Divided by five

Thinking about echinoderms: the sea stars, the sea urchins, the sand dollars, the cucumbers that I see at low tide.

Sand dollar, alive. Dead ones lose the blackish hairs. Look closely: they move!

Green sea urchin, Strongylocentrotus droebachiensis. (What a name for a small critter!)

Everything else, the crabs and the fish and even the worms are arranged in a way that seems "normal" to us; with two similar, sometimes identical, halves: whether they have six, ten, fourteen or umpteen feet, they come arranged in pairs.

But the echinoderms are arranged with five points around a central axis. Usually; some of the sea stars may have more, from 6 up to 50. But it's usually 5.

No reason: Ma Nature does what she wants.

But reading about them on Wikipedia, I discovered this:

Although adult echinoderms possess pentaradial, or five-sided, symmetry, echinoderm larvae are ciliated, free-swimming organisms that organize in bilateral symmetry which makes them look like embryonic chordates. Later, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis. (Wikipedia; Echinoderms:Anatomy and Physiology)

Ma Nature does what Ma Nature wants to.

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Estaba pensando en los equinodermos, tales como las estrellas de mar, los erizos, los dólares de arena y los pepinos de mar, todos los que veo en las playas cuando la marea está baja.

Fotos:

1. Un dólar de arena, también conocido como galleta de mar. Está vivo; muertos, pierden los pelos negros rapidamente. Míralos de cerca; están en continuo movimiento.
2. Un erizo de mar Strongylocentrotus droebachiensis. Un nombre muy largo para un animal tan chico.

Todo lo demás; los cangrejos y los pescaditos y aun los gusanos están organizados en una forma que a nosotros nos parece "normal"; con dos mitades por lo menos similares, muchas veces idénticas. Aunque tengan o seis, o diez, catorce o una multitud de patas, se organizan en pares.

Pero los equinodermos se organizan con 5 regiones alrededor de un eje central. Casi siempre; algunas de las estrellas de mar tienen más brazos, desde 6 hasta 50. Pero por regla general, tienen 5 brazos o lados o regiones.

No se puede saber el porque; la Madre Naturaleza hace lo que quiere.

Pero leyendo las páginas en Wikipedia, encontré esto:

Los equinodermos son animales originalmente con simetría radial, menos las larvas, que poseen simetría bilateral. (Wikipedia/es)

El sitio en inglés explica con más detalle:

Aunque los equinodermos adultos poseen una simetría pentaradial, o sea de cinco lados, las larvas son organismos con cilia que se organizan en simetría bilateral, lo que las hace parecer cordatos embriónicos. Más tarde, el lado izquierdo del cuerpo crece, abandonando el lado derecho, el cual finalmente es absorbido. El lado izquierdo desde entonces crece en forma de simetría pentaradial, en la cual el cuerpo se organiza alrededor de un eje central.

La Madre Naturaleza hace lo que le da la gana.

Flowery pancake animal

At low tide, well out towards the bottom of the intertidal zone, we often find sand dollars. They're usually hard and whitish; the bare, empty skeletons of the living animal. Rarely, we see a live one. These are black, and have spines all over, top, bottom, and edges. When I look closely, I can see the spines moving.

We found one of these last week, and I brought it home to add to the aquarium, where it settled in happily, burrowing slowly under the sand.

I fished it out two days later, to see if it had survived the transfer. It had; it was trying to walk out of my hand, on its spine "feet".

Top side. One edge is broken.

Under the light, it wasn't really black. And where the edge is regrowing, the spines had a pink tinge to them. When I had a photo, I checked the coloring; the main colour is muted down so it looks brown or black, but when I saturated the photo, the spines were all red.

Detail, central "flower".

I looked at the spines under my little microscope. They're beautiful, transparent as glass, but with dotted lines running the length of each spine. There are two types; long, tapered pillars, and between them, short, thin hairs with a knob on top. And all of them, all the time, are swaying in all directions.

They are far too small for my camera, but I found a photo on the web that is more or less what I saw.

Microphotograph from the University of Queensland, AU. I've lightened it up for better visibility.

Back in the tank, the spines went to work, sliding the sand dollar over and under the sand until it disappeared from view.

For a flat plate, moving on microscopic fur, the dollar manages to cover a lot of territory. Here's a time-lapse video of one in California, from mrjustin5 on YouTube.

This was about 25 min(ute)s of footage condensed into 1 min 30 s.

A mini-Roomba! All it needs is a mini-kitten.

The sand dollar can live up to 10 years. This one is about 3 inches across, half the size of the largest I've seen, so it's probably still young. I hope it does well in the tank, away from its hungriest predators, starfish and gulls. Hermit crabs will eat them, too, so I'll watch for that and remove it if necessary.