From migratory birds to newly hatched turtles making their way to the sea, studies have shown that animals use the Earth’s magnetic field to orient themselves.

Crayfish are no different. However, their use of magnetic cues is influenced by the number of symbiotic worms that live on the crustaceans’ bodies.

“This is the first study to demonstrate that crayfish can detect and respond to the earth’s magnetic field. What I think makes this study really unique is that it is the first to study the effect of symbionts on magnetoreception,” said Bryan Brown, associate professor in the Department of Biological Sciences in the College of Science.

Brown studies large-scale aquatic community ecology. His work focuses on how multiple species interact in aquatic habitats and how those interactions are altered by changing environmental conditions. Brown has studied crayfish symbiosis for more than 20 years; last year, he completed a 17-day kayaking trip to assess invasive crayfish species.

In a study published in Scientific Reports, Brown and a team of researchers from Virginia Tech looked at the connection between symbionts and crayfish’s ability to magnetically orient themselves.

The researchers found that ectosymbionts — species that live on the outside of the host’s body and have mutually beneficial relationships with their hosts — affect a crayfish’s use of the Earth’s magnetic field as a directional reference.

Lukas Landler and James Skelton, both Ph.D. students who graduated in 2015 from the Department of Biological Sciences, combined their areas of expertise to develop this study. Landler, who earned his Ph.D. under John Phillips, studied the neuronal basis of magnetoreception, while Skelton, one of Brown’s Ph.D. students, studied the interspecific relationships in aquatic macroinvertebrate species. Phillips, a professor in the same department, specializes in magnetic field detection and sensory ecology.

James Skelton and Lukas Landler
James Skelton (left) and Lukas Landler (right) collecting crayfish on Sinking Creek in Newport, Virginia. Photo courtesy of James Skelton.

“Magnetoreception is one of the more mysterious things about animals, because nobody fully knows how that mechanism works,” said Brown, who is also an affiliated faculty member of the Global Change Center, housed within the Fralin Life Sciences Institute.

It is well established that animals use a variety of directional cues, including sun position, star patterns, polarized skylight, and the Earth’s magnetic field to guide their movements. What is less understood is why a wide variety of animals, when not actively moving, spontaneously align themselves along roughly the north-south axis relative to the magnetic field.

Worms and crayfish live in symbiosis with one another — the worms feed on parasites and keep the crayfish's exoskeleton clean, especially its gills. In exchange, the worms get food, protection, and access to favorable habitat. The natural density of worms found on crayfish tends to be in the moderate range, which is most beneficial for the crayfish.

Crayfish with symbiotic worm on its head.
Crayfish with symbiotic worm on its head. Photo courtesy of Bryan Brown.

“Our previous work has shown that along the gradient from no worms to high density, the relationship between host and ectosymbionts goes from mutualism to parasitism,” Brown said. “In parallel with this change in the relationship between crayfish hosts and worms, the response of crayfish to the magnetic field goes from quadramodal alignment to bimodal alignment to a random distribution as the density of worms increases.”

Quadramodal alignment is consistent with systematic search of the area surrounding a fixed reference point to which the searching individual returns after each foray.  Consistent with the quadramodal response being part of an "active" response, crayfish without ectosymbionts showed significantly higher levels of activity than those in the other two groups. In contrast, bimodal alignment is indicative of a resting state in which standardizing the “projection” of the outside world onto the visual system may make it easier for the crayfish to detect and identify novel features of its surroundings. For example, this would help the crayfish lying in wait under the edge of a rock to distinguish between the approach of a potential predator versus that of potential prey, a distinction critical to the crayfish’s survival that must be made before deciding whether or not to leave the safety of its refuge.

“Symbioses are really complicated. To make sense of them, ecologists tend to pigeonhole them into familiar little boxes like 'cleaning symbiosis.' But if we stop there, some fascinating and important nuances are lost. I love this study because it shows that these worms don't just clean crayfish. At higher densities the worms become a little annoying, and being annoying has real effects on how crayfish behave, which stimuli they respond to, and perhaps how well they can find their way home. It shows just how intimate and complex these interactions really are,” Skelton said.

At high densities, ectosymbionts can injure the crayfish, feeding on gill tissue when all the organic matter that worms normally consume has been removed by other worms. As a consequence, the researchers speculate that at high densities of ectosymbionts, crayfish may seek out a safe refuge or burrow where they can safely groom to reduce the worm population. If so, visual features may be more useful than the magnetic field in finding the entrance. However, further research is clearly needed to pin down how the ectosymbionts directly influence the crayfish’s behaviors.

“As of now, there are very few conservation efforts directed at symbiotic organisms,” Brown said. “Every organism has symbionts; the more we know about them, the more important they appear to be.”

This study furthers the scientific understanding of the evolutionary forces shaping sensory systems, how symbionts influence a host’s response to magnetic cues, and how symbiotic interactions affects the host’s and ectosymbiont’s fitness.

This study was funded by the National Science Foundation, the Society for Integrative and Comparative Biology, the Global Change Center (formerly Organismal Biology and Ecology) Seed Grant, and the Virginia Tech Graduate Research and Development Program.

-Written by Rasha Aridi

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