Darkness is an excellent device for directors to use to frighten the audience. We fear not knowing what's going on around us, and our visibility is severely limited at night. In this instance there are only two light sources, the bonfire and the moon. Both of these are quite weak, and the fire source disappears completely when two of the young people, Chrissie and Cassidy, run from the group. The bonfire being the initial dominant light source is interesting.
Fire is another primal component, and has a number of differentiating signifiers.
The teenagers are protected by this light, but their safety is compromised when they leave the proximity of the fire. However, although it can provide warmth and comfort, it is often used in the visualisation of hell, and the use of the fire could also be read as a warning signal of what is to come. Spielberg is again making us psychologically unstable, keeping us unsure of our enemies identities. Later in the movie it will be the town's Mayor, who refuses to close the beaches for financial reasons, who is ultimately responsible for several further shark attacks. Spielberg is warning us to be aware and observe the environment around us before committing ourselves to a preconceived safety.