Off to the Shore for a Symmetry Beach Party

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Written by minustide

Welcome to my small celebration of symmetry. This beach party’s a renewal of kinship with animals we share symmetry with and a reminder that symmetry varies in ways worth appreciating. An in-depth survey of symmetry would expose developmental and evolutionary mysteries better left to zoology and comparative anatomy courses. So instead, let’s head to the shore, where examining a few examples from the world of bilateral animals will serve as a foundation for recognizing and enjoying less familiar layouts.

Bilateral symmetry
Most of us are familiar with bilateral symmetry because that’s what people and their most common pets have. A bilateral body can be divided into roughly mirror images along only a single plane. Animals with bilateral symmetry have left and right sides, what we sometimes call handedness.

Symmetry in some fishes is straightforward, but what about flounders? They have both eyes on the same side?
Barnacles are biateral
Close up of the head and front part of the worm in my hand.
Worms have bilateral pride
Molluscs raise pesky questions. Can you tell left from right on a clam? And what about snails? Their shells and bodies coil as they grow
Colonial forms, like pyrosomes, can take shapes that don’t appear bilateral. The tiny zooids inside these spherical cylinders have bilateral symmetry


Radial symmetry
This cast of characters gets along without handedness. Like pizza, their bodies can be divided into equal parts along a few to many planes.

Nobody argues radial symmetry with a giant green
Crystal jellies lay out like spokes on a bicycle wheel
Echinoderms, like this cucumber, are important players on Team Radial
Colonial forms, like ostrich plumes, can take shapes that don’t appear radial. Tiny polyps in these feather-like structures have radial symmetry
How many planes of symmetry do you see in this washed up sand dollar?


Biradial symmetry
Ctenophores lean to the biradial. They have eight rows of cilia for locomotion. They appear radial (imagine dividing the body like an eight-piece pizza). But, the analogy falls apart; they have a pair of feeding tentacles originating down by the mouth. With that, I’m on thinner symmetry ice than I like.

Sea gooseberries, washed ashore here, are spherical with eight rows of cilia and a pair of feeding tentacles


A tip of my cap to asymmetry
thoutershores is an inclusive place, so there’s no way I’m celebrating symmetry without inviting the sponges. There’s no plane of symmetry by which sponges can be divided into two equal parts.

A velvety asymmetrical beauty
A good-sized patch surrounding a couple giant green anemones and some red seaweeds
Purple encrusting sponge, proliferating asymmetrically
Halichondria panicea dares you to detect a plane of symmetry

My symmetry party illustrates that examples of the main types of animal symmetry abound on the intertidal shore. Variations on the bilateral form are dazzling. There are fast movers among them and sessile forms too. Speaking of sessile forms, how about the radial crowd at the tide pools? Starfishes and anemones aren’t using their pizza pie symmetry for speed. Neither are the pelagic jellies; they’ve got good movement, but they do not speed, and being a bit at the mercy of the currents, they tend to get tossed up on the beach. So head over there. It won’t be long ’til you find a beachcast jelly. While you’re there, keep an eye out for symmetrically enigmatic ctenophores.

Symmetry


I updated this post on September 20, 2022.

27 Comments

  3. This made me immeasurably happy. The photos are so wonderful and the idea of a symmetry beach party sounds like the most 🙂

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  5. I don’t know about anybody else Steve but I know I’m always on the lookout for enigmatic cteophores LOL. But seriously, a beautiful treatise on the symmetry (and asymmetry) with which nature gifts some of her many creatures. Well done!

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    1. I’m so glad you liked my post, and I wish to tell you, I’m not sure how your nice comment slipped away from me. Thanks you so much! The ostrich plumes are a favorite of mine too.

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    1. Hi, Margaret! I’m so sorry I let your very nice comment slip. My bad and I apologize. Good luck getting out to the shore, but please don’t do it ’til it’s safe!

      Reply

  8. Fabulous post, Steve. I love how you break down the classifications and give us marvelous images to accompany them. Your knowledge of marine life is very impressive!!

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