Hygric stresses and strategies in maintaining the association between crayfish and ectosymbiotic worms across vastly different environments

Abstract

Crayfish and their ectosymbiotic worms may be a valuable model system for studying strategies to maintain symbiotic relationships strained by host-induced environmental shifts. Ectosymbiotic branchiobdellidan worms are thought to be dependent on crayfish hosts for reproduction. However, many crayfish species leave surface waters to excavate and reside in subterranean burrows. During this semi-terrestrial phase, crayfish are frequently out of water, subjecting their associated worms to desiccation. Water balance characteristics of Cambarincola mesochoreus, an ectosymbiont of Procambarus clarkii, were examined to elucidate strategies for surviving emersion while maintaining host associations. Emersed worms were characterized by a high water loss rate and low tolerance for dehydration. A lack of direct water contact eventually prompted a coiled, immobile dormant state where water loss was temporarily suppressed. This dormant state was broken by addition of water and re-entered by subsequent emersion. Neither group effects by clustering of many worms, nor water vapor absorption were utilized as mechanisms to increase desiccation hardiness. When crayfish leave surface or subterranean water, worm survival while maintaining host contact is likely facilitated by facultative quiescence. However, this strategy only allows for survival during relatively short-term, cyclic bouts of emersion. Survival of longer-term emersion periods would require worms to leave hosts and remain behind in surface or subterranean water. These strategies likely incur counterbalancing trade-offs: increased mortality but maintenance of host association versus increased survival but temporary or permanent loss of host association and sites for reproduction.

Transmission and terrestrial dispersal of non-native ectosymbionts on invasive crayfish

“…The terrestrial walking speed of signal crayfish is unknown, but adult male red swamp crayfish (Procambarus clarkii Girard, 1852), another invasive North American species, walk on average 58 m h -1 (Ramalho & Anastácio, 2014). At this rate, we estimate X. victoriensis could survive terrestrial migration up to 43.5 m, although this does not take into account variable environmental conditions (Marques et al, 2014;Ramalho & Anastácio, 2014;Yoder et al, 2016). It is also possible that branchiobdellidans could tolerate longer periods of desiccation in vivo by retreating into host crevices or gill chambers, but for X. victoriensis this is unlikely given that this species is a chelae specialist (Gelder & Hall, 1990;James et al, 2015b).…”