Symbiotic coral-dwelling crabs on Bonaire’s reefs

Dr. S.E.T. van der Meij and S. Bähr (GELIFES, University of Groningen , Groningen, The Netherlands)

Bonaire, Dutch Caribbean – 26 July – 17 August 2019

Coral reefs encompass the highest biodiversity of any marine ecosystem. Stony corals provide a large number of habitats that are home to various invertebrate species, many of which live in a close symbiotic relationship with a specific host coral. In an environment that is teeming with symbiotic relationships the minute and easily overlooked coral-dwelling gall crabs of the family Cryptochiridae represent a prime example of obligate symbiosis. These crabs are named after their close relationship with their coral hosts; they cause the development of a gall by modifying their hosts’ tissue.

The reefs along Bonaire’s coast are amongst the healthiest reefs in the Caribbean, however they are relatively understudied compared to nearby Curaçao and other islands with a strong research infrastructure. In this study we aimed to collect occurrence data of three Atlantic gall crab species (Fig. 1) over depth intervals along the Bonaire coast, to better understand the distribution of symbiotic species on coral reefs. In combination with coral cover measurements the results of this study provided us with baseline data about the status of Bonaire’s reef. Moreover, using the same approach as a similar study carried out on the reefs off Curaçao in 2015, it allows us to compare the results between the two islands.

We studied 25 dive sites along the west coast of Bonaire and around Klein Bonaire using 5-m2 belt transects (10m x 0.5m) at three different depths (8m, 12m, 18m), resulting in a surveyed area of 375 m2. Within each transect we measured the diameter of all corals known to host gall crabs, and counted the number of dwellings on each individual colony.

Our observations of the crabs Troglocarcinus corallicola and Kroppcarcinus siderastreicola represent new species records for Bonaire. Direct comparisons of coral cover data and gall crab density between Curaçao and Bonaire provides interesting insights in the status of the two reef systems. On Bonaire 19 out of 22 known host corals were observed in the transects, compared to 15 out of 22 of Curaçao. Strikingly, the number of examined coral colonies on Bonaire was with 4023 more than twice as high as the number of colonies examined on Curaçao (1874 colonies). The crab density of Opecarcinus hypostegus was with 6,3 crabs per m2, however, exactly the same at both localities. Troglocarcinus corallicola had with 5,22 crabs/m2 a slightly higher density on Bonaire than on the reefs off Curaçao (4,02 crabs/m2). Unfortunately the dwelling data for K. siderastreicola had to be excluded from the analyses due to some misidentifications of their dwellings in part of the study. 

Our results suggest that Bonaire’s reef system has a significantly higher coral cover than the reefs on Curaçao. Using further statistical analyses we will check for significant species level differences between the two islands, as well as between the various dive sites off Bonaire. Certain sites are heavily impacted by divers or industry (e.g. cruise ships, salines), whereas others are further away from human activities or rarely visited by divers – and we are keen to see if this impacts the distribution of the symbiotic gall crabs.

Figure 1: Dwellings with gall crabs found on corals off Bonaire. (A) Opercarcinus hypostegus in Agaricia lamarcki, (B) Kroppcarcinus siderastreicola in Siderastrea siderea, (C) Troglocarcinus corallicola in Orbicella annularis. Pictures: Pieter de Groot.

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