Cephalopods—the class of marine mollusks that includes octopus, squid, cuttlefish and nautiluses—are amazing animals unlike anything that walks on land. The world-renowned Monterey Bay Aquarium set out to shed new light on these incredible invertebrates with an expansive exhibit that opened this past April called Tentacles: The Astounding Lives of Octopuses, Squid and Cuttlefishes. Senior Aquarist Chris Payne took us on a walk-through of the 3700-square-foot exhibition to explain a little more about what makes these sea creatures so unique.

1. Although the differences between the species can be stark, they all have a basic body structure in common: A head with eyes, a mantle that contains all their internal organs, a siphon for scooting around the oceans that works by expelling a jet of water, and a powerful parrot-like beak with a file-like tongue called a radula. Additionally, they are most overtly identified by a "foot" that has evolved into either tentacles or arms—but those two appendages are not the same. Octopuses have eight arms, covered in hundreds of suckers, but no tentacles. Both squid and cuttlefishes have eight arms and then an additional two feeding tentacles that shoot out to grab prey and pull it in towards their arms. And nautiluses have up to a hundred tentacles, and no arms. For animals that have both, the distinction is how the appendages are used. "They use the feeding tentacles to attack their prey—shoot the two tentacles out, grab whatever it is they’re feeding on, pull the food into their arms, and then the arms will help manipulate the prey," Payne says.

2. They grow really fast. Most cephalopods only live for a year or two—even the giant pacific octopus has a maximum life span of just five years. (The exception, as always, is the nautilus, which is not even sexually mature until it's 15 years old.) Since they're born small, out of pearl-sized eggs, to reach their (admittedly variable) adult size, cephalopods exhibit an extremely fast growth rate.

3. To mate, the male cephalopod reaches into the female's mantle with one of his arms and deposits the sperm. But this doesn't necessarily mean the babies will be his. "Cephalopods can store the sperm and decide if they want to mate with one individual male or another individual male," Payne explains. Once mom has picked the perfect genes to pass on, she lays her eggs—usually 15 to 30 egg capsules with four to six embryos each.

4. And then she waits. During gestation, mom will guard the eggs and fan them to keep the embryos oxygenated. Her dedication to their safety is admirable and made significantly more so by the fact that during this time—which is typically several months but can sometimes span (at an extreme maximum) over four years—she does not eat at all. "They're expending as much energy as they can until the last egg and then that’s it. They die. There’s no round two," Payne says. He stipulates that, while this is typical, there are some species of squid and cuttlefish that can lay several clutches of eggs.

5. All cephalodpods used to have hard outer shells but only nautiluses retained them over the millennia. Without this physical shield, squids, octopuses, and cuttlefishes all developed ink sacks to serve as a defensive tactic. Since they are significantly slower than other cephalopods, nautiluses feed primarily on scraps, including shrimp and lobster molts, which provide the calcium they need to maintain their striking, striped, spiral shells.

6. The shells themselves are fascinating. They are an incredible natural example of a logarithmic spiral and give the nautilus the ability to regulate its buoyancy by taking in different amounts of water and gasses into the various chambers (the nautilus' mantle extends only partially into the shell).

7. All cephalopods, except (of course) nautilus, can change not only the color of their skin but also the texture. Sometimes this is a reaction to emotional stimulus—a flurry of cuttlefish turn black when they've been disturbed and the giant pacific octopus goes deep red when we open her tank, perhaps thinking we've brought food—but usually it's a matter of camouflage. Octopuses, in particular, spend most of their lives lying in wait to ambush potential prey, and to give themselves quite the competitive advantage, they alter their appearance to almost perfectly match the color and texture of their surroundings, from sandy ocean floors to rocky coral crevices.

8. The mimic octopus takes his shape-shifting a step further, transforming to resemble not just scenery but other animals as well. It's really better explained with a video:

9. Octopuses are certainly the smartest of the invertebrates (have you seen some of the cool things they've been caught on video doing?) but they are still an invertebrate and they put that lack of backbone to use. Their malleable bodies can fit through any space large enough to accommodate their beak.

All photos courtesy of Monterey Bay Aquarium.