The caudal luminous organs of lanternfishes: general innervation and ultrastructure.

Neuroanatomical, light and electron microscopic investigations of the caudal luminous organs of two lanternfish species, Stenobrachius leucopsarus and Parvilux ingens, were conducted in a search for morphological correlates underlying their luminescent behavior and control mechanisms. Complex neural pathways involving the spinal nerves and the sympathetic nerve chain of the caudal peduncle are associated with profuse segmental innervation to both the supracaudal and infracaudal organs. Neural composition of these segmental subunits indicates that pre-ganglionic (spinal) as well as post-ganglionic (sympathetic) fibers are involved in the neural control of luminescence of these organs. Neuro-photocyte units, in which multiple nerve branches are sandwiched between two lamellar photocytes and establish large surface areas of close appposition, as well as gap junctions apparently interconnecting all photocytes throughout the luminous organs, may account for the very rapid and simultaneous displays of spontaneous or electrically driven luminescence. The organization of the caudal luminous organs is compared with that of lanternfish photophores. Relatively few granular and agranular synaptic vesicles are present in some nerve processes of the photocyte units, suggesting that adrenergic neurotransmission as well as electrotonic spread of excitation may be involved at the neuro-photocyte junctions.