Ultrastructure and Neuronal Control of Luminous Cells in the Copepod Gaussia princeps

The physiology of light production in copepods is largely unknown. The mesopelagic copepod Gaussia princeps possesses luminous glands, each consisting of a single large cell discharging through a cuticular pore. Slow flashes external to the cuticle are triggered from excised abdomens by electrical stimulation of the ventral nerve cord. Each luminous cell contains UV fluorescent secretory vesicles distally, which are secreted through a valved cuticular pore. Each luminous cell, except for the most proximal portion, is surrounded by a cellular sheath, which appears to form the distal valve. Luminous cells have a stem containing small, electron-lucent precursors to secretory vesicles proximal to the fluorescent vesicles. Nerve terminals, filled with large synaptic vesicles, are associated with the unsheathed proximal cell membrane. Gap junctions interconnect the nerve terminals, and possibly serve to accelerate conduction to the luminous cell to achieve a synchronous effector output.