by B. N. Sullivan
...and you definitely can't swallow it!
That is the message that fishes in the Spiny Puffer family attempt to transmit to potential predators. While other creatures defend themselves from predators by fleeing, or hiding, or camouflage, members of this family (Diodontidae) inflate themselves. In addition, evolution has armed these guys with another feature: rigid spines all over their bodies that are erected when the fish inflates.
Ain't nobody gonna swallow these babies!
When Spiny Puffers are molested or feel threatened, they open their mouths and draw sea water into their stomachs, which are capable of expanding greatly -- so greatly that the stomach and its watery contents can virtually fill the whole fish, squishing the rest of its organs up against its backbone. Its skin is stretchy, which also helps it to expand like a balloon.
The Spiny Puffer's spines actually are like specialized scales. When the puffer is not inflated, most of the spines lie more or less flat against the skin, but when the skin stretches during inflation, the spines go upright.
Both of the Spiny Puffers on this page are Caribbean species. I photographed them on two separate night dives in the Cayman Islands. Both photos are 1:2 macro shots. To give you an idea of their size, each of these fish was approximately 12-14 inches (30-35 cm) in length. [Click on either photo for a larger view.]
The common name of the fish in the top photo is 'Balloonfish' (Diodon holocanthus). This species has fairly long spines compared to the species in the second photo at left. The fish in the second photo is called the 'Porcupinefish' (Diodon hystrix). Its spines are really short and stubby compared to its cousin above.
Both fish pictured are partially inflated. Each was pottering along in a shallow reef area when we spotted them. In each case, Jerry shined his light on the fish so that I could approach closely and aim the camera. These are touchy critters, and that was enough to induce them to begin to inflate, so in each instance I snapped two frames and retreated before they freaked out.
Puffers are not very streamlined, even when they are not inflated, so they are not fast swimmers. Once they inflate they really are ungainly. Their little pectoral fins will flutter, but they don't attain much in the way of forward motion. To survive, they rely entirely on making themselves look unappealing as prey.
Some divers intentionally harass or even try to grab puffers, just to see them inflate. This is quite a mean thing to do. Remember, inflation is a defense. If the fish begins to inflate, that means it is alarmed. If it puffs out to its maximum, it is really scared! This behavior may be amusing to divers, but it really stresses the poor fish.
If you see a puffer and it begins to inflate, move away from it to let it know you are not a threat. Don't terrorize the puffers!