Tasty but Protected—First Evidence of Chemical Defense in Oribatid Mites

Abstract

Oribatid mites (Acari, Oribatida) represent one of the most abundant and speciose groups of microarthropods in the decomposer food webs of soils, but little is known of their top-down regulation by predators. Oribatids are relatively long-lived and have numerous morphological defensive adaptations, and so have been proposed to live in 'enemy-free space'. Most also possess a pair of large exocrine oil glands that produce species-specific mixtures of hydrocarbons, terpenes, aromatics, and alkaloids with presumably allomonal functions, although their adaptive value has never been tested empirically. We developed a protocol that discharges the oil glands of the model oribatid species, Archegozetes longisetosus. and offered 'disarmed' individuals as prey to polyphagous Stenus beetles (Staphylinidae), using untreated mites as controls. Stenus juno fed on disarmed mites with behavioral sequences and success rates similar to those observed when they prey on springtails, a common prey. In contrast, mites from the control group with full glands were almost completely rejected; contact with the gland region elicited a strong reaction and cleaning behavior in the beetle. This is the first evidence of an adaptive value of oribatid mite oil gland secretions for chemical defense. The protocol of discharging oil glands should facilitate future studies on top-down control of oribatid mites that aim to differentiate between morphological and chemical aspects of defensive strategies.

Storage and release of hydrogen cyanide in a chelicerate ( Oribatula tibialis )

“…The large oil glands, which represent the major exocrine system in oribatid mites and which secrete MNH in O. tibialis, are potent organs for chemical defense, thus playing an important role in the structuring of feeding interactions in soil food webs (24)(25)(26). As with arthropod defensive compounds in general, most known oil gland secretions of oribatid mites-mainly hydrocarbons, terpenes, aromatics, esters, and alkaloids-are probably "class II compounds" (38); i.e., they irritate or repel potential predators without substantial harm (22)(23)(24)(25)(26)(27)(28). By contrast, benzoyl cyanide and HCN are "class I compounds" (38), true and harmful toxins (3,6,39,40).…”

“…Here, we demonstrate cyanogenesis in a mite of the order Oribatida, a diverse and mostly soil-dwelling group of decomposers that discharge myriad defenserelated semiochemicals from a pair of large exocrine opisthonotal oil glands (22)(23)(24)(25)(26)(27)(28). The common and widespread species Oribatula tibialis stores HCN as the natural product mandelonitrile hexanoate (MNH) and releases HCN upon disturbance via two different chemical pathways.…”

Specialized feeding of Euconnus pubicollis (Coleoptera: Staphylinidae: Scydmaeninae) on oribatid mites: Prey preferences and hunting behaviour

“…Some oribatid mites (Ptyctima) are capable of retracting their legs into the idiosoma and "encapsulation" by flexing ventrally the large, shield-like prodorsum; this so called ptychoid defence mechanism enables them to protect soft and vulnerable body parts and expose to a predator only heavily sclerotized cuticle (Schmelzle et al, 2008(Schmelzle et al, , 2009(Schmelzle et al, , 2010. Other oribatids are known to have exocrine oil glands (or opisthonotal glands) that produce secretions, which in some taxa (belonging to families Collohmanniidae and Trhypochthoniidae) provide chemical protection against some beetle predators (Raspotnig, 2006;Heethoff et al, 2011;Heethoff & Raspotnig, 2012a, b). Recently Saporito et al (2007Saporito et al ( , 2011 provided evidence that oribatids of the family Scheloribatidae contain toxic alkaloids.…”

Triggering chemical defense in an oribatid mite using artificial stimuli

“…by shaking or using a pincer, or by the action of potential predators), and determining whether a chemically defended prey itself-or isolated/synthesized secretions or single compounds from complex secretions-is repellent to the predator (e.g. Eisner 2003;Eisner et al 2004Eisner et al , 2005Raspotnig 2006;Heethoff et al 2011a;Souza and Willemart 2011). However, the real adaptive value of defensive secretions can only be quantified by generating chemically 'disarmed' specimens and by presenting these specimens to the same predators (Heethoff et al 2011a;Souza and Willemart 2011;Heethoff and Raspotnig 2012).…”

“…Tomita et al (2003) mechanically disturbed specimens of the astigmatid mite Tyroborus lini Oudemans by shaking until oil glands were completely empty and Raspotnig (2006) performed a similar procedure with the oribatid mite Collohmannia gigantea Sellnick, but no such study included predation experiments. Only Heethoff and Raspotnig (2012) and Heethoff et al (2011a), working with oribatid mites, used chemically disarmed specimens in predation experiments.…”