The filament injection organ of Myxobolus cerebralis detects a potential fish host via receptors that recognize chemical compounds present in fish mucus.

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The actinospore phase of a myoxozoan parasite emerges from its invertebrate worm host and must infect a fish host in order to complete its life cycle. The actinospore features a single-use filament ejection organ, which it uses to anchor itself to fish to implant new parasite cells. Since premature triggering of the filament capsule would mean death for the individual actinospore, they must respond to chemical cues originating specifically from fish. Moreover, the chemical trigger must be a compound that fish secrete involuntarily so as to reduce the likelihood of the fish developing resistance. The epidermal mucus coating of a salmon, a host species for Myxobolus cerebralis, is well adapted to supplement its swimming, ion/gas exchange, defense against physical damage, and immune system. M. cerebralis takes advantage of chemical compounds present in this mucus to signal that a fish host is nearby, triggering release of the parasite's anchoring-filament. This adaptation is extremely effective in identifying a suitable fish host because the signaling compounds in the mucus are associated with critical metabolic processes, so the fish must produce them. They are also very water insoluble, so their detection by the parasite at certain concentrations indicate extreme proximity to host fish. An interesting aspect of the trigger mechanism is that it is not species-specific, just fish-specific. This may be an adaptation to increase the likelihood of adaptation to new host species.

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"Natural, host-derived chemical molecule identified as a recognition cue for the phylum Myxozoa. The actinospores of these parasites attach to fish hosts via polar filaments that are extruded upon mechanical stimulation after preceding recognition of a chemical trigger contained in surface mucus...the free nucleosides inosine, 2'-deoxyinosine and guanosine. These nucleosides also activated sporoplasm emission. Nucleosides appear to be appropriate cues for rapid host recognition by the waterborne parasite stages since they are continuously released into surface mucus...Myxozoa, a phylum of parasitic metazoan organisms that live in invertebrate and various vertebrate hosts up to the mammalian level. Most myxozoans known to date feature a life cycle involving an oligochaete worm and a teleost fish, the latter which they invade by actinospores...[actinospores] possess apically located polar capsules to attach to their fish host...From these capsules, a polar filament is discharged for anchorage...polar filament discharge is an 'all or nothing' process...passively floating actinospore stages need to avoid accidental or non-specific discharge of their polar filaments. This includes contact not only with dead matter, but also with manifold aquatic invertebrates and their secretions including those of their oligochaete hosts in which they develop. Thus the actinospores need to chemically detect a possible host not only very quickly, but also very efficiently...They are able to discern fish mucus from other habitat-specific targets very well, while the radius of action in which attachment to the host is achievable is limited to the length of the polar filament." (Kallert et al. 2011:271-272).

"Inosine and its 2'-deoxygenated species as active polar filament discharge triggers in all three myxozoans tested. Hence, inosine could elicit a significant discharge rate that was comparable to that reached by the use of native mucus substrate." (Kallert et al. 2011:274).

"The sugar moiety seems to be crucial for recognition...The presence of electrolytes and amino acids potentially aids in keeping inosine in solution, which could explain the higher effect of the natural isolates...Actinospores never discharge their filaments in the vicinity of their mucus-producing oligochaete hosts...[because] nucleobase metabolites are not excreted via the skin in oligochaetes or other invertebrates. However, fish cannot avoid its release, a very beneficial fact for parasites. Another advantage of these compounds as indicators of fish hosts is their hydrophobicity making them almost absent as solutes in waters. That means, when an elevated inosine concentration is recognized, the probability for having close contact with a living fish is very high...Furthermore, common (e.g. DNA-derived) nucleobase substances would be too unspecific and abundant in aquatic habitats, while inosine accumulation is confined to animal surfaces with indigenous cellular breakdown and ATP degradation...myxozoan actinospores utilize recognition cues that are inherently excreted by living fish and are reliable signals for differentiation between fish mucus and secretions of other aquatic organisms, plant material and dead matter." (Kallert et al. 2011:275).

Journal article
No shot in the dark: Myxozoans chemically detect fresh fishInternational Journal for ParasitologyNovember 17, 2010
Dennis M. Kallert, Walter Bauer, Wilfried Haas, Mansour El-Matbouli

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