I am thinking that if you do a PhD in a very specific area (i.e. an specific receptor in a rare bacteria) you'll close a lot of doors for future work opportunities. Am I right? I find difficult to imagine a company hiring somebody who only knows about an specific protein (even if he knows a lot about it). Any postdoc there?

Most PhD's are specific by their very nature. After your PhD you may never use the facts that you pick up along the way about the specific receptor in your rare bacteria, but they are only a small part of your PhD experience. In addition to large amounts of general background knowledge you will learn how to design experiments, how to interpret results, how to write research papers, library research, internet research and a variety of laboratory techniques. Companies will be interested in your general scientific ability and the techniques you have used which can be applied to all sorts of research problems. Many universities are begining to offer courses under the New Route PhD, which have an extra year of training in research methods and theory.

I want to do a phd but still need to define the topic in behavioural ecology, more precisely the adaptive nature of visual signals, the cause function and evolution of colour patterns... what career is open to me around the world, except for academia?