You raise an interesting point. Perhaps we shouldn't stipulate that a symbol must be understood the first time - the meaning of most symbols are probably taught

You raise an interesting point. Perhaps we shouldn't stipulate that a symbol must be understood the first time - the meaning of most symbols are probably taught in some way - but rather that once learned, the meaning is unambiguous. The silhouette of a shark on a beach sign is quite unambiguous, whereas an artist's illustration of a shark may be open to interpretation. In other words, perhaps it is the level of ambiguity which distinguishes symbols from other graphic forms.