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.
The Camelback Shrimp is recognizably different from the Peppermint Shrimp, in that it has a hump on its back. This shrimp isn't a good choice for reef tanks, but can be used to devour AEFW off infected corals, according to one site. Set up a hospital tank, place six of these shrimp in the tank, and place an infected coral in their midst for exactly 30 minutes, not a minute longer.
Feb 2014: I discovered this tiny hitchhiker in the rockwork. More than likely Tridacna hippopus sp., or Bear Claw clam.
Here's a pretty good view of what it looks like, if you happen to know the precise spot in my reef. It's tiny as can be.
Peppermint Shrimp - Lysmata wurdemanni -are often sought after because they tend to eat small aiptasia in the reef. While most LFS will carry these, some are possibly mistakenly sold as Peppermints when in fact they are Camelback shrimp. Camelbacks have a hump that is quite distinct when the animal is viewed from the side rather than from above (as pictured here). They do not harm most corals and fish tend to ignore them. They are usually only visible late at night with a flashlight, as they hide in the rockwork all day long.