Abstract
BackgroundMechanical defenses are very common and diverse in prey species, for example in oribatid mites. Here, the probably most complex form of morphological defense is known as ptychoidy, that enables the animals to completely retract the appendages into a secondary cavity and encapsulate themselves. The two groups of ptychoid mites constituting the Ptyctima, i.e. Euphthiracaroidea and Phthiracaroidea, have a hardened cuticle and are well protected against similar sized predators. Euphthiracaroidea additionally feature predator-repelling secretions. Since both taxa evolved within the glandulate group of Oribatida, the question remains why Phthiracaroidea lost this additional protection. In earlier predation bioassays, chemically disarmed specimens of Euphthiracaroidea were cracked by the staphylinid beetle Othius punctulatus, whereas equally sized specimens of Phthiracaroidea survived. We thus hypothesized that Phthiracaroidea can withstand significantly more force than Euphthiracaroidea and that the specific body form in each group is key in understanding the loss of chemical defense in Phthiracaroidea. To measure force resistance, we adapted the principle of machines applying compressive forces for very small animals and tested the two ptyctimous taxa as well as the soft-bodied mite Archegozetes longisetosus.ResultsSome Phthiracaroidea individuals sustained about 560,000 times their body weight. Their mean resistance was about three times higher, and their mean breaking point in relation to body weight nearly two times higher than Euphthiracaroidea individuals. The breaking point increased with body weight and differed significantly between the two taxa. Across taxa, the absolute force resistance increased sublinearly (with a 0.781 power term) with the animal’s body weight. Force resistance of A. longisetosus was inferior in all tests (about half that of Euphthiracaroidea after accounting for body weight). As an important determinant of mechanical resistance in ptychoid mites, the individuals’ cuticle thickness increased sublinearly with body diameter and body mass as well and did not differ significantly between the taxa.ConclusionWe showed the feasibility of the force resistance measurement method, and our results were consistent with the hypothesis that Phthiracaroidea compensated its lack of chemical secretions by a heavier mechanical resistance based on a different body form and associated build-up of hemolymph pressure (defensive trade-off).
Highlights
Mechanical defenses are very common and diverse in prey species, for example in oribatid mites
The breaking point was significantly different between Phthiracaroidea and Euphthiracaroidea
The exact mode of failure is not observable, since crushing of the mites happens within one frame of the video at normal temporal resolution (60 FPS), but viewing the remains (Additional file 4: Figure S1), we found that especially larger Phthiracaroidea were much more shattered (Additional file 4: Figure S1E, F) than Euphthiracaroidea (Additional file 4: Figure S1A-C)
Summary
Mechanical defenses are very common and diverse in prey species, for example in oribatid mites. They are speciose and abundant [49], and constitute valuable potential prey for predators that naturally are abundant in soil ([34, 45, 55]; Mollemann and Walter, 2001; [37, 56, 72]). In such scenarios with pronounced predation pressure, oribatid mites developed diverse and effective defensive strategies ([35, 37] and references within), and they are assumed to live in a largely ‘enemy-free space’ [41]. Morphological defenses of oribatid mites include a thick, hardened, and in some cases biomineralized cuticle [3, 38, 39], wing-like tecta protecting the legs (pteromorphs; [45, 55]), and erectile setae [36]
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