Well, aside from being cute and frisky little creatures, green spotted puffer fish are actually a subject of much research these days. The green spotted pufferfish, Tetraodon nigroviridis, has a genome of 350 megabases, about an 8th the size of the human genome and the smallest genome known of all vertebrates. That’s not to say they have less genes, but rather they have less space between the genes. They have very small introns—the regions of genes which are not used to code proteins. In humans and other vertebrates, the entire gene, introns included, is transcribed into mRNA; then the introns are spliced out by the cell machinery before translation (protein creation) begins. Some introns have unknown functions, but others exist so that alternative splicing—cutting the mRNA in different ways to create different proteins—can take place. Puffer fish not only have fewer and smaller introns, but they also have less DNA in between genes. One example is transposable elements, moveable genetic sequences that may have arisen due to interaction with viruses. These make up 45% of the sequence of the human genome, while green spotted puffers only contain about 3.8% transposable elements in their genomes. Scientists are now asking why puffers did not develop long non-coding regions like most vertebrates, and what sort of cell machinery exists to prevent their coming about. Basically, how do these little guys keep their DNA so tidy?
For more info. on the compact genome of Tetraodon nigroviridis, check out this cool site: http://www.cns.fr/externe/English/Projets/Projet_C/organisme_C.html#ecologie