though a small fish , its heavily toothed mouth is so wide that it can swallow whole most of the smaller reef fishes , but its supreme weapon lies in its erectile dorsal fins , the first few of which , acting on contact like hypodermic needles , are fed by poison glands containing enough dotoxin to kill a man if they merely graze him in a vulnerable spot-in an artery , for instance , or over the heart or in the groin .