To reduce the problem of Valonia (bubble algae), many people recommend the Emerald Crab as the solution. They are known to pick these off the rocks and consume them. I've added them to my reef often, and results vary. I've seen them consume bubble algae, as well as decimate a troublesome spot of Caulerpa. Reports vary with these crabs, sometimes good and sometimes bad. Reef safe, usually.
Usually expensive, the Skunk Cleaner Shrimp - Lysmata amboinensis - adds color and flair to any reef. Fish will swim right up and allow the Cleaner Shrimp to 'clean' its body of all types of parasites, so those with Tangs would be wise to have one or two in their tank. In pairs, they breed quite often, and the released eggs will feed your tank's inhabitants. Reef-safe.
This shrimp is carrying eggs (the bright green mass under its belly/tail section.
This interesting pair live together. The Yasha Goby - Stonogobiops yasha - guards and signals the Candy Stripe Pistol Shrimp - Alpheus randalli - when it is safe and when it is time to duck for cover. The shrimp always keeps at least one antennae on the fish, and when it feels a sudden twitch it knows to retreat into its hole in the sand.
Blood Shrimp are simply gorgeous. Often, they are fairly expensive at the LFS (local fish store). I've had one for almost a year, and occasionally see it in my 280g reef. It hides all the time, and late at night I can see it briefly if I feed something nearby. I wouldn't recommend this for any tank, but rather for one where you can enjoy this creature if hiding spaces are limited. If the tank contains anything that will make the shrimp fearful, you'll rarely see it. Reef-safe.
This is a Red Legged Hermit crab. It was trying to right itself in this image, and almost pulled itself completely out of its shell. Please do not confuse this one with a Scarlet Hermit crab, which is vivid red. The red-legged ones are considered safer than blue legged hermits. Hermit crabs are known to kill snails to get their shells, so I'll label this one Reef-safe-ish. They are a good member of your clean up crew.
This is a Black Fingered Mud crab - Xanthid crab. He arrived on a coral when small, but had grown quickly. This guy is black in real life; this picture is a bit misleading. His pinchers remind me of a Mithrax crab, but it has 'teeth' within the claws. Because he can't be trusted, he was transferred into my refugium where he'd be trouble-free. Xanthids are Not Reef-safe.
If you like small elusive crabs, the Porcelain Crab - Petrolisthes spp - fits the bill. The white ones tend to reside near or in an anemone, using the anemone's tentacles for protection. The crab has pinchers and can catch meaty foods, but it also has twin fans that it holds out to trap particulates floating by. A clownfish may tolerate this neighboring crab, but this depends on the fish's territorial attitude. These crabs can be tough to locate in a bigger tank, and once discovered elicit a smile for sure.
When it comes to keeping the tank clean, Blue Legged Crabs are a good choice. They keep the substrate stirred up as they meander throughout the tank, and they can consume some algae if it is within reason. They are known to kill snails to steal their shell for themselves. After all, these crabs do grow, and their home becomes too small. Reef-safe.
Photo by Tod Bogard (buzz123)
This is a Spiny Lobster. Its colors are vivid, with a deep shade of purple on its arms. Richard emailed me with his own experience, which follows: "I really never fed it anything specific; it would eat flake foods, pellet foods especially anything that was meaty (would go crazy with chopped up shrimp, scallop or pre-packaged frozen foods). It would only go after weak fish, but never healthy ones. I had to be careful introducing new fish. Most behavior was nocturnal; during the daytime it would usually only will come out for feeding.
If you want to see guests react strongly, get a Coral Banded Shrimp (or Banded Coral Shrimp) - Stenopus hispidus. Molting every few weeks can give you the momentary impression that suddenly you have two, their molt closely resembling the original creature. It will consume Peppermint Shrimp, Arrow Crabs, and Flame Scallops unfortunately, but my Cleaner Shrimp has eluded it for some time. The bright colors and its agility make it a fascinating addition to any reef tank.