Gall crabs

Coral reefs harbour an immense diversity of species, many of which live in association with one or more host organisms. Stony corals (Scleractinia) have been documented to host nearly 900 organisms, of which > 310 are decapod crustaceans. This associated fauna is also involuntary host to a diverse parasite fauna. Coral-dwelling gall crabs (Cryptochiridae) are obligate symbionts of scleractinian corals; however, records of their parasites are scarce, with only two named epicaridean isopod species known. Fieldwork focused on cryptochirids in the Indo-West Pacific and Caribbean yielded diverse new collections of both hosts and parasites. This new material was compared with material deposited in museum collections. Both previously described species were encountered in the newly collected material, as well as a new genus and species of bopyrid, a new genus and species of cryptoniscoid, four new species of Danalia, and the first record of a rhizocephalan parasitizing a gall crab, which is also described as a new species. Parasitism of gall crabs is more widespread than what could be assumed based on literature records, and all parasite species appear to be specific to hosts in this crab family. This is the first review of hypersymbioses between stony corals, gall crabs and the crabs' parasites and hyperparasites

Curaçao, Dutch Caribbean, is home to three species of gall crabs belonging to Cryptochiridae, a family obligatorily associated with a wide range of scleractinian host species. Gall crabs are reliant on their host coral; females are sedentary and never leave their dwelling (van der Meij 2014a). One of the three Atlantic gall crab species is Opecarcinus hypostegus (Shaw and Hopkins, 1977), which inhabits corals of the genus Agaricia. Corals of this genus are abundant in the photic zone (<30 m), but also in the mesophotic zone (30–150 m), where they predominantly belong to Agaricia grahamae Wells, 1973 and Agaricia lamarcki Milne- Edwards and Haime, 1851. The latter was found to be most abundant at depths of 25–60 m (Bongaerts et al. 2013). 

During a survey on 31 March, 2014, with the manned CuraSub submersible launched from Substation Curaçao (12°05 ́04.14 ̋N, 68°53 ́53.16 ̋W), a colony of A. lamarcki was observed at approximately 60 m depth (Panel B) exhibiting the characteristic tunnel formed by O. hypostegus. The shape of this tunnel is virtually identical to those found with O. hypostegus crabs in A. lamarcki at shallower depths (Panel A: Slangenbaai, Curaçao). Hence, we contend that this is strong evidence of O. hypostegus at mesophotic depth. 

The present finding is relevant in the light of the “deep reef refugia” hypothesis, which states that mesophotic reefs may act as a refuge in the face of global reef decline (Bongaerts et al. 2010), as it furthers our knowledge on the communities that presently thrive on these deep reefs. 

Coral-associated invertebrates dominate the biodiversity of coral reefs. Some of the associations involving symbiotic invertebrates remain unknown or little studied. This holds true even for relatively wellstudied coral reefs, like those in the Caribbean Sea. Coral gall crabs (Cryptochiridae), obligate symbionts of stony corals, form a much-overlooked component of coral reef communities. Most recent studies on the Atlantic members of Cryptochiridae have been conducted off Brazil and little recent data have become available from the Caribbean region. During fieldwork off Curaçao (southern Caribbean Sea), eight new host coral species, belonging to four coral families, were recorded for three cryptochirid species. Kroppcarcinus siderastreicola Badaro, Neves, Castro and Johnsson, 2012, previously only known from Brazil, and Opecarcinus hypostegus (Shaw and Hopkins, 1977) are new additions to the fauna of Curaçao. Besides the new hosts and geographic range extensions, a free-living male Troglocarcinus corallicola Verrill, 1908 was observed visiting a female of the same species lodged in her gall in an Orbicella annularis (Ellis and Solander, 1786) colony. This is the first photodocumented record of the “visiting” mating system in Cryptochiridae.