Life-cycle transitions connecting larval and juvenile stages in metazoans are orchestrated by neuroendocrine signals including neuropeptides and hormones. In marine invertebrate life cycles, which often consist of planktonic larval and benthic adult stages, settlement of the free-swimming larva to the sea floor in response to environmental cues is a key life cycle transition. Settlement is regulated by a specialized sensory-neurosecretory system, the larval apical organ. The neuroendocrine mechanisms through which the apical organ transduces environmental cues into behavioral responses during settlement are not yet understood. Here we show that myoinhibitory peptide (MIP)/allatostatin-B, a pleiotropic neuropeptide widespread among protostomes, regulates larval settlement in the marine annelid Platynereis dumerilii. MIP is expressed in chemosensory-neurosecretory cells in the annelid larval apical organ and signals to its receptor, an orthologue of the Drosophila sex peptide receptor, expressed in neighboring apical organ cells. We demonstrate by morpholino-mediated knockdown that MIP signals via this receptor to trigger settlement. These results reveal a role for a conserved MIP receptor-ligand pair in regulating marine annelid settlement.M etazoan life cycles show great diversity in larval, juvenile, and adult forms, as well as in the timing and ecological context of the transitions between these forms. In many animal species, neuroendocrine signals involving hormones and neuropeptides regulate life cycle transitions (1-3). Environmental cues are often important instructors of the timing of life cycle transitions (4), and can affect behavioral, physiological, or morphological change via neuroendocrine signaling (5).Marine invertebrate larval settlement is a prime example of the strong link between environmental cues and the timing of life-cycle transitions. Marine invertebrate life cycles often consist of a free-swimming (i.e., pelagic) larval stage that settles to the ocean floor and metamorphoses into a bottom-dwelling (i.e., benthic) juvenile (6-8). In many invertebrate larvae, a pelagicbenthic transition is induced by chemical cues from the environment (9, 10). Larval settlement commonly includes the cessation of swimming and the appearance of substrate exploratory behavior, including crawling on or attachment to the substrate (11-14). In diverse ciliated marine larvae (15), the apical organ, an anterior cluster of larval sensory neurons (16) with a strong neurosecretory character (17-20), has been implicated in the detection of cues for the initiation of larval settlement (21). Although molecular markers of the apical organ have been described (22-24), our knowledge of the neuroendocrine mechanisms with which apical organ cells transmit signals to initiate larval settlement behavior is incomplete.Here, we identify a conserved myoinhibitory peptide (MIP)/ allatostatin-B receptor-ligand pair as a regulator of larval settlement behavior in the marine polychaete annelid Platynereis dumerilii. MIPs are pleiotro...

New methods for clearing and expansion of biological objects create large, transparent samples that can be rapidly imaged using lightsheet microscopy. Resulting image acquisitions are terabytes in size and consist of many large, unaligned image tiles that suffer from optical distortions. We developed the BigStitcher software that efficiently handles and reconstructs large multi-tile, multi-view acquisitions compensating all major optical effects, thereby making single-cell resolved whole-organ datasets amenable to biological studies.Sample clearing [chung, Hama] and expansion microscopy (ExM) [exp] are powerful protocols that create large, transparent volumes of whole tissues and organisms. Using lightsheet microscopy, these samples can be imaged with subcellular resolution in their entirety within a few hours [Tomer]. These acquisitions have the potential to be powerful tools for whole-tissue and whole-organism studies since they preserve endogenous fluorescent proteins and are compatible with most staining methods (Supplementary Fig. 1).However, raw data acquired by the microscope is not directly suitable for visualization and analysis. Many large, overlapping three-dimensional (3d) image tiles are collected that amount to many terabytes in size and require image alignment ( Fig. 1d-m). Due to sample-induced scattering of the lightsheet in the direction of illumination [scat], 3d image tiles are typically acquired twice while alternating illumination from opposing directions to achieve full coverage ( Fig. 1d and Supplementary Fig. 2). Similarly, emitted light is distorted by the sample, effectively limiting maximal imaging depth at which useful data can be collected (Fig. 1n). Additionally, sample-induced light refractions cause depth-and wavelength-dependent aberrations in the acquired images (Fig. 1j,k). To reconstruct these complex datasets and make the data transparently accessible we developed the BigStitcher software. It enables interactive visualization using BigDataViewer [bdv], fast and precise alignment, real-time fusion, deconvolution, as well as support for alignment of multitile acquisition taken from different physical orientations, so called multi-tile views, thereby effectively doubling the size of specimens that can be imaged (Fig. 1n), and in the case of orthogonal views rendering the resolution isotropic.

Animals use spatial differences in environmental light levels for visual navigation; however, how light inputs are translated into coordinated motor outputs remains poorly understood. Here we reconstruct the neuronal connectome of a four-eye visual circuit in the larva of the annelid Platynereis using serial-section transmission electron microscopy. In this 71-neuron circuit, photoreceptors connect via three layers of interneurons to motorneurons, which innervate trunk muscles. By combining eye ablations with behavioral experiments, we show that the circuit compares light on either side of the body and stimulates body bending upon left-right light imbalance during visual phototaxis. We also identified an interneuron motif that enhances sensitivity to different light intensity contrasts. The Platynereis eye circuit has the hallmarks of a visual system, including spatial light detection and contrast modulation, illustrating how image-forming eyes may have evolved via intermediate stages contrasting only a light and a dark field during a simple visual task.DOI: http://dx.doi.org/10.7554/eLife.02730.001

Developmental programs have the fidelity to form neural circuits with the same structure and function among individuals of the same species. It is less well understood, however, to what extent entire neural circuits of different individuals are similar. Previously, we reported the neuronal connectome of the visual eye circuit from the head of a Platynereis dumerilii larva (Randel et al., 2014). We now report a full-body serial section transmission electron microscopy (ssTEM) dataset of another larva of the same age, for which we describe the connectome of the visual eyes and the larval eyespots. Anatomical comparisons and quantitative analyses of the two circuits reveal a high inter-individual stereotypy of the cell complement, neuronal projections, and synaptic connectivity, including the left-right asymmetry in the connectivity of some neurons. Our work shows the extent to which the eye circuitry in Platynereis larvae is hard-wired.DOI: http://dx.doi.org/10.7554/eLife.08069.001