Tuesday, October 20, 2020

Spines and cog wheels and five teeth

On my last trip to the beach, I had on my "shopping" list: some eelgrass with roots, because the hermits love to climb it and to eat whatever grows on it, hydroids and bryozoans and the like; some fresh sea lettuce, if I could find it, because the crabs had eaten all of the last batch; a bit of kelp, to keep my plumose anemone healthy, and a length of kelp stipe, because both the crabs and the hermits eat it, all the more enthusiastically as it disintegrates; a small stone with barnacles for the snails to eat; some snails for the anemones to eat. The little underwater community demands variety!

In the latest offerings torn up by the tide from the sea floor, I found the kelp and a good handful of eelgrass, and at the last minute, a nice blade of fresh sea lettuce. It all went in my bag. There were no barnacles this time; the carnivorous snails will have to make do with mussels for now.

Washing off the sea lettuce at home, I discovered that it was hiding a tiny sea urchin; lucky for it that I collected that piece; otherwise, tossed up by the tide and abandoned, the urchin would have died.

It's cruising around the upper levels of the wall of the aquarium, eating algae. And showing off its five-pointed star mouth, chomping away.

Mouth side of the urchin. About 1 cm. across the whole animal. (It's a baby.)

The arrangement of five teeth is called "Aristotle's lantern", not because they look like a light, but because Aristotle first described them. They open and shut, scraping at the algae; sometimes they can be used to scrape a hole for the urchin, even in rock.

The test, or hard shell of the urchin has spines; they're jointed, so they can rotate. Here you can see the purple joints on the spines. The urchin "walks" on its spines, its tube feet, and even its teeth. The tube feet protrude through holes in the test, and are soft and flexible. Each one ends in a round sucker; these are being used to hold onto the glass, and to move about. Quite rapidly, it turns out.

Zooming in. The suckers on the tube feet are like little cog wheels.

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En la última vuelta a la playa, my lista de "compras" consistía en: hierba marina "Zostera", con raices y todo, porque a los ermitaños les gusta trepar en lo alto y además comen lo que crece en la hierba, sea briozoos o hidroides u otras algas; un poco de alga "kelp" para mantener sana a mi anémona plumosa, y una sección del tallo porque los cangrejos y los ermitaños lo comen con gusto, tanto mayor cuando ya está desbaratándose; un poco de lechuga marina, porque los cangrejos ya comieron lo que les traje hace una semana; y una piedrita con bálanos para los caracoles carnívoros. ¡Mi pequeña comunidad acuática pide una dieta variada!

Entre lo que aventaron las olas, encontré la hierba, el kelp, y por fin, un trozo de lechuga marina. No había piedras con bálanos esta vez; los caracoles se las van a tener que arreglar con los mejillones.

Limpiando la lechuga marina en casa, descubrí que protegía un pequeño erizo de mar. Tuvo suerte; aventado a la arena por las olas, se hubiera muerto. Ahora da vueltas en la parte superior del acuario, comiendo algas.

Saqué fotos de su boca, ya que come con la boca al vidrio.

Tiene cinco "dientes"; se le llama a este conjunto la linterna de Aristóteles, porque fue descrito por el filósofo antiguo. Estos raspan a la superficie, cortando algas. Pero son suficientemente fuertes para también excavar hoyos para protección del animal, incluso en roca.

El animal se mueve usando sus espinas articuladas, vistas aquí, color crema con segmentos color morado. También para caminar (¡y bastante rápido que se mueve!) usa los pies ambulacrales y hasta esos cinco dientes.

Los pies ambulacrales son tubitos que se extienden desde poros en el caparazón. Son muy flexibles, y terminan con un botón que se adhiere a las superficies. En la segunda foto se puede ver como terminan con una ruedita con dientes.

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