Harlequin Shrimp - Hymenocera elegans - are gorgeous creatures. I've never owned one myself, but was able to get a good picture of one while visiting a LFS. These shrimp only eat starfish, and will otherwise perish in our tanks. If you choose to own this shrimp, you'll have to buy a starfish or two each month just to keep it well-nourished. There is a chance it will eat the tiny white starfish (Asterina), but since I've got several Serpent starfish already in reef, I'm not willing to buy this particular shrimp to see if that is true.
This Commensal Shrimp - Periclemens holthuisi - was hanging out in a carpet anemone at the LFS, and I had to take its picture. The anemone seemed unaware of its presence, and due to the lack of coloration of the shrimp, it was nearly invisible. Reef-safe.
The orange worm is a predatory worm that needs to be extracted once discovered in your reef. It exudes a toxin to stun its prey, and devours it. If spooked, it retracts back into the safety of the rockwork via that same toxin-slime that it builds in its wake. It will appear to be bright orange when spotted in the tank during flashlight duty. Avoid shining the light directly on this worm if you hope to catch it, and have forceps handy.
If you like to buy coral as a colony rather than a frag, you'll often times also inherit acro crabs. These little guys rarely stray out of the coral head, because they remain safe within the branches. They catch food that drifts through the coral, and keep the coral clean of detritus, nuisance algae, and other waste. Their symbiotic relationship is extremely beneficial to both the coral and the crab, because the branches keep the crab safe from becoming fish food. Reef-safe.
This guy is an Arrow Crab. Originally I bought one because I was tired of the abundance of bristleworms in my tank, his favorite meal. However, I had a Coral Banded Shrimp and he killed the Arrow Crab relatively quickly. After many years, the CBS died, so I got a new Arrow Crab. Mainly I enjoy this creature because of its delicate structure and blue pinchers, and because it is not a crab that likes to hide. Some report it will kill fish, but it would have to be smaller fish if at all. I've never had any negative experiences. Reef-safe-ish.
The T. gigas clam can grow to be several feet in size. The one in this picture was acquired at 23" wide by a local fish store for their 1000g display. A big tank is required by anyone wanting to care for this species of clam. It's going to need a lot of space, and as you might have surmised will be utilizing calcium constantly. This type of clam belongs to expert reefers, and even those people find them challenging.
I always read suggestions from others that Urchins are good algae eaters. Well, this urchin - Arbacia punctulata - sure seems to love coralline, the purplish algae I actually want! Oh well, its not doing too much damage over all, with 120lbs of LR in the tank. It travels over the rockwork and the glass, and hasn't been problem at all. It does knock over smaller frags like a bulldozer sometimes. Reef-safe.
When I was young, we had Flame Scallops in my parent's tanks. I love their vivid color, but apparently they just don't fare well in captivity. My first one was consumed by my Coral Banded Shrimp. Finally, I was bored with all the green in my refugium, and since these clams are not photo-synthetic, lighting wasn't an issue. I put it in there, and it remained quite content living in that predator-free zone for about a year. I fed phytoplankton every other day, which it filtered from the water column.
I always wanted a clam. At MACNA 2002, I saw about a hundred for sale, and asked many reefers what they thought would live under 3x55w Power Compact lighting. They agreed that my best choice was a Derasa. However, they were huge! This small blue Tridacna maxima called out to me and I put it at the very top of the tank for a few months. It jumped to the substrate, and has been happy as a clam ever since - for nearly five years. I fed phytoplankton to that tank every other day.
This is a common hitchhiker that you'll find in a variety of stony corals. This is a Barnacle, in the mollusc family, which will retract and extend the featherduster-like catcher's mitt used to capture particulates in the water. The coral in the background is a Galaxia. I've seen these in SPS coral, as well as Psammacora, Porites, and more. I've never observed any damage whatsoever by these creatures; rather they seem to be a symbiotic creature that lives in harmony with its host. I'd call it Reef-safe.