Spreading of deterrency as a means of chemical defense among aquatic organisms inhabiting the coral reefs of Vietnam

Comatulids (Crinoidea, Comatulida) chemically defend against coral fish by themselves, without assistance from their symbionts

Symbiotic associations between small animals and relatively large sessile invertebrates that use taste deterrents for protection are widespread in the marine environment. To determine whether the symbionts are involved in the chemical protection of their hosts, the palatability of ten species of comatulids and six species of their symbionts was evaluated. Taste attractiveness was determined by offering agar pellets flavoured with extracts of comatulids and their symbionts for four coral reef fish species. Five species of symbiont were highly palatable, and one was indifferent to the taste. Almost all comatulids were distasteful, while their aversiveness was different for different fish. These findings indicate that comatulids chemically defend themselves without assistance from symbionts, and the taste deterrents are not universal and can only be effective against particular predators. The presence of tasteful symbionts reduces the security of their hosts by provoking attacks of predators and may impact on the individual and population fitness of comatulids. However, the chemical protection of comatulids is useful for symbionts and undoubtedly increases their survival. Obtained results allows the relationship between comatulids and their symbionts considered commensalism. Most likely, similar relationships can be established in many other associations, where symbionts inhabit chemically defended coral reef invertebrates.