Phylogenetic ecology of gall crabs (Cryptochiridae) as associates of mushroom corals (Fungiidae)

The nature of the relationship between an associated organism and its host is usually unknown. In this study, the obligate associations between gall crabs and mushroom corals are reviewed from a phylogenetic perspective. Based on field surveys, examination of museum material, and a literature review, a total of 35 fungiid species have been found that act as hosts for four gall crab species. Fungiid-associated gall crabs appear to be more geographically widespread than previously known, with new records showing their occurrences from the Red Sea and western Indian Ocean all the way to the central Pacific Ocean. The obligate nature of the association between cryptochirids and their hosts makes them an ideal model taxon to test for possible cospeciation events. The congruence between their phylogenies was tested by using the program Jane 4.0, resulting in cospeciation and duplication events between the crabs and their host corals. The sharing of several closely related host coral species by the same gall crab species or genus may provide support to models indicating phylogenetic relationships within the Scleractinia. Historical records of fungiid-associated gall crabs were examined by searching coral collections from various institutions, including Naturalis Biodiversity Center, Royal Belgian Institute of Natural Sciences, University of Milano-Bicocca, and American Museum of Natural History. Additional records were obtained from literature and field observations. Gall crab identifications were based on Fize and Serène (1957), Kropp (1990), and van der Meij (2015a), while coral identifications were based on Hoeksema (1989), Gittenberger et al. (2011), and Benzoni et al. (2012). Fieldwork was conducted in various parts of the Indo-Pacific, including Spermonde Archipelago, Makassar Strait, Raja Ampat, Bunaken, Ternate, Lembeh Strait, Semporna, Kudat, Nha Trang, Great Barrier Reef, New Caledonia, Loyalty Islands, Payar Island, Tioman Island, and the Maldives. Mushroom corals were sampled for gall crabs, attempting to sample as many host species as possible from deep to shallow reef zones. The corals were sampled in the same way as described above after being photographed with a Canon 400D camera equipped with a 50 mm Sigma macro-lens. Cophylogenetic analyses based on host association data were tested by using the program Jane 4.0, based on the coral phylogenies in the studies of Gittenberger et al. (2011) and Benzoni et al. (2012). The results showed cospeciation and duplication events between the crabs and their host corals. The sharing of several closely related host coral species by the same gall crab species or genus may provide support to models indicating phylogenetic relationships within the Scleractinia. The study of fungiid-associated gall crabs has provided new insights into the phylogenetic ecology of these organisms. The results suggest that the obligate associations between gall crabs and mushroom corals are more widespread than previously known, and that cospeciation events have occurred between the crabs and their host corals. Further research is needed to fully understand the phylogenetic relationships between gall crabs and their host corals.