Fig. 4

Serotonin immunoreactivity (5-HT-lir) in the veliger larvae of Azumapecten farreri. Yellow—5-HT-lir; mangeta (a–d) or white (a1–d1)—cilia, α-tubulin immunoreactivity; blue (a–d) or white (a2–d2)—cell nuclei, DAPI. a The early veliger at 48 hpf. The apical organ (AO) comprises five 5-HT-lir cells with thin neurites running to the velum (v). Inserts: a1 Five cells with thin neurites (arrows). a2 Five cells with thin neurites (arrow) extending to the velum. b The middle veliger stage at 72 hpf. Cells of the AO are surrounded by neuropil and extend neurites (arrows) to the velum. Inserts: b1 Four paired cells of the AO (asterisks) and neurites extend to velum (arrow). b2 The neuropil of the AO with extending neurites (arrow). c The late veliger stage at 7 dpf. The AO is transformed into a cerebral ganglion (CG), which has thin neurites extending to the velum. From CG, neurites extend to the dorsal (arrow) and ventral parts of the larvae (arrow). Likewise on the ventral side of the larvae, neurites form a net of an enteric nervous system (ens). Inserts: c1 5-HT-lir neurites extend from CG to the dorsal side of the larvae (arrow) and to the velum (arrow) and c2 to the ventral side of the larvae and enteric nervous system (ens). d The pediveliger stage is 30 dpf. CG 5-HT-lir neurites extend to the dorsal part of larvae and to the ventral part of the larvae and enteric nervous system (ens). The two neurons located in the ventral part of the larvae are likely components of the prospective pleural ganglion (PlG). Inserts: d1 5-HT-lir innervation extends from CG to the dorsal side of the larva’ s body. d2 5-HT-lir cells like part of PlG. Additional abbreviations: a—anus, m—mouth, st—stomach. The orientation a- anterior, d—dorsal, p—posterior, v—ventral. The orientation a- anterior, d—dorsal, p—posterior, v—ventral. Scale bars = 20 μm