Ectosymbionts alter spontaneous responses to the Earth’s magnetic field in a crustacean

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.

Magnetic body alignment in migratory songbirds: A computer vision approach

“…Sample size to be one component of a goal-oriented response, e.g. honeybees (Martin and Lindauer, 1977), crayfish (Landler et al, 2019), flies and mice (Painter et al, 2013). A possible explanation of magnetic bi-axial orientation could be the presence of quadrimodal components of the complex 3D patterns generated by the lightbased radical-pair magnetoreception mechanism (Hore and Mouritsen, 2016).…”

The importance of time of day for magnetic body alignment in songbirds

“…However, in Bianco et al ( 2019a ), the two nocturnal migrant species chiffchaff ( Phylloscopus collybita ) and European robin ( Erithacus rubecula ) exhibited a bi-axial response during evening hours, whereas the diurnal migrant species the dunnock ( Prunella modularis ) showed an axial response along the N–S magnetic axis (Bianco et al 2019a , b ). Axial orientation (i.e., either along the magnetic axis or orthogonally to it) and bi-axial orientation are equally common in body alignment studies (Begall et al 2013 ; Burda et al 2020 ) and probably a direct consequence of the mechanism of light-based radical-pair magnetoreception (Phillips et al 2010 ; Hore and Mouritsen 2016 ; Landler et al 2019 ). However, it still remains to be explained what mechanism/-s determine the specific body direction in magnetic body alignment in all tested species (e.g., Malkemper et al 2016 ).…”