mini-mouse

Last Friday found me back at the beach, taking advantage of a late low tide under the black, windswept sky. The fog had cleared, rain ceased; it was a tolerable forty degrees out despite the breeze coming off the bay. Myriad stars glittered overhead, and the moon, a day short of fullness, shone bright through patchy clouds. I rode my bike along the Alki Trail, Wellingtons in my pannier and headlamp ’round my noggin.

It was not a particularly low low tide, just enough to expose the twin rock jetties jutting west into the sound. There I found the usual anemones, kelp crabs, and jumbled ocher stars; also stout shrimp, sea cucumbers, whelks, and limpets amid the glistening rocks. There was a dead river otter washed up on shore—a saddening sight, but it seemed to have passed on naturally, as far as I could tell. As I waded through the shallows, headlamp beaming off the choppy surface, I noticed a number of stones festooned with ghostly white, filamentous spirals: eggs of a nudibranch, an aeolid by the looks of it.

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Note the egg-whorl

Commonest among Puget Sound’s intertidal nudibranchs is the shaggy mouse (Aeolidia papillosa), drowned slug-rodent of the sea. Two to four inches long, colored in creamy browns and eggshell whites, it sports tapered rhinophores that, with a generous imagination, could be said to resemble pointy mouse ears. The shaggy mouse is in fact common throughout its entire circumboreal range, found in number along the North Pacific and North Atlantic coasts as well as near Chile, Argentina, and the Falkland Islands. Its genus name honors Aeolus, Greek god of the winds and son of Poseidon.

Like all members of the nudibranch family Aeolidiidae, shaggy mice lack gills and instead rely on cone-shaped extensions of their digestive tract, called cerata, for gas exchange. (Keratos is Greek for “horn”.) These tentacle-like projections often cover the backs of aeolids in a flowing coat, waving in the current like wind-blown tresses, and are sometimes tipped with poisonous barbs (more on this in an older post). The cerata of shaggy mice are lanceolate and fine, giving them an almost furred appearance.

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In the intertidal sound, the shaggy mouse’s favorite prey is Anthopleura elegantissima, the green-and-pink aggregate anemone. Spewing a preprandial mucus that seems to relax the triggers on the anemone’s touchy nematocysts, the nudibranch munches first at the unarmed stem, taking dainty bites with its thirty- to fifty-toothed radula. The anemone is all but powerless to resist. Occasionally the poor creature is consumed in one sitting, tentacles and all—shaggy mice are known to eat their weight daily in cnidarians. It is impressive enough that aeolids will eat anemones and cannily appropriate their stinging defense, but perhaps even more incredible is the fact that some species, including the shaggy mouse, also steal the symbiotic algae of their victims for personal gain. Many anemones such as A. elegantissima harbor zooxanthellic algae in their tissues; these endosymbionts photosynthesize for their hosts in what is essentially an exchange of nutrients for protection. In devouring the anemones, shaggy mice ingest these algal guests and apparently treat them with at least a modicum of hospitality, for they continue to photosynthesize for several days within the cerata of their new hosts.

The eggs of shaggy mice, minutely plentiful and attached in gauzy whorls to rocks and seaweed, are characteristic of nudibranch spawn the world over: products of hermaphroditic slug-sex laid and left to tidal vagaries; gelatinous, often pale, their vulnerability exemplifying the hands-off parenting practice of “quantity over quality”. The egg-whorl is a mortal gamble of which the vast majority of participants will lose. Twenty or so days after laying, thousands of larvae will hatch from a brood and develop, free-floating, in the water column; less than one percent of these will reach adulthood. Such is the cruelty and the enormity of the sea.

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