Fig. 1

Development stages of Platynereis dumerilii. A Temporal development of Platynereis beginning with fertilization (0 hpf), spiral cleavage stages (2 to 12 hpf), early and late trochophore (24 and 48 hpf), and nectochaete (72 hpf) larval stages. The first cell division begins shortly after 2 hpf, followed by a period of spiral cleavages, and then a transition to a bilaterally symmetrical pattern of cell divisions after 7 hpf. B Unequal spiral cleavage pattern of Platynereis embryos. Schematics depict animal-pole views of spiral cleavage stages. The two small circles in the center of the 4- to 16-cell stages represent the two polar bodies. The 4-cell stage shows the unequal size and nomenclature of the four quadrants/founder cells (A–D). The 8-cell stage shows the animal-pole 1st micromeres (1a, 1b, 1c, and 1d; or 1q) and their vegetal-pole daughter cells (1A, 1B, 1C, and 1D; or 1M). The 16-cell stage depicts the daughter cell pairs of the first micromeres (1q ¹ and 1q ²). The 49-cell stage indicates the cell progeny contributed from each quadrant (dark lines) and highlights the cleavage pattern of the progeny of the first micromeres indicating the nomenclature for the 1a progeny, the first micromere of the A quadrant. Each of the four quadrants generates one small rosette cell (1q ¹¹¹) and a larger daughter cell (1q ¹¹²) whose progeny will form the anterior head region. These cells form the ‘annelid cross’ (white) and are surrounded by cells (green) that will form the ciliated ring/prototroch of the larvae. The 66-cell stage highlights the first bilaterally symmetrical cleavage in the 2d cell lineage giving rise to 2d¹¹²¹ and 2d¹¹²² cells whose progeny will form the trunk ectoderm. C Schematics of three larval stages, the early (24 hpf) and late (48 hpf) trochophore, and the mid nectochaete (72 hpf), ventral views with anterior to the top. The prototroch is a ciliated ring of cells located between the anterior head region/episphere and the posterior trunk region/hyposphere. The episphere harbors the apical organ, eyes, and brain. The trunk region contains the three larval segments including the chaetal sacs. The pygidium includes the posterior growth zone where new segments are added. The stomodeum is located on the ventral side adjacent to the prototroch. Chaetal sacs are three segmental pairs of primordia that give rise to appendages, the parapodia. By the nectochaete stage, parapodia are well established and the head region becomes distinct. Abbreviations: A antenna, AC anal cirri, AT apical tuft, GC larval gland cells, LA larval eyes, P prototroch, Pa palps, PC peristomial cirrus, S stomodeum, SS setal (chaetal) sacs. Schematics are modified from Fischer and Dorresteijn 2004 [43] and Pruitt et al. 2014 [54]