilateral, radial, biradial and spherical.

Bilateral Symmetry

Bilateral is the most common type of symmetry found in 99% of organisms—both above and below the water. In bilateral symmetry, an organism can be divided longitudinally (lengthwise) into roughly mirroring halves. Popular marine species that exhibit bilateral symmetry include polar bears, sea otters, sea turtles and fish! Even the octopus, despite having eight arms, has bilateral symmetry.

Radial Symmetry

ethandaniels_jellies (1) — **sea jellies © Ethan Daniels**

Radial symmetry occurs when an organism can be divided by planes to produce roughly identical pieces, around a central axis, like pieces of a pie. Organisms that exhibit radial symmetry don’t have an easily distinguishable right or left side like we do. Instead, they have either a distinct top and bottom or front and back surface—for example, a sea urchin doesn’t have a clear right and left side but does have a distinct top and bottom.

Additionally, there are even more ways to classify radial symmetry depending on how many identical divisions can be made. Tetramerism is a term used to describe organisms that can be divided into four equal parts, such as one of my favorite species, the sea jelly. In fact, juvenile moon jellies have been shown to reorganize their arms after an injury so that they are once again symmetrical, which is probably better for moving through the water. Pentamerism describes organisms that can be divided into five equal parts. Among ocean species, the only organisms that exhibit pentamerism are echinoderms, like sea stars, sea urchins and sand dollars. But even echinoderms aren’t always considered pentamerism—many begin as bilateral organisms and gain pentaradial symmetry as they grow to adult form. Additional forms of radial symmetry included hexamerism (six equal parts) and octamerism (eight equal parts).

Biradial Symmetry

800px-Ctenophora-Comb-jelly-Euplokamis-sp.-02 — **Comb jelly © Wikimedia Commons**

Biradial symmetry is a combination of radial and bilateral symmetry where an organism is still divided into mirroring halves, but they are not limited to being divided longitudinally, or ‘down the middle.’ Ctenophores, such as the comb jelly, are the perfect examples of biradial symmetry—there are two different ways to divide comb jellies in order to get equal halves.

Spherical Symmetry

Spherical symmetry is one of the rarest types of symmetry in organisms. Spherical symmetry occurs when an organism can be divided into mirroring halves by any line that passes through its center. There are only two types of organisms that exhibit this symmetry, those found in the mostly ocean-dwelling Radiolaria and freshwater Heliozoa, as well as the freshwater green algae, Volvox.

Finally, there are a few ocean animals that are outside the box when it comes to symmetry. Some sponges are asymmetrical, which means they have no symmetry. Another fun exception is what we call flatfishes—they start life out as larvae with bilateral symmetry but grow up into adorably asymmetric adults. Because flatfishes spend most of their lives with one side of their bodies flat against the seafloor, they’ve decided to go rogue on symmetry. As flatfish larvae grow, they undergo a dramatic transformation where they flatten out, one side becomes pale, and one eye migrates over to the other side of the head.