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

Abstract

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. In coral communities, which are characterized by extremely high species diversity and an unusually wide range of biotic relationships among organisms, the symbiotic coexistence of species is very common 1-3. At least 860 symbionts were recorded in only one host group, the scleractinian corals 4. A comparable diversity of symbionts is associated with other host groups of coral reef animals, comatulids or feather stars (Сrinoidea, Comatulida) 5-14. Comatulids are sessile animals with limited mobility. To reduce their vulnerability to predators, they have adopted various morphological, behavioural, chemical and other protective adaptations. These may include body rigidity, dense crown, cryptic mode of life, fast regeneration, aversive taste, and toxicity 15-18. Based on field and aquaria observations, it has been assumed that fish avoid feeding on comatulids 19. Later, the repulsive taste of some comatulids for several fish species was confirmed experimentally 20,21. There are no fish species specialized in feeding on comatulids; nevertheless, sometimes fish attack them, and findings of comatulid fragments in the digestive tracts of some fishes are well documented 19,22-24. Diverse symbiotic communities comprising tens of species have been found in all studied comatulids on the coral reefs of South Vietnam 25,26. Among their symbionts are polychaetes, gastropods, crabs, shrimps, brittle stars and fish. The total number of symbiotic species recorded just in one bay was close to seventy 25. Up to ten species and more than fifty individuals of symbionts can inhabit one host specimen simultaneously 13. ...

Species-specific metabolites mediate host selection and larval recruitment of the symbiotic seastar shrimp

“…Some asteroid species exclusively produce sulphated saponins while others only produce non-sulphated saponins, and some produce both 59 – 61 . Moreover, saponins are known to play a crucial role in chemical defence in marine environments and have strong ichthyotoxic properties that effectively deter potential predators 62 . Therefore, symbiotic fauna must evolve mechanisms to counteract the harmful effects of asterosaponins.…”