This is a 9 day old zebrafish embryo stained with a dye called Alcian blue that binds to cartilage. The embryo is a mutant called nacre, which produces eye pigment (very dark blobs on either side of the skeleton-like cartilage) but lacks pigment elsewhere, making the staining easier to see. Stains like this are useful for monitoring how cartilage develops.
For a look at how zebrafish are used to study pattern formation during development see this Signal to Noise article: http://sitn.hms.harvard.edu/
To get a more in-depth view on how zebrafish are being used to study the effects of the Deepwater Horizon oil spill see here: http://www.biomedcentral.com/1741-7007/10/40
Art contributed by Katherine Rogers