We propose an inanimate system composed of camphor boats in an annular water channel in order to understand the collective motions. The boats move on the water surface spontaneously and interact with one another through the concentration of the camphor molecules on the water. We observed several modes of collective motion, e.g., behaviors analogous to traffic flow or an ant trail. Our system provides a convenient experimental setup for the investigation of a variety of collective motions.

When an ensemble of self-propelled camphor boats move in a one-dimensional channel, they exhibit a variety of collective behaviors. Under certain conditions, the boats tend to cluster together and move in a relatively tight formation. This type of behavior, referred to as clustering or swarming here, is one of three types recently observed in experiment. Similar clustering behavior is also reproduced in simulations based on a simple theoretical model. Here we examine this model to determine the clustering mechanism and the conditions under which clustering occurs. We also propose a method of quantifying the behavior that may be used in future experimental work.

Sea urchins are used as a model organism for research on developmental biology and gene regulatory networks during early development. Gene knockdown by microinjection of morpholino antisense oligonucleotide (MASO) has been used to analyze gene function in early sea urchin embryos. However, as the effect of MASO is not long lasting, it is impossible to perturb genes expressed during late development by MASO. Recent advances in genome editing technologies have enabled gene modification in various organisms. We previously reported genome editing in the sea urchin Hemicentrotus pulcherrimus using zinc‐finger nuclease (ZFN) and transcription activator‐like effector nuclease (TALEN); however, the efficiencies of these technologies were not satisfactory. Here, we applied clustered regularly interspaced short palindromic repeat (CRISPR)‐CRISPR‐associated nuclease 9 (Cas9) technology to knock out the Pks1 gene in H. pulcherrimus. When sgRNAs targeting Pks1, which is required for the biosynthesis of larval pigment, were microinjected into fertilized eggs with SpCas9 mRNA, high‐efficiency mutagenesis was achieved within 24 hr post fertilization and SpCas9/sgRNA‐injected pluteus larvae had an albino phenotype. One of the sgRNAs yielded 100% mutagenesis efficiency, and no off‐target effect was detected. In addition, the albino phenotype was maintained in juvenile sea urchins after metamorphosis, and the knockout sea urchins survived for at least one year and grew to albino adult sea urchins. These findings suggest that knockout adult sea urchins were successfully established and the CRISPR‐Cas9 system is a feasible method for analyzing gene functions from late developmental to adult stage.

The concept of self-propelled objects is important for the understanding of biological mobility, as well as for the development of autonomous devices in medicine and engineering. In this study, a simple self-propelled object, driven by a difference in surface tension, was found to exhibit intermittent self-motion (alternately in motion and at rest) in an annular water channel, with resting positions and features of motion in subsequent cycles remaining almost the same as those previously visited; that is, memories of the resting positions and features of motion were observed. The occurrence of the memory phenomenon was found to depend on the relationship between the resting time and the period for one lap of the annular channel. The mechanism of memory is discussed in terms of the distribution of surface-active molecules and local surface tension at the resting positions.

We investigate the collective motion of symmetric self-propelled objects that are driven by a difference in the surface tension. The objects move around an annular water channel spontaneously and interact through the camphor layer that develops on the water surface. We found that two collective motion modes, discrete and continuous density waves, are generated depending on the number of self-propelled objects. The two modes are characterized by examining the local and global dynamics, and the collective motion mechanism is discussed in relation to the distribution of camphor concentration in the annular water channel. We conclude that the difference between these two modes originates from that of the driving mechanism that pushes a camphor paper away from a cluster, through which mechanism density waves are generated and maintained.