By: minustide
Photo Challenge, Symmetry, , , , , , ,

Off to the Shore for a Symmetry Beach Party

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. 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
  • 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 get 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 pie). The analogy falls apart, though; 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
  • 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 can be found 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 over 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’re not speed burners, 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 enigmatic cteophores.

Symmetry