Characteristics of the infestation of Seriatopora corals by the coral gall crab Hapalocarcinus marsupialis Stimpson, 1859 on the great reef of toliara, Madagascar

This study describes the association between the obligatory symbiont coral gall crab Hapalocarcinus marsupialis and its stony coral hosts Seriatopora sp. within the Great Reef of Toliara in Madagascar and attempts to discuss their symbiotic status through comparison with previous studies. These corals are inhabited by crabs living in galls that can be categorised in four distinct morphological stages, where the first one corresponds to a small bud and the last one represents a completely closed gall surrounding the crab inside. Within the reef, 563 colonies of Seriatopora species were observed by scuba-diving at ten different stations: 37.8 % of them were infested by H. marsupialis, with a total of 763 galls, and with a majority of stage 4 galls. Galls are monopolised by females that can have different morphologies. Females store the spermin two spermathecae and are fertilised when their morphology and size are similar to males and the gall is not closed. Histological observations coupled with scanning electronic microscopy analyses show that closed galls are made of an external living tissue, a mid skeletal layer and an internal living tissue. The internal living tissue includes polyps similar to the external tissue, some of them being sexually mature. Nitrogen and carbon isotopic signatures confirmed that these crabs are filter-feeders and do not feed on their host. This association perfectly highlights the difficulties to define the symbiotic status of a symbiont if one considers inflexible the three categories of symbiosis commonly defined.