Sticky flatworms (Platyhelminthes) kill armored harvestmen (Arachnida, Opiliones) but are not immune to the prey′s weapons

Abstract

Asking how prey respond to different species of predators helps understanding the repertoire of defenses prey exhibit. This approach may also add to the knowledge on the efficiency of prey′s defenses against each predator and allows studying the costs of being attacked by different predators. Here, we studied interactions between the predatory terrestrial flatworm Cephaloflexa bergi and the harvestman Mischonyx cuspidatus. The flatworm quickly strikes at the prey′s leg with its head, then crawls towards the prey′s body releasing mucus and thus subdues the harvestman. Harvestmen may release defensive chemicals and pinch the flatworm with spines on legs IV, sometimes cutting the predator in two pieces. We also experimentally assessed the benefits of harvestmen′ defense and the cost of being attacked by testing whether the chemical defense of the harvestmen is efficient against the flatworm and whether the harvestmen that had survived a flatworm attack would have their locomotion abilities hampered. Both hypotheses were corroborated. Previous studies on interactions between harvestmen and other predators, such as spiders and scorpions, had shown that defensive secretions and mechanical retaliation were seldom used and were often ineffective. Our results point to the exact opposite and suggest that different predators exert different selective pressures culminating in the array of defenses exhibited by the prey.

Habituation to a predatory stimulus in a harvester (Arachnida, Opiliones)

“…The composition of this defensive chemical is a blend of several components, including benzoquinones and phenols among others (for more details see Hara et al 2005 ). The release of this compound, though efficient (Machado et al 2005 ; Silva et al 2018 ), is costly (Nazareth and Machado 2015 ; Nazareth et al 2016 ), which may justify why none of the animals released chemicals more than once on the same day.…”

“…Mainly males can nip an aggressor by rapidly flexing both legs IV. Particularly in Mischonyx squalidus , nipping has been documented to pierce, cut, and repel predators (Dias and Willemart 2013 ; Segovia et al 2015 ; Silva et al 2018 ) and pierce human skin (GFP personal observation). This behavior can be reliably elicited by holding the harvester´s body dorso-ventrally with tweezers, a procedure that mimics capture by a bird (Gnaspini and Hara 2007 and references therein).…”

Chemical communication and ecology in Eupnoi harvestmen

“…When tested in lab trials, such shield deterred ants, wandering spiders and frogs but not opossums (Machado et al 2005). Secretions have also experimentally demonstrated to be successful to deter flatworm attacks in Myschonyx cuspidatus (Gonyleptidae) (Silva et al 2018). Besides the chemical shielding and jettison, some behaviors may enhance the secretion as defense.…”

The hard task of a short-tailed mouse opossum (Monodelphis) to prey a harvestman (Arachnida: Opiliones)

“…Some laboratorial experiments manipulated secretions extracted from harvestmen, injecting it in possible offensive agents like other arthropods, flatworms, and rats (e.g. Machado et al, 2005;Gnaspini & Hara, 2007;Silva et al, 2018). Therefore, it is unclear how such secretions act in nature leading the harvestman to escape predation as its last line of defense (Gnaspini & Hara, 2007;Pomini et al, 2010).…”