Host-related Morphological Variation of Dwellings Inhabited by the Crab Domecia acanthophora in the corals Acropora palmata and Millepora complanata (Southern Caribbean)

Brachyuran crabs of various families are known as obligate associates of stony corals, with many of these species living as endosymbionts inside the skeleton of their hosts [...]

Brachyuran crabs of various families are known as obligate associates of stony corals, with many of these species living as endosymbionts inside the skeleton of their hosts [1]. In particular, coral gall crabs (Cryptochiridae) have been well studied in tropical coral reefs around the world. These crabs can be recognized by the shape of their dwellings (or pits), which may be crescent-shaped or resemble a slit, a canopy, a basket, or a gall, depending on the identity and morphology of their host, and on the position inside the host's skeleton [2][3][4][5][6][7]. Cryptochirids are each known to be associated with a few scleractinian host species (Anthozoa: Scleractinia) or only one [2][3][4][5][6][7]. Crabs of the species Latopilumnus tubicolus Türkay and Schuhmacher, 1985 (Pilumnidae), have so far only been reported as endosymbionts of the Indo-Pacific scleractinian Tubastraea micranthus (Ehrenberg, 1834) [8]. Their dwellings are unique because they start in one of the coral's calyces from where they penetrate deep inside the coral branches [8], becoming long and tubular, whereas the pits (or cysts) of cryptochirids remain relatively shallow [2][3][4][5][6][7].
Other records of crabs living inside stony corals, concern unidentified species of the genera Tetralia Dana, 1851 (Tetraliidae) and Cymo De Haan, 1833 (Xanthidae) that live in association with Indo-Pacific Acropora spp. [9]. The pits constructed by Cymo sp. show an oval margin that becomes increasingly thick, resembling a collar on top of flattened, fused Acropora branches, whereas those of Tetralia sp. resemble a slit in between bifurcating, round Acropora branches, which eventually develop into galls [9]. Dwellings made by Atlantic Platypodiella spp. (Xanthidae) are usually oval but can start as shallow depressions inside hosts of the genus Palythoa (Anthozoa: Zoantharia) [10,11], which are encrusting, colonial sea anemones related to scleractinians; they are leathery in appearance with sand particles inside their tissue instead of a calcareous skeleton.
The pits made by Cymo sp. resemble most those of the Atlantic crab Domecia acanthophora (Desbonne and Schramm, 1867) (Domeciidae) in flat branches of the scleractinian Acropora palmata (Lamarck, 1816) observed at Puerto Rico (Greater Antilles) and Venezuela (Southern Caribbean) [12,13]. Here, the same crab was also found in Acropora cervicornis (Lamarck, 1816) and Acropora prolifera (Lamarck, 1816), while it was also observed wandering on corals of other species [12,13]. Domecia acanthophora has also been reported as associated fauna of fire corals, like Millepora alcicornis Linneaus, 1758 at Brazil [14] and Millepora spp, at Yucatán, México [15]. However, no information is available on the morphology of Domecia pits in Millepora and its possible difference with Acropora.
Therefore, we present information on Domecia dwellings found in six corals of A. palmata at Curaçao (<2 m depth; 2015) and two corals of Millepora complanata Lamarck, 1816 at Klein Bonaire (2-3 m depth; 2019). Curaçao and Bonaire (next to Klein Bonaire) are major Dutch Caribbean islands, situated 50 km apart and 70-80 km off the coast of Venezuela. The observations were made during biodiversity surveys down to 30 m depth using the roving diving technique.
The Domecia pits in A. palmata were up to 2 cm long ( Figure 1). Those at the margins of coral branches had outlines that were not fully closed (Figure 1d,e), whereas pits away from the margins were enclosed and showed a thickened, smooth periphery, like a collar around an open callus (Figure 1h-i). An intermediate form consisted of a fully enclosed pit without a thickened margin (Figure 1f). The crabs could still be observed when they were inside the dwelling (Figure 1d-i) and also could easily be collected from an incomplete pit at a branch margin (Figure 1b).
The Domecia dwellings in M. complanata were different in shape, usually consisting of folds in the coral's branches (Figure 2). They were most easily observed at the upper margins of the coral's vertical plates (Figure 2d-f) and did not show a collar-like thickening at the margin. Some dwellings occurred at a side of a plate (Figure 2b). One crab was found inside a narrow crevice (Figure 2c). In all cases, the crabs were easy to spot (Figure 2c-f). Interestingly, this association was only observed in exposed reef habitats at Klein Bonaire. It seemed that the crabs altered the shape of some of the Millepora corals by giving their branches a more contorted appearance (Figure 2b). Alternatively, the particular form of M. complanata inhabited by Domecia may also be a result of environmental factors (surge, currents, and turbulence) to which these organisms are exposed at Klein Bonaire.  The present information may facilitate recognition of endosymbiotic crab fauna in Acropora and Millepora corals, including fossil ones [4]. Further studies are required to determine whether Atlantic Domecia crabs associated with different hosts all belong to D. acanthophora or, alternatively, represent distinct but closely related species.