Symbiosis between the coral gall crab Hapalocarcinus marsupialis (Decapoda: Cryptochiridae) and the stony coral Seriatopora hystrix (Hexacorallia: Scleractinia) within the Great Reef of Toliara, Madagascar

The parasitic marsupial crab Hapalocarcinus marsupialis is found in abundance on
the stony coral Seriatopora hystrix within the Great Reef of Toliara, Madagascar.
The female crabs have a modified abdomen that forms a large pouch located
under the cephalothorax and where they can brood more than 500 eggs. Their life
cycle is divided into two distinct parts: a free-swimming larval phase and an adult
parasitic phase. When a larva settles down on its host, it induces a gall formation
that grows in four distinct stages where the first one corresponds to a small bud
and the last one represent the gall completely closed and surrounding the crab
inside. Presence of the parasite does not affect the coral skeleton or their
reproduction, because mature polyps were found.
On the reef, 563 colonies of S. hystrix were observed by scuba-diving on different
stations: 37.8% of them were infested by these crabs with a total of 763 galls. Galls
morphology is representative of the age of the infestation on a site and the maturity
of the crab inside. Galls are monopolized by females which have a similar
morphology to males at the juvenile stage. They grow simultaneously with the gall
and males visit them in order to mate before the gall closure. Females are fertilized
by different males and store the sperm into two spermathecae, mating being
probably a powerful stimulus that induces a fast growth of their abdomen. In
parallel, the gall closes until the crab is stuck inside.
Thanks to the isotopic compositions of carbon and nitrogen, this study shows that
the crabs feed mostly on particles carried by the current thanks to the small
openings located all around the gall.