In the final exams for our undergraduate zoology degrees, my fellow-majors and I were given an assortment of petri dishes, each of them containing an animal. Our task was to classify the creatures to the phylum level. Now, more than a decade later, I can conjure up only two of the test dishes. The first contained a dead cockroach (phylum: Arthropoda). The other contained a rock in a thin layer of water, with a green, slimy film on one of its faces. Midway through the allotted time, the invigilator observed aloud that many of us seemed to be trying to classify the rock. It was, he assured us, a rock. The unspoken corollary: we should perhaps focus instead on the slime.
The second dish turned out to contain a sponge (phylum: Porifera), an animal that looks like the antithesis of an animal. Sponges sit immobile, anchored to rock or rooted in sediment, filtering particles from water. There are around eighty-five hundred known species, and they live in both oceans and freshwater. Sponges exist in a rainbow of hues and can, in the case of the Caribbean giant barrel sponge, grow up to eight feet in diameter. All of them share the same basic body plan: a measly two layers of cells, enveloping a jelly-like filling. They lack digestive systems and circulatory systems. They have no left or right, no front or back. Good luck finding a sponge’s head—it doesn’t exist.
But this simplicity is deceptive. A sponge essentially carves organs out of negative space, using its layers and jelly to delineate a complex network of channels and pores, which transport nutrients and waste much like a human kidney or bloodstream. This Spartan anatomy is so efficient that a single sponge can filter up to a thousand times its body volume of water in one day. Off the coast of Canada, reefs of glass sponges (so named for their silicate skeletons) can clean more than five hundred vertical feet of overlying water. And, if they take in dirt or toxins, sponges can clear themselves out with a languorous sneeze.
Sponges are smothered in a diverse coterie of bacteria and other microbes. All animals, including humans, form partnerships with such organisms, but a sponge’s partners produce an exceptionally vast pharmacopeia, perhaps to help it cope without an immune system of its own. Humans have repeatedly raided this chemical larder…
Read the whole post here.