Abstract
Magnetic sensing is used to structure every-day, non-migratory behaviours in many animals. We show that crayfish exhibit robust spontaneous magnetic alignment responses. These magnetic behaviours are altered by interactions with Branchiobdellidan worms, which are obligate ectosymbionts. Branchiobdellidan worms have previously been shown to have positive effects on host growth when present at moderate densities, and negative effects at relatively high densities. Here we show that crayfish with moderate densities of symbionts aligned bimodally along the magnetic northeast-southwest axis, similar to passive magnetic alignment responses observed across a range of stationary vertebrates. In contrast, crayfish with high symbiont densities failed to exhibit consistent alignment relative to the magnetic field. Crayfish without symbionts shifted exhibited quadramodal magnetic alignment and were more active. These behavioural changes suggest a change in the organization of spatial behaviour with increasing ectosymbiont densities. We propose that the increased activity and a switch to quadramodal magnetic alignment may be associated with the use of systematic search strategies. Such a strategy could increase contact-rates with conspecifics in order to replenish the beneficial ectosymbionts that only disperse between hosts during direct contact. Our results demonstrate that crayfish perceive and respond to magnetic fields, and that symbionts influence magnetically structured spatial behaviour of their hosts.
Highlights
The available evidence suggests that sensitivity to the geomagnetic field is widespread among motile animals and plays a fundamental role in organizing spatial behaviour[1]
Naturally occurring differences in symbiont density on individual crayfish explained much of the variation in crayfish spontaneous magnetic alignment (SMA) responses
This study is the first study to demonstrate that crayfish can detect and respond to the Earth’s magnetic field
Summary
The available evidence suggests that sensitivity to the geomagnetic field is widespread among motile animals and plays a fundamental role in organizing spatial behaviour[1]. Since magnetic perception appears to play an integral role in spatial behaviours, could it be possible that symbionts alter magnetic field responses in animals? Fish plagued by ectoparasites will seek cleaning stations inhabited by mutualistic partners that remove harmful ectoparasites[16,17,18] This example illustrates two important drivers of symbiont effects on host fitness and resultant behaviour; first, potential negative effects of ectoparasites and secondly the benefit of interacting with cleaner species that remove ectoparasites[19]. Fleas change mammalian spatial behaviour by introducing distracting sensory stimuli that may alter movement in relation to resources or potential predators[7] Regardless of causality, these examples clearly indicate that our understanding of a host’s spatial behaviour is often incomplete without consideration of its symbionts. We characterize spontaneous magnetic alignment in a population of freshwater crayfish (Cambarus appalachiensis) and experimentally test for the effects of external annelid symbionts on crayfish magnetic behaviour
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