Fig. 2

Eurpymna scolopes embryonic development and denomination of axes. a Schematic overview of E. scolopes normal embryonic development after Lee et al. [25]. Development takes about 21 days at 24 °C water temperature. Except for the hatchling, embryos are oriented with dorsal to the top and anterior to the left. The hatchling is shown in a posterior view, which in the physiological orientation of the adult will become the ventral side of the animal. b Embryonic (anteroposterior, AP) versus adult (dorsoventral, DV) body axes. As opposed to the physiological orientation of the adult animal, the mantle is considered dorsal and the mouth ventral during embryonic development, while the future dorsal side is considered anterior and the future ventral side posterior in the embryo. c Terminology of the embryonic arm’s spatial organization with respect to the embryonic body axes (inset) used in this study. Distal is defined as the tip, proximal as the base of the appendage, the side bearing suckers is considered as oral and the opposite side, as aboral. The side facing the early mouth primordium is regarded as anterior and the one facing the funnel as posterior. Grey rectangles indicate the position of the sections through the arms shown in this study (frontal and sagittal). I – V denotes the arm pairs in the order they are spatially positioned along the AP axis; e, eye; fu, funnel; fi, fin; m, mantle