This is a Whelk, a snail that consumes dead livestock on the reef. It will also attempt to attack and kill snails. These are carnivores. I had at least six of these in my 55g reef for the frist year, and never had a problem. After carefully acclimating a tiny Golden Maxima clam, the next day I found all the whelks finishing off my beautiful clam. They definitely attacked it - as clams are part of their diet - I only learned later.
One popular critter that helps with algae issues is the Tuxedo Urchin. As with most urchins, the biggest problem is how they may knock things over in your tank. They also can pick up small bits of rock and loose frags and carry them elsewhere. Reef-safe.
Scarlet Hermit Crabs are useful to keep corals and live rock clean of nuisance algae. They also help process detritus, stir the sand, and give you hours of enjoyment watching them navigate through your reef tank carrying their home on their back. I have a few of these and they do no harm. Hermit crabs are known to be opportunistic, and can kill a snail to steal the shell for a newer larger home. Reef-safe.
Hermit crabs come in various sizes and colors. This Red Legged Hermit crab would be a good addition to your clean-up crew. Don't confuse this with a Scarlet Hermit Crab, whose legs are vivid red. Hermit crabs tend to munch on algae, but will also look for meaty foods in the tank after a recent feeding. Hermit crabs take shells to use as their homes, and are known to kill snails to 'upgrade'. Still, this is normal and not a reason to fear getting a few for your reef tank. Reef-safe.
Photo by Darrin Trager
This adorable creature is a Pom Pom Crab. It is very small, and doesn't typically survive long in captivity. The pom poms are actually tiny anemones. The crab puts them on like gloves! When the crab molts (shedding its exoskeleton) it recovers the anemones and puts them on the ends of it claws again. When it gets in a defensive situation, it waves the pompoms to scare away the predator. Reef-safe.
This little guy came in on an SPS coral. Doug was unsure if was safe to keep, and did his research. It turns out it is a Trapezia Lutea commensural crab for Pocillipora coral. Reef-safe.
This tiny hermit crab is a Paguritta gracilipes from Indonesia. It is very very small, living in the former home of christmas tree worm. In the above picture, you can see the claws of a neighboring hermit crab in the foreground, so territorial issues don't appear to be a problem. It is unclear if they kill the worm to get the home, or if they move in once the hole is vacant. They don't seem to be picky eaters, and have been observed to eat brineshrimp and whatever floats by. My guess is that the long antennae act as nets to catch small particles of food.
Sometimes you'll need live food for your tank. If you have Lionfish or even larger seahorses, contact your LFS (local fish store) to see if they sell Ghost Shrimp. These need to be kept in fresh water with an airstone, and then added to the display tank when it is time to feed. They will live for a brief period in saltwater, long enough for hungry fish to hunt them down.
Back in 2006, I noticed this Pavona coral had a tiny dweller. She didn't come out and crawl around; during the daytime she withdrew more deeply into the coral's protection. It's called a Gall crab, because it creates a gall (or cave) in the coral it hosts within. I never saw any reason to be concerned by this crab, primarily since it never left its home. It captured any food within reach. According to one source, the female is trapped in the gall while the male can come and go as he pleases. They breed in their cave where the female protects and raises their young.
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.