Gall crabs are a type of crab that you may not see in your corals or wish to. You may have to face these tiny crabs in your aquarium journey and don’t know what that parasite is in corals.
Here I’ll talk everything you need to know about the gall crab, their characteristics, how harmful they are and what to do if you see them in your corals.
Habitat and distribution
Gall crabs, known for living closely with reef-building corals, can be found across the Indo-West Pacific, including locations like Hong Kong, Malaysia, and the Red Sea. These crabs thrive from shallow waters to depths of about 30 meters, even along the Saudi Arabian coast.
Gall crab in coral
Gall crabs live inside corals, with the coral growing around them to form a protective structure known as a gall. Females stay inside these galls permanently, while males are more mobile. These crabs can be found in various coral species, including Trachyphyllia geoffroyi, mushroom corals like Lobactis scutaria, and scleractinian corals such as Stylophora pistillata and Pocillopora verrucosa. Their relationship with corals is symbiotic, with crabs creating larger galls as they grow. Despite being harmless to corals, they remain vulnerable if their host coral dies, staying even in dead coral structures.
Are they reef safe?
Researchers have found that four species of gall crabs live in 35 different coral species. They dig into stony corals without causing significant harm, making them generally reef-safe. In terms of impact, gall crabs aren’t bad for coral health—they form part of the coral ecosystem without damaging it. So, while they might seem unusual, they coexist peacefully with their coral hosts.
Gall crab species
Crabs from the family Cryptochiridae are commonly called gall crabs. There are twenty-one genera, with the most popular being Opecarcinus, which contains nine species and originated around 15–6 million years ago.
The crabs likely started in the Indo-West Pacific and later moved to the Atlantic, adapting to new environments in the Caribbean. Here are some species I found:
- Lithoscaptus doughnut—Hong Kong
- Lithoscaptus semperi—Indonesia and Malaysia
- Fungicola fagei—Hawaii; prefers Lobactis scutaria coral
- Fungicola syzygia—Indo-West Pacific
- Opecarcinus hypostegus—Curaçao; prefers Agaricia lamarcki and Agaricia undata corals
- Kroppcarcinus siderastreicola
- Troglocarcinus corallicola
- Fizesereneia panda—Red Sea; prefers Lobophyllia coral and Symphyllia recta
- Hapalocarcinus marsupialis—Egyptian Red Sea coast
- Opecarcinus aurantius—Egyptian Red Sea coast
- Pseudocryptochirus viridis
Gall crab characteristics
Gall crabs are less than 1 cm in size, with the females being twice the size of males
Gall crab grows through five stages, with larger crabs and more egg-carrying females in later stages. Females mature at around 2.5 mm, marked by a larger abdomen, developed ovaries, and egg-laying. After maturing, they live about 10 months, spending 80% of that time carrying eggs and producing at least eight broods, showing a strong focus on reproduction.
Gall crabs have a pretty interesting diet—they mainly snack on the tissue and mucus produced by their coral hosts. Coral mucus is like their favorite treat, but they also munch on detritus, making sure to take advantage of whatever organic matter is around. This allows them to stay close to their coral home while getting all the food they need.
Gall crab in reef tank; what to do?
Gall crabs occupied your elegance coral or any other coral; came as a hitchhiker that you didn’t notice. I suggest keeping them, as they are not harmful and destructive like the gorilla crabs. However, most of the aquarists suggest getting rid of them.
If you see the corals are dying because of them, remove them with tongs or spears; they might die in this process. If you want the crab alive and keep it in another tank, you can dip the infested coral in freshwater, and the crab will come out. Here are some more tricks on how you can catch them.
