No, you're right about what a null is. Null is null and that's about all. It really only has meaning in a database context. There's now something in .NET called a nullable type, which is kind of taking null into the language, though I still have some doubts about those types, and have yet to find a need to use one.

In general, we offer up an explicit value for unknown for those cases that really are unknown, and the sex of a fish certainly can fall into that category. Some fish you can only tell by dissection, but you happened to pick a good example of one that is difficult....and then there are the wrasse, which can change gender. Fortunately, I don't have to deal with them. I suppose that for such a species, you'd have to say not just the sex but the time of observation, because the sex is time dependent. Pretty strange fish. The fish I work with are much simpler. If you can get your hands on them, you can often tell by looking at them.

There are people who have policies of not allowing nulls in databases. I don't believe in that policy. I think that null is a perfectly valid value...or a lack of a value, or whatever.