Developmental and Microbiological Analysis of the Inception of Bioluminescent Symbiosis in the Marine FishNuchequula nuchalis(Perciformes: Leiognathidae)

Abstract

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.

Common trends in mutualism revealed by model associations between invertebrates and bacteria: Table 1

“…However, another widespread basis for cooperation between species is defense against predators and pathogens. Such is the case for bioluminescent microbes that colonize the light organs of numerous marine fishes and squid (Dunlap, 1985; Dunlap et al , 2004; Dunlap et al , 2008; McFall-Ngai, 2008b). Among bioluminescent symbioses, the mutualism between the Hawaiian bobtail squid Euprymna scolopes and Vibrio fischeri is the best studied, and indeed represents one of the most developed model systems to study mutualism (McFall-Ngai, 2008b).…”

Larval development of the Common Ponyfish, Leiognathus equulus (Teleostei: Leiognathidae)

“…Light pigment is scattered over the ventral surfaces of the gas bladder, and very little pigment is scattered over the lateral surfaces. The light organ appears to develop similarly to that in Nuchequula nuchalis, but in L. equulus is more bulbous laterally, and both the light organ and gas bladder are more advanced in development at a similar body size (Haneda and Tsuji 1976;Dunlap et al 2008).…”

“…Pigment on the fin spines, the preopercular spines, and the spine-like angle of the lower jaw and cleithral symphysis may make these prominent structures even more visually prominent, and could thus serve a defensive function. Based on dissection of three specimens, the light organ development of L. equulus is very similar to that of Nuchequula nuchalis (see Dunlap et al 2008). In both species, the light organ is well developed in larvae as small as 5.2 mm.…”

“…Flexion occurs at a small size (2.8-4.0 mm), by which time a very long premaxillary process and small teeth have developed. The 'fin lock' mechanism is functional early in the postflexion stage (Trnski and Leis 2004), and the luminescent organ is present from about 5 mm (Dunlap et al 2008).…”

“…Descriptions of larval development are available for only a few identified species (all Morph 2): Nuchequula nuchalis (as Leiognathus nuchalis) (Fujita 1960;Myoung and Kim 1984;Kinoshita 1988;Haque and Ozawa 1995), Secutor ruconius (Connell 2009), Equulites rivulatus (Kinoshita 1988;Haque and Ozawa 1995) and another species likely to be Equulites elongatus (as Leiognathus elongatus) (Haque and Ozawa 1995). Development of the light organ is described in Nuchequula nuchalis (see Dunlap et al 2008). Here we describe the larval development of Leiognathus equulus based on 24 specimens, ranging from 4.6 to 15.8 mm, from aquaculture farms in southern Taiwan.…”