Communication in order to share information between conspecifics is very common in nature. Just think about male birds singing very loudly early in the morning with the dual purpose of telling conspecific males that this spot is taken as well as getting information through to the females what a hunk he is, or the male monkey baring his canines in order to shape up the behavior of rowdy monkey teenagers.
This kind of communication between conspecifics often have quantitative elements. The information is not of an either or character (strong-weak, sexy-not sexy, angry-not angry) but rather about the level of the issue (not the strongest territory owner in town but pretty strong, not the Brad Pitt of songbirds but not to bad, seriously pissed and going to beat you up big time if you don’t comply rather than just angry). However, it is rare with that kind of communication between species, where different species do communicate with different levels of a signal. It does in some instances occur, though, with the marine environment giving one of the best examples.
As most of you following those blogs know, there is very interesting symbiosis between shrimp gobies and different species of shrimp. These gobies and their associated more or less blind shrimp lives on sand flats, where the gobies keep watch for predators and the shrimp digs the deep holes that both need in order to get shelter from predators. The shrimp gets information from the goby about current risk levels by always touching the posterior back fin of the goby with its its long antennae. The goby sends this information by wiggling its posterior dorsal fin, which the very sensitive shrimp antennae easily pick up. And, which is kind of amazing given the intellectual capacity of a goby and a shrimp, this information clearly is qualitative, in the sense that the shrimp gets information not about danger vs no danger, but about the specific level of danger. The intensity and maybe to some degree the frequency of the fin wiggle tells the shrimp the magnitude of the danger, which lets the shrimp make informed changes in its behavior, maximizing its output of work.
Such a high quality information exchange between different species seems to be quite rare. Obviously the co-evolution between species need too be very tight in order for such systems to evolve. Right now I can not think of any other symbiotic systems with such an elaborate information exchange. This could be due to that it is very rare, but it could also be caused by a post dive memory dysfunction on my behalf. I will get back with an update if a number of examples pop up after a good nights sleep!