Discharge Trigger Responds to Fish’s Chemical Cues

Biological Strategy

Myxobolus

AskNature Team

Sense Chemicals (Odor, Taste, etc.) From the Environment

Chemicals are important for signaling and communication among living systems, either intentionally (such as when two living systems try to find one another) or unintentionally (such as when a plant emits a chemical signal that an herbivore can use to find a tasty bite). They are also important for other uses, such as navigating or finding sources for minerals. But chemical signals are often relatively weak and disperse when moving through water or gases. Therefore, detecting them requires specialized abilities, including a way to determine where they are coming from. A well known example of sensing chemicals can be seen in ants following a pheromone trail laid down by others in their colony to locate a quality and abundant food source.

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Respond to Signals

To interact with its environment, a living system must not only sense a variety of signals, but also respond to them. To be energy- and material-efficient, those responses must be appropriate to the signal. This generally requires sensing thresholds to trigger an appropriate level of response (for example, hiding under a shrub versus running away to avoid a predator). Response strategies are tied to a specific signal and often have a response threshold, which determines how strong a signal must be to warrant expending energy to respond. One example is a plant that lives in arid regions in South Africa. Its seed capsules remain closed until rainfall triggers them to open to release the seeds. But the plant only responds to a second rainfall, thus protecting against releasing its seeds before there is enough sustained water for them to grow.

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Cnidarians (Jellyfish, Coral)

Phylum Cnidaria (“stinging thread”): Jellyfish, coral, sea anemones, hydra

Over 99% of cnidarians live in marine ecosystems, with only 1% found in fresh water. They have two distinct body plans: medusa, which moves and is the familiar jellyfish shape, and polyp, which is stationary and the classic sea anemone shape. Members of this phyla have egg-shaped, free-swimming larvae, and can release capsules called cnidae. The most well-known type of cnidae are nematocysts, which are harpoon-like stinging cells that are the bane of many a trip to the beach.

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

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 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.