Trigger (Balistapus undulatus) fish have an unusual defence mechanism; A strong dorsal spine which can be locked in place by a smaller spine,. When approached by a predator it can dart into a small opening and 'trigger' this mechanism thus locking itself inside the opening. Most are solitary, and they have strong teeth which allow them to feast on most things including molluscs and sea urchins.
The yellow tang (Zebrasoma flavescens) belongs to the family Acanthuridae, which includes the surgeonfishes and the other tangs. Yellow tangs are good fish for beginning saltwater aquarists because they are able to tolerate fluctuating water conditions better than other tangs or surgeonfishes (up to a certain point).