Pagurus bernhardus
Pagurus bernhardus are unique in that although they live their whole lives in their shells, they do not actually grow their own shell - they simply swap out the shell for a larger one they find on the bottom. This fact alone makes hermit crabs a very interesting crustacean to keep in a reef aquarium. Furthermore, hermit crabs also act as cleaners, and the bottom of the aquarium, where they are kept, will always be clean.
Pagurus bernhardus are a type of crustacean and they are more closely related to lobsters than to real crabs. These creatures have distinctive crab-like paws and claws, but the rest of their body is soft and curved - which is why they need a shell to protect themselves from predators. Because these crabs do not grow their own shells (as snails do), they are forced to use discarded shells that they find near reefs on the ocean floor. Because they constantly change shells as they grow, these crabs have been nicknamed "hermit crabs".
There are many different species of hermit crabs, but not all of them are recommended for a reef aquarium. Hermit crabs, for the most part, are excellent inhabitants of reef aquariums because they harvest the food that fish leave behind - many species eat algae, small snails and detritus in the aquarium - they also help aerate the sand by sifting through it when they feed. Some hermit crabs do eat small fish, and even larger fish can become prey to them if they swim too close. For these reasons, you should be careful when choosing a hermit crab for your reef aquarium.
The different hermit crabs reach different sizes, but for the most part they are between 5 and 6.5 cm in size. The ideal water conditions for hermit crabs are similar to most reef aquarium inhabitants - they prefer 22 - 26° C water temperature, pH 8.1 - 8.4 and specific gravity (salinity) 1.023 - 1.025. Water quality should be monitored using an effective filtration system. Also, 1/5 of the aquarium water should be changed weekly with fresh water.
Sand or coral flakes should be used as substrate. Large rocks can be placed at the bottom for the hermit crab to hide in.
The menu of Pagurus bernhardus consists of various foods fed to marine fish, algae and detritus. However, keep in mind that you may need to supplement their diet with dried algae or pieces of shrimp if they are short on food.
Reproduction
The Pagurus bernhardus do not reproduce under aquarium conditions.
In natural conditions, the females bear eggs on their abdomen. The eggs are very small, sometimes one female has up to 15,000 eggs on her belly. The eggs incubate for 7 days and the hatching larvae live an independent life from the first days. Females spawn continuously throughout the year.
The Pagurus bernhardus have a life span of about 10 years.
