Nocturnal Predation of Christmas Tree Worms by a Batwing Coral Crab at Bonaire (Southern Caribbean)

Christmas tree worms (Serpulidae: Spirobranchus) occur in shallow parts of coral reefs, where they live as associates of a large number of stony coral species [...]

During another night dive, two days later, the crab was no longer present but the extent of damage to the worm tubes and the coral was evident (Figure 1b). The worm in the longest of the two tubes was gone, while the other worm had survived in a part of the tube that was inside the host coral. Its operculum appeared lost (Figure 1b).
This observation is interesting because little is known about species predating on Christmas tree worms (see above), while also hardly anything has been published on the diet of the Batwing coral crab. Carpilius corallinus is well known for its nocturnal activities [11] and it is the only West Atlantic member of Carpiliidae, a family of three congeneric species [12]. All three species possess an enlarged right cheliped (claw-bearing leg), with a blunt molariform tooth found proximally on the cutting edge of the pollex, which is the fixed 'finger' of the claw.
In laboratory conditions in Guam, the Indo-West Pacific species C. convexus (Forskål, 1775) and C. maculatus (Linnaeus, 1758) have been observed to use their major claw to crush shells of various species of gastropods [13]. The latter crab species has also been reported as predator of a commercially important abalone, Haliotis asinina Linnaeus, 1758, in the Philippines [14], and was found in the field between the remains of freshly-killed gastropods on two separate occasions in Guam [15].
Individuals of the West Atlantic C. corallinus were also found to be feeding on gastropods in captivity, while they were also fed with sardines [16]. In another case, a female individual in an aquarium was observed to break apart shells inhabited by hermit crabs in an attempt to remove them from their homes [11]. Only one published record was found on the diet of C. corallinus in its natural environment, consisting of Diadema sea urchins [11]. There is also unpublished data concerning C. corallinus feeding on sea urchins, as well as on a topshell, Calliostoma javanicum (Lamarck, 1822), all from Bonaire (E.M., pers. obs.).
It seems that information on the diet of Carpilius species is rare, but considering the armor of previously reported prey species, the crushing of serpulid worm tubes seems to be within their capacity when they use their right claw. The crab at Bonaire was, however, using its slender left claw to feed from the worm tube. We do not know if the crab had initially crushed the tube using its specialized right claw and continued feeding using its left claw, or if the crab initially used its left claw to break the tube.
The extent of damage on worm tubes is striking (Figure 1). In spite of many dives on Bonaire, this kind of harm was not reported before. Because Spirobranchus tubes may easily become covered by coral tissue and algae [3,6,7], it is possible that damaged worm tubes may get unnoticed due to similar overgrowth. All in all, we do not expect Spirobranchus to be a regular part of the diet of Carpilius corallinus. The present observation and previously published information suggest that Carpilius species are not prey specific. More research on the diet and foraging behavior of these commercially important crab species will teach us more about their role in the food chains of coral reefs.