Many marine fish harbor luminous bacteria as bioluminescent symbionts. Despite the diversity, abundance, and ecological importance of these fish and their apparent dependence on luminous bacteria for survival and reproduction, little is known about developmental and microbiological events surrounding the inception of their symbioses. To gain insight on these issues, we examined wild-caught larvae of the leiognathid fish Nuchequula nuchalis, a species that harbors Photobacterium leiognathi as its symbiont, for the presence, developmental state, and microbiological status of the fish's internal, supraesophageal light organ. Nascent light organs were evident in the smallest specimens obtained, flexion larvae of 6.0 to 6.5 mm in notochord length (NL), a developmental stage at which the stomach had not yet differentiated and the nascent gasbladder had not established an interface with the light organ. Light organs of certain of the specimens in this size range apparently lacked bacteria, whereas light organs of other specimens of 6.5 mm in NL and of all larger specimens harbored large populations of bacteria, representatives of which were identified as P. leiognathi. Bacteria identified as Vibrio harveyi were also present in the light organ of one larval specimen. Light organ populations were composed typically of two or three genetically distinct strain types of P. leiognathi, similar to the situation in adult fish, and the same strain type was only rarely found in light organs of different larval, juvenile, or adult specimens. Light organs of larvae carried a smaller proportion of strains merodiploid for the lux-rib operon, 79 of 249 strains, than those of adults (75 of 91 strains). These results indicate that light organs of N. nuchalis flexion and postflexion larvae of 6.0 to 6.7 mm in NL are at an early stage of development and that inception of the symbiosis apparently occurs in flexion larvae of 6.0 to 6.5 mm in NL. Ontogeny of the light organ therefore apparently precedes acquisition of the symbiotic bacteria. Furthermore, bacterial populations in larval light organs near inception of the symbiosis are genetically diverse, like those of adult fish.

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Re-examination of two deep-sea ophidiid species, Bassozetus compressus (Günther 1878) and Bassozetus elongatus Smith and Radcliffe in Radcliffe 1913, based on specimens from the West Pacific, Atlantic and Indian Oceans, disclosed no morphologically significant differences amongst those from the West Pacific, including the lectotype of B. compressus and holotype of B. elongatus. Two characters considered to distinguish between the species [pelvic-fin length to standard length (SL) ratio and position of a small process on the sagittal otolith] were found to be invalid, leading to the conclusion that B. compressus is a senior synonym of B. elongatus. The remaining specimens from the Atlantic and Indian Oceans, previously reported as B. compressus or B. elongatus, represented one or more as-yet undescribed species. A detailed redescription of B. compressus is provided on the basis of the West Pacific specimens, the species being distinguishable from all other congeners by the following combination of characters: dorsal-fin rays 118-127; pectoral-fin rays 22-26; oblique scales ca. 18-20; vertebrae 11-13 ? 54-58 = 66-69; distance between base of pelvic fin to origin of anal fin 15.8-20.5 % SL; eye diameter 2.0-2.7 times in snout length; a median basibranchial tooth patch; anterior margin of sagittal otolith with a small process, dorsal margin smooth. Two specimens of B. compressus, collected from off Uotsurijima Island, Senkaku Islands and Kumejima Island, Okinawa Islands, represent the first record of this species from Japanese waters.