The pom pom or boxer crab belongs to the genus Lybia with 10 species of small crabs.
Boxer crabs live with their best friends
The pom poms these little crabs carry around are not actually part of them but they hold small anemones. How cute is that? They are like best friends spending all day together and both profit from their relationship (mutualism) but they can survive without each other.
The crabs swing the poisonous anemones around as defense mechanism and catch food using them. With its mouth parts or the first pair of legs the crab gets the food particles from the tentacles of the anemones while enough remains for the anemones. They also profit from the movement and a constant food supply.
Boxer crab and anemone have adapted for this symbiotic relationship. The crab has much smaller claws which would not be of much use to defend itself anymore but are more suitable for holding the anemones. On the other side the anemone lost its pseudo tentacles used by other species to photosynthesize as they rely on the higher food supply being carried around by the crab. Indeed a very fascinating symbiosis and they actually do look like they belong together.
Three different species of anemone are suitable to be carried around by the pom pom crab. If they can’t find any of them they might replace the anemone with another sessile animal they find like a sponge or coral.
Carrying eggs on the belly
When very lucky it is possible to see a boxer crab with an orange-red big belly during a dive.
These are the eggs which are attached to the female’s abdomen for around 2 weeks until the little ones hatch. The females are under a higher risk during this time as they have to come out more often from their hiding place to feed. After hatching the new boxer crabs are free swimming as larvae in a first stage of their development.
Where to find them?
The crab found by scuba divers is mostly the Lybia tessellata living in the shallow tropical waters in Australasia.
The boxer crab is normally hard to find during the day as they are hiding under rocks in shallow waters. When you see your guide starting to turn rocks at 3 m depth you might know what he’s looking for. They are hard to photograph as well as they want to hide again immediately after they have been discovered by annoying divers.
If you do want to photograph them try night dives when the chances are higher that they come out. The best I have seen them was in Ambon where loads of them were sitting on coral blocks in the evenings. To avoid that they run away try not to shine the torch directly at them or use red light.
Boxer Crab in Creation Wiki. Accessed June 14, 2016 on http://creationwiki.org/Boxer_crab
Lybia H. Milne Edwards, 1834 in World Register of Marine Species. Accessed June 14, 2016 on http://www.marinespecies.org/aphia.php?p=taxdetails&id=206564
Lybia tessellata in Wikipedia. Accessed June 14, 2016 on https://en.wikipedia.org/wiki/Lybia_tessellata