Further insight on carapid?holothuroid relationships

Abstract

Carapidae (or pearlfish) are eel-like fishes that live inside different invertebrates, such as holothurians, sea stars, or bivalves. Those of the genus Carapus are commensal and use their host as a shelter, while Encheliophis species are parasitic and eat the host's gonads. In areas where they live in sympatry, C. boraborensis, C. homei, C. mourlani and E. gracilis are able to inhabit the same host species. Infestation is considered as monospecific when several conspecifics are observed in the same host. However, many aspects of this particular relation remain obscure, e.g. communication between carapids and the defence systems of the different protagonists (carapids and hosts). Experiments have been conducted in the field and laboratory to investigate several aspects of the carapids' relationships with their hosts. Sampling carried out in the Bay of Opunohu (Moorea, French Polynesia) determined the sex ratio of C. boraborensis (3:1) and C. homei (1:1) and their distribution rate within different Echinodermata. Our study showed that neither species was capable of determining whether a heterospecific already occupied a sea cucumber or not. They were, however, able to locate the sea cucumber's cloaca, due to the excurrent resulting from respiration. The sea cucumber's defence system (Cuverian tubules) minimises predator attacks, but is not effective against carapid intrusion. The Carapidae defence system is twofold. Due to a passive system related to the sea cucumber's low cloacal position, the Cuverian tubules are not expelled when fish enter the cloaca. Moreover, carapids resist sea cucumber toxins better than other reef fish. Their increased resistance might be related to their gills rather than to their mucus coating; however, the latter may assist the fish in resisting the sticky substances emitted by the Cuverian tubules.

Effects of Holothuroid Ichtyotoxic Saponins on the Gills of Free-Living Fishes and Symbiotic Pearlfishes

“…They placed Cuvierian tubules of three individuals of B. argus with the pearlfishes C. homei and C. boraborensis and some tropical fishes used as controls, in one aquarium. They observed the death of all the fishes after less than 5 min, except for the carapids, which resisted up to 160 minutes (Parmentier and Vandewalle, 2005). The experiments detailed here show similar results.…”

“…Rather, it may be due to the higher concentration of saponins present in the Cuvierian tubules, as shown by Van Dyck et al (2011). Parmentier and Vandewalle (2005) were the first to test carapid resistance to the presence of Cuvierian tubules. They placed Cuvierian tubules of three individuals of B. argus with the pearlfishes C. homei and C. boraborensis and some tropical fishes used as controls, in one aquarium.…”

“…These predators are then in close contact with saponins, which should be enough to repel most of them. Oddly, carapids penetrate the cloacum of holothuroids and place themselves in the respiratory trees without provoking the expulsion of Cuvierian tubules (Parmentier and Vandewalle, 2005). In the laboratory, carapids were unaffected by the glue of the tubules and swam between them without getting stuck.…”

“…Pearlfishes are less sensitive to the ichtyotoxic holothuroid saponins than are other fishes (Parmentier and Vandewalle, 2005). Parmentier and Vandewalle (2005) found that the time required for carapids to die when exposed to holothuroid saponins is 2 to 16 times longer than for other fishes and suggested that the resistance of pearlfishes to holothuroid toxins may be related to specializations of the gills.…”

“…Parmentier and Vandewalle (2005) found that the time required for carapids to die when exposed to holothuroid saponins is 2 to 16 times longer than for other fishes and suggested that the resistance of pearlfishes to holothuroid toxins may be related to specializations of the gills. Mucus coating may also assist the fish in resisting the sticky substances emitted by the Cuvierian tubules (Trott, 1970).…”