Biology of the oribatid mite Mucronothrus nasalis (Acari: Oribatida: Trhypochthoniidae) from a small coldwater springbrook in eastern Canada

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Abstract
A population of the aquatic oribatid mite Mucronothrus nasalis, living in a cold, hardwater springbrook near Toronto, Canada, was sampled monthly from July 1985 to July 1986 using a Surber sampler. Of nine sampling locations situated along the full 60 m length of the springbrook, the source had by far the highest density of mites. Mites were most abundant in winter, reaching 1 individual/1.3 cm2 of substrate at the source, and least abundant in summer. Males were present, but uncommon, usually composing less than 10% of the adult population. Most collected females were gravid, regardless of season. Population age structure was inverted, with older instars being more abundant. There were no pulsed increases in density of immatures relative to adults, suggesting that reproduction is not strongly seasonal. Both adults and immatures appear to be generalized grazers on the "aufwuchs" community of unicellular and filamentous algae, fungi, and other organisms coating substrate surfaces, but also feed on decaying higher plant material. New geographic records show a transcontinental distribution in Canada, and the first records for the United States and Australia are given. Newly recorded habitat types include riverbeds and the profundal zone of lakes. Out of water behavior suggests that terrestrial dispersal is unlikely, supporting the hypothesis that the species predates the breakup of the supercontinent Pangea, with local distributions perhaps dating from Pleistocene events. The life history of this species appears more K-selected than that of another aquatic oribatid mite, Hydrozetes lemnae.

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