Reef Tank Janitors: Hermit Crabs and Snails

Blue legged hermit crab
RevolverOcelot/Wikimedia Commons/CC 3.0

Hermit crabs and snails are usually the first janitors that aquarists place in their aquariums because they are some of the best animals to have for controlling common algae problems.

About Hermit Crabs

Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca

Did you know that hermit crabs are scavengers? Yep, most species will eat just about anything they can find. For this reason, they make ideal cleaners for a reef tank, as long as you choose a Reef Safe Hermit Crab.

Properly chosen hermit crabs should have no negative impact on a reef system. In fact, they are solely beneficial. Small species that do not grow more than a couple of inches in size are most desirable, as they usually do not disturb other tank life, and they are able to get into tiny cracks and crevices where algae grow that larger hermits cannot access. They can also access spaces under rocks and corals where detritus or debris accumulated to remove it.

Large species such as the Anemone Carrying Hermit (Dardanus pedunculatus), Yellow Hairy Hermit (Aniculus maximus) and Halloween Hermit (Trizopagurus strigatus) are undesirable as reef janitors, as they may cause unwanted damage to your reef system. These types of hermits can disrupt tank life by climbing on everything, and because of the large clumsy, bulky shells they live in, cause the toppling of rockscape arrangements and the moving of corals. Besides, they may attack or eat other tank inhabitants. If you desire to keep large hermit crabs, do so only in a tank of suitable environment and size, and remember they will outgrow their shells. You need to provide them with new housing (larger shells) as they molt and grow, otherwise, they may attack other shelled animals to get a new shell. One commonly imported species that has this trait is the Clibanarius vittatus, most often sold as the Striped Hermit Crab. Do you need shells for your hermit(s)? Here are a few suppliers you can check out.

Popular Algae Eating Hermit Crabs

  • Small hermit crabs of the Genus Calcinus found in Hawaii are extremely efficient little critters. Some remain very small, only 1-1.5 cm in size, while other species in this group reach a length of fewer than two inches. Because of their tiny size, these hermit crabs can really get into those small spaces in a reef tank that other hermits cannot. The Left-Handed or Dwarf Zebra Hermit Crab is one of the best examples of a great reef safe algae eater.
  • The Blue Legged Hermit (Clibanarius tricolor), as well as other similar species are quite popular, but some such as the Anomura sp. will kill Astraea snails to obtain their shells.
  • The Red Legged Hermit (Clibanarius digueti) is said by some to be a much better algae eater than the Blue Legged Hermit, less aggressive, and has been reported to eat red slime algae.
  • Aside from eating algae, the golf ball sized Blue Eyed Hermit Crab (Paguristes erythrops) spends its time stirring up the top layer of the substrate of the aquarium.
  • The Scarlet Hermit Crab (Paguristes cadenanti) is one of the most popular hermits with reef keepers, because of its colorful appearance, and because it will eat all kinds of algae, such as red, green and brown slimes, as well as green hair algae.

About Snails

First of all, learn how to identify and stay away from all types of predatory shelled snails. These are stinging, venomous animals that either bore holes into their prey and rasp out their flesh, or kill their prey with venom, usually injected by means of a harpoon, and eat the prey whole. The largest and most obvious of the venomous species are all in the genus Conus, whose venom is not only lethal to other marine life but can be exceptionally lethal to humans! Snails in the predatory category are not generally sold in fish stores, but sometimes they can ride in as hitchhikers with live rock collected in the wild.

Three of the most common marine snail species used for controlling algae in saltwater aquariums and reef tanks are the Astraea/Astrea, Turban/Turbo, and Trochus/Trocus, with many varieties found worldwide. Let's take a closer look at each of these groups.

  • According to Julian Sprung's Reef Aquarium Manual, Volume One, Astraea sp. are the ideal snail to be placed in your aquarium as soon as ammonia and nitrite levels reach acceptable levels (less than 1 ppm). Introduced as soon as possible to a new aquarium, that has reached this cycling phase, these snails effectively limit the development of all microalgae. In other words, they are good at eating diatoms but will consume Red Slime and Green Hair algae as well. The Astraea tecta found in Florida and Caribbean waters inhabits rocky intertidal regions and is said to be quite adept at removing algae from rock surfaces.
  • There are numerous species of Turbans, referred to as Turbo Snails, and Trochus snails worldwide that feed solely on algae, making them perfect candidates for algae control. These types of snails are less adept at dealing with irregular surfaces, so they usually divide their time between cleaning the glass and digging in the sand for detritus.
  • Another good glass polisher is the tiny Black Nerite (Pipipi) snail (Nerita picea) found in Hawaii. It only reaches a size of about 1.5 cm and spends its time living along the shallow rocky and coral rubble covered inter tidal regions of the shoreline, in cohabitation with small hermit crabs of Genus Caleinus. The N. picea likes to reside on the aquarium glass in search of algae to eat during night time hours but will spend some time roaming around to aquarium. Close relatives are N. neglectus, that grows to the size of a thumbnail, and N. polita lives in the sand during daylight hours and grows to about 1-1/2 inches. These two species like to crawl out of the aquarium, therefore, they are not good choices.