For animals with swim bladders--an organ that contains air to maintain neutral buoyancy in many (but not all) fish--a rapid change in pressure will cause the swim bladder to expand, literally pushing its eyes from its head and stomach out its mouth (ala, the video above). It is not a pretty site... Unlike the rockfish living at 100+ m, humans are acclimated to life at 1 atm pressure. When we venture into the ocean for a snorkel or SCUBA dive, we can feel the pressure increase in our head and the must equalize ("pop our ears") to eliminate this pressure we feel. This may sound strange to the aquatic greenhorn, but it is easy :) In the SCUBA diver, however, equalizing is not the entire story because the diver is breathing compressed gas. This air--mostly nitrogen (~80%) and oxygen (~20%) just like air on land--becomes more and more compressed as you descend deeper. However, the partial pressure of these gases, or relative proportions of these gases in solution (i.e., the "partial pressure"), remains the same since the system is confined--i.e., your SCUBA tank. But, as you move deeper this air within the tank becomes more and more compressed, therefore more dissolved gases are moving into your blood, lungs and tissues than would happen at the surface. This will also drain your SCUBA tank faster since you are breathing a more compressed air in each breath. Just ask any diver how long they can breathe on their tank at 1 m vs. 30 m? Spoiler: you can dive at 1m for almost an hour, but may only spend a few minutes at depths > 30m before having to return to shallow depths.
Now back to our fish/diver barotrauma....If a diver, or a fish, makes a rapid ascent the gas in its tissues will expand. This expanded gas volume causes the air in the fish's swim bladder and our lungs to expand. This can lead to serious trauma. In divers, the increased gas volume may lead to lung over expansion and the formation of embolisms in vessels (usually in the arteries) where air bubbles obstruct the flow of blood. In order to prevent barotrauma divers always ascend at a slow rate to allow for gas to leave the blood and to prevent gas expansion in the body. Also during deep dives it is recommended to have decompression stops at ~ 5 m to allow dissolved gases to be released from your lungs (and blood) before returning to the surface where pressure is the least. In the absence of these safety procedures a diver is at risk of developing decompression sickness, or the BENDS. The bends describes the condition where compressed gases in blood and tissues expands once external pressure is reduced, in this case, upon heading to the surface. This is one of many reasons why normal SCUBA diving are not advised to go below 40 - 50 m.
But how is it that humans can get the bends but other marine mammals don't? Well the complicated answer is, yeah, other animals CAN get the bends by performing repeated dives, for example, to avoid predators or to hunt (Read about research on this topic from the journal Science). However, the bends in marine mammals may be rare since marine mammals are efficient divers, possessing special adaptations affording them protections from barotrauma. Check out the video below to learn more about whales and the bends!
Pressure as it relates to animals and SCUBA divers
I have been a SCUBA diver for nearly two decades. As a young man snorkeling, SCUBA, and surfing were the activities that motivated me to pursue marine biology as a career. Today, as a grad student I am still pursuing this elusive "career"--and yes, sometimes I do feel like Captain Ahab. SCUBA diving is safe and fun, and I firmly believe it offers a profound opportunity to foster a respect and appreciation for the ocean. So now that I'm all inspired from the TED talk, I'd like to discuss some cool points about diving, pressure, and how this affects marine organisms and humans, "in the vein (or artery)" of the aforementioned video.
Now enough of the factoids--go LEARN TO DIVE! This is something anyone can do. Today, you can take diving education classes and earn a SCUBA diving certification in most cities around the world. Learn more about SCUBA and dive safety at Divers Alert Network (DAN) and become certified to dive by taking classes through PADI or NAUI diving groups.