Sea Urchins - Dangerous to Touch
Kirt Edblom/Flickr/CC 2.0
Like coral, sea urchins do not pose a danger to conscientious, controlled divers. However, a diver who is out of control or unaware of his or her surroundings may accidentally touch an urchin, which will result in quite a shock. Sea urchin spines are sharp and brittle and can easily penetrate a wetsuit and break off under a diver's skin. In addition, certain species of sea urchins defend themselves by injecting a painful venom into creatures that touch or attack them. As long as a diver is careful to touch nothing while underwater, he or she can be certain to avoid a sea urchin sting.
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Triggerfish - Dangerous
Christian Jensen/Flickr/CC 2.0
Some species of triggerfish are friendly, and others defend their territory against intruders. An example of a highly aggressive triggerfish is the Titan Triggerfish. Titan Triggerfish are found in the Indo-Pacific Ocean. They are quite large — over a foot long — and have specialized teeth and powerful jaws. Titan Triggerfish will defend their nests and territory violently, biting and butting at intruders.
These fish have been known to seriously injure divers and should not to be taken lightly. Many experienced divers are more nervous around Titan Triggerfish than any other species. Dive briefings in locations with dangerous triggerfish usually include a clear explanation of how to identify the triggerfish, and what actions to take if an aggressive triggerfish is spotted. Stay with the dive guide and follow his or her advice. In many cases, guides can help divers to avoid dangerous triggerfish territories.