Fig. 1

SEM images showing the formation of the shell field in Lo. goshimai. All panels are dorsal views with anterior to the top (except a, since the dorsal side could not be readily determined at the developmental stage). Panels e’-f’ are images with higher magnifications showing the details of the shell field; note that they were obtained from different individuals from those shown in e and f. A shell field (sf) is discriminable with the development of some cells with surface elaborations at 7 hpf (the arrowhead in d). The peripheral cells started to arrange in an imaginary circle since 7 hpf (dashed curves in d-f) and exhibited obvious wedge-shapes since 8 hpf (highlighted by the orange arrows in e’ and f’). The shell formation started at around 8 hpf and a shell plate is evident at 9 hpf (double arrowheads in e’, f and f’). The yellow arrow in f’ indicate the apparent correlation between surface protrusions and the shell plate. Note that although we label the prototroch (pt) in all panels, it was not fully developed at 4 hpf. Bars represent 20 μm