Thursday, September 16, 2010

'Anonymous Blobs'

Image and caption for "Chapter 3: The Fact of Variation" of the evolution book.


For example, in the top section of Figure 3-2 we see a schoolbook diagram of the lifecycle of the moon jellyfish (Aurelia aurita). At (A) you see the adult form of the jellyfish, which has released a proto-offspring, the planula larva (B) after about a week of growth. The planula settles onto a rock (C), where it develops into the scyphistoma (D and E) with tentacles allowing it to feed on passing nutrients. Eventually the scyphistoma develop into the strobila, a sort of stack of immature jellyfish on a stalk (F) that can bud off offspring (ephyra) for several years. When one of the ephyra are released (G), if all goes well, they develop into the immature medusa (H) which grows to adulthood (A). Note that this schematic drawing shows a single, largely-symmetrical ephyra (G). Seeing such a depiction in a textbook, we internalize a fixed image in our mind: this is how Aurelia ephyra look. But look at the lower part of the diagram, which shows the actual appearance of twelve Aurelia ephyra captured off the coast of California in 1996. Note that no two are identical; for reasons we'll examine in this chapter, there is always variation--even in this world of ‘anonymous blobs’.

Text and image (c) Cameron McPherson Smith 2010.

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