Artist's depiction of a megalodon chasing two blue whales (image credit: Karen Carr, the Virginia Museum of Natural History)

What I find most interesting about this subclass is the body make-up these creatures share. They do not have a bony skeleton like humans; their skeleton is made up of cartilage—a dense connective tissue that is tough yet elastic. Their grace, speed, and great success as predators come from the fluidity of their movement.

This also makes it difficult for Ichthyologists to agree on taxonomy within this class as fossilized remains of cartilaginous fish are often poor. The oldest fossil on record of a megalodon, 18 million years old, is the only bone in the skeleton: a tooth. From the tooth, several researchers have tried to reconstruct the jaw and piece together what the body would look like. The tooth also suggests what this massive prehistoric shark preyed upon: There are bite marks on whales, dolphins, porpoises and sea turtles.

As the sharks, rays, and guitarfish pass by in the lagoon, I imagine the shadow of their giant cousin, the megalodon.

Mary Collins School teacher Blythe Shelley touching
a leopard shark at the Aquarium of the Bay

So, how do the Bay's leopard sharks, soupfin sharks, sevengill sharks, spiny dogfish, and other shark species differ from "non-shark" fishes? Here are a few key distinctions:

#1. You could say that sharks don't have a bad bone in their bodies. In fact, sharks don't have any bones in their bodies. Sharks-along with their relatives skates, rays, and ratfish-belong to a diverse class of fish that have cartilaginous skeletons, unlike the bony skeletons of other fish.

#2. Body shape. If you look at most fish head on, they have a generally oval shape. Sharks, in contrast, tend to be more triangular with a wide, flat under-surface. Their broad pectoral fins give them lift as they move through the water, not unlike the wings of an airplane. This hydrodynamic shape is key to keeping sharks afloat (you'll see why as we move on to difference #3).

#3. Besides bones, sharks lack the air-filled swim bladders that most fish use for buoyancy (If sharks are airplanes, does that mean bony fish are hot air balloons?) Instead, sharks keep afloat with the help of a large, low-density liver, their unique body designs, and the physics of forward motion. If a shark stops swimming it won't necessarily drown-only some sharks need to swim to breath-but it will sink!

#4. While most fish have gills tucked behind a bony flap called an operculum, sharks exhale water through gill slits located behind their head. Five gill slits are typical, but some sharks -like the sevengill shark found in the Bay-have more. Most sharks use ram ventilation to breath, swimming constantly with their mouths open to keep water flowing over their gills. Bottom dwelling sharks, whose mouths may be buried in the sand, inhale water through an opening on the top of their head called a spiracle and pump water past their gills.

#5. A shark's skin is covered with tiny dermal denticles that differ from scales on most fish. As their name indicates, they bear a physiological similarity to teeth. Their unique structure helps reduce drag as the shark moves through the water-in fact, sharkskin helped inspire the high-tech swimsuits we saw at the Summer Olympics.

#6. Most fish spawn by releasing large numbers of unfertilized eggs and sperm into the water. Sharks, in contrast, reproduce via internal fertilization. Depending on the species, they then lay a much smaller number of fertilized eggs, or carry the eggs inside until they hatch, giving birth to live pups.

Old Adobe Elementary teacher Juliet James examining shark teethSadly, these unique creatures are declining all over the world due to overfishing, pollution, loss of habitat from coastal development, and climate change. And that's bad news not just for sharks but also for their ecosystems. Like lions and wolves, most sharks sit atop the food chain as apex predators; thus their disappearance can trigger a cascade of disruption up and down the chain.

All the more reason for us to study up.

From fossils and gemstones to bird eggs and a stuffed Kodiak bear, it takes a lot of creativity to pack their collection. Everything seems to have special requirements. Their fish collection is made up of 200,000 jars – all filled with alcohol. And since it's a flammable liquid, they'll need a licensed hazmat driver to take it across town.

The live animals take extra care, of course. In this story, we followed the move of three black tip reef sharks. They hadn't been fed in a few days (so they wouldn't make any, um, deposits in their holding tank) and they weren't easy for the staff to catch, but they made it safely to their new exhibit. Even the largest of the three, F3 as she's known, made it ok, despite being a little groggy at first from too much oxygen.

As curator Bart Shepherd put it, their new Philippine Coral Reef Exhibit is a giant science experiment. The water for the 200,000 gallon tank comes straight from the Pacific Ocean through a four mile pipe. But most impressively, the Academy has been growing coral just for the exhibit. Just managing the water chemistry alone has been a major project, but now several dozen colonies of coral are happily planted in their new home.

Make sure to check out the audio slide show for this story to see how the new building is shaping up. And check out a few of the posts from QUEST science blogger Cat Aboudara, who is an Academy staff member, for more details on what it takes (here, here and here).

Watch the "Moving Day" audio slide show online, as well as find additional links and resources.

Lauren Sommer is an Associate Media Producer for QUEST.

The San Francisco Bay is much more of a mystery to scientists than I, at least, had realized. Why? It's simply too hard to peer into. There's no point in scuba diving. The bay is thick with sediment, much of it a legacy of gold mining explosions in the Delta. So if you want to know what's swimming around in those murky waters, you have to go fishing.

At first glance, this struck me as both laborious and tough on the animals – catch and release may spare lives, but not without putting a lot of stress on whatever's on the other end of the line. But when you think about how heavily we humans use the bay – sewage leaks, oil spills, urban runoff, coastal development — it becomes clear we have to take a closer look at how its inhabitants are faring. Sharks are at the top of the food chain, which means they're a great indicator of how everything underneath them is doing.

Of course, tagging is only worth the effort if you catch enough animals to have meaningful data – which means this project requires tenacity on the part of Aquarium researchers. For updates (as well as info on what to do if you catch a tagged shark) check out the Aquarium's website. Also, here's the radio piece we did on the same project.

Watch the "Cool Critters: Sharks of the Bay" TV Story online, as well as find additional links and resources. Also don't miss our behind-the-scenes photos for this story.

Amy Standen is a Reporter for QUEST and Radio News at KQED-FM.