Ultrastructure of neuro-spirocyte synapses in the sea anemone Aiptasia pallida (Cnidaria, Anthozoa, Zoantharia)

Using transmission electron microscopy of serially sectioned tentacles from the sea anemone Aiptasia pallida, we located and characterized two types o f neuro-spirocyte synapses. Clear vesicles were observed at 10 synapses and dense-cored vesicles at five synapses. The diameters of vesicles at each n euro-spirocyte synapse were averaged; clear vesicles ranged from 49-89 nm i n diameter, whereas the dense-cored vesicles ranged from 97-120 nm in diame ter. One sequential pair of synapses included a neuro-spirocyte synapse wit h clear vesicles (81 nm) and a neuro-neuronal synapse with dense-cored vesi cles (168 nm). A second synapse on the same cell had dense-cored vesicles ( 103 nm). An Antho-RFamide-labeled ganglion cell and three different neurite s were observed adjacent to spirocytes, but no neuro-spirocyte synapses wer e present. Many of the spirocytes also were immunoreactive to Antho-RFamide . The presence of sequential neuro-neuro-spirocyte synapses suggests that s ynaptic modulation may be involved in the neural control of spirocyst disch arge. The occurrence of either dense-cored or clear vesicles at neuro-spiro cyte synapses suggests that at least two types of neurotransmitter substanc es control the discharge of spirocysts in sea anemones. (C) 1999 Wiley-Liss , Inc.