SignificanceSingle-celled microorganisms are important in ecosystems, and their behaviors impact the Earth’s environments. To survive in harsh environments, these organisms frequently act as though exercising discretion. How do they achieve such intelligent behaviors? In this work, we focused on the accumulation of ciliates on solid/fluid interfaces, where they can obtain sufficient nutrients and a stable environment. This phenomenon is not described in the standard hydrodynamics of microswimmers. Our experiment and simulation revealed that simple principles, the anisotropic shape of the cell and the mechanosensing nature of cilia, induce the accumulation of ciliates on solid/fluid interfaces. The contribution of our work is that a simple response of the cellular apparatus and fluid dynamics explain the apparently clever behavior of ciliates.

Some types of ciliates accumulate on solid/fluid interfaces. This behavior is advantageous to survival in nature due to the presence of sufficient nutrition and stable environments. Recently, the accumulating mechanisms of Tetrahymena pyriformis at the interface were investigated. The synergy of the ellipsoidal shape of the cell body and the mechanosensing feature of the cilia allow for cells to slide on interfaces, and the sliding behavior leads to cell accumulation on the interfaces. Here, to examine the generality of the sliding behavior of ciliates, we characterized the behavior of Paramecium caudatum, which is a commonly studied ciliate. Our experimental and numerical results confirmed that P. caudatum also slid on the solid/fluid interface by using the same mechanism as T. pyriformis. In addition, we evaluated the effects of cellular ellipticity on their behaviors near the wall with a phase diagram produced via numerical simulation.

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For analysing spatial distribution of maternal proteins in an amphibian egg, monoclonal antibodies specific to certain regions were raised. One monoclonal antibody was found (MoAB Xa5B6) which reacted specifically with the animal hemisphere of the mature Xenopus laevis egg. The maternal protein that reacted with the MoAb Xa5B6 was shown to be distributed asymmetrically along the dorso-ventral axis in the upper region of the equatorial zone of the fertilized egg. At late blastula stage, the antigen protein could be observed clearly in both the marginal zone and animal cap. It was localized predominantly in mesodermal and ectodermal cells of late neurula embryos. The Xa5B6 antigen accumulated during oogenesis. The distribution pattern of maternal protein was remarkably different in the developmental stages of the oocyte. The pattern in the mature oocyte was completely different from that of the immature egg in which the antigen was located in the radial striations of the oocyte cytoplasm. After maturation, the distribution pattern changed drastically to an animal-vegetal polarization and the striation labellings were no longer observed. By Western blot examination, it was confirmed that the amounts of antigen protein were constant during early embryogenesis and the mesoectoderm contained a greater amount of antigens than the endoderm at late blastula. The antibody detected two bands of approximately 70 × 10 and 30 × 10 Mr by Western blot analysis. The latter molecule may possibly be a degrading moiety of the former. The results were discussed in relation to establishment of animal-vegetal (A/V) and dorso-ventral (D/V) polarization at the molecular level.

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