This week's fish is Xiphophorus maculatus, the platyfish. Aquarium hobbyists are probably familiar with this species. It's also a very important research organism. Notice the dark patches near the tail. Those are caused by cells named "macromelanophores". Crossing one of these with a X. helleri, then backcrosing the resultant hyrid offspring to the X. maculatus results in four categories of offspring, one quarter of which develop malignant melanoma (skin cancer) and die around the age of sexual maturity (a few months). This is not increased susceptibility to cancer, or some probablistic cancer rate, this is genetically determined cancer. The macromelanophores basically proliferate out of control, invading other tissues and messing up tissues and organs.
This genetic cancer has made Xiphophorus as a genus a much-valued research organism for people studying melanoma and other types of cancers. It also raises some interesting questions about speciation, population dynamics and ecology for these fishes that have not been as thoroughly researched.
They're also very pretty, in my opinion. Note the lack of head-mounted laser weapons, however. I admit, this is not the greatest picture out there, but it nicely shows the macromelanophores. Blatantly stolen from http://www.aquarium-holgen.nl/overzicht.htm, which is in Dutch, so I couldn't read the "don't steal my pics" message, if there is one.
This genus forms the other half of the comparative work I did in my Master's. I used published genomic resources (microsatellites on a linkage map) in a Xiphophorus mapping cross as the foundation of my work in Poecilia reticulata, last week's Friday fish.