Marine Symbioses: Metazoans and Microbes (V fischeri) (Figure 6(c)) and several species of Photobacterium (P leiognathi and P phosphoreum) The necessity of the association for host survival is less well defined than the nutrient-exchange symbioses However, hosts lacking the ability to produce light for prey attraction or predator avoidance will likely not be successful over time From a symbiont perspective, the culturable strains are considered facultative and free-living under nutrient-rich conditions, but several strains remain resistant to culturing, suggesting that they are highly specialized for survival inside the host light organ However, most bacterial species are currently unculturable, so this feature does not guarantee that they are in an obligate association with their host As a mutualism, the hosts gain a variety of functions that will be highlighted shortly, and the bacteria are provided with a nutrient-enriched environment for growth, allowing populations to survive in much greater densities than in the open ocean Density becomes an important consideration for light production by bacteria, since a bacterium is only several microns long The amount of light produced by a single cell quickly becomes a waste of valuable energetic resources However, when enough cells are present in a small area such as a host’s light organ, the light output becomes substantial But how bacteria know when enough cells are present? The regulation of light production includes a mechanism for quorum sensing, allowing the cells to chemically detect density (also important in pathogenesis) Quorum sensing involves the continual release of a chemical into the environment When the chemical, an autoinducer, reaches a critical concentration, it triggers the induction of a metabolic process In the case of light production, the autoinducer turns on the production of the luciferase enzyme, thereby turning on luminescence (Figure 6(d)) Fish Light production in bony fish is often autogenic, but those that host bacterial symbionts have developed a variety of symbiosis structures (Haygood, 1993) Several of these host organs occur as outpockets of the gut, which suggests that colonization occurs during feeding from free-living populations (confirmed by culturing) Others house the bacteria in highly specialized superficial structures that are open to the environment However, their symbionts have not been seen free-living (nor cultured), so colonization mechanisms are unknown The location of the light organ and direction of light emission provides some information about function, with a few representative examples described in the following paragraphs Ponyfish, or Leiognathids, reside in shallower, near-shore waters in the Indo-Pacific Ocean and are characterized by a protrusible mouth and a circumesophageal light organ (Figure 6(e)) The light organ is colonized by P leiognathi, and emission of light by the light organ is controlled by highly modified structures in the host fish The light organ is positioned next to the gas bladder, with a dorsal shutter that controls the entrance of light into the gas bladder The gas bladder is silvered over to allow the light to be reflected to the posterior end and out through the ventral side of the fish body 123 near a transparent anal fin (Mcfall-Ngai and Dunlap, 1984) Other lateral shutters are present around the light organ to control lateral light emission The position of light emission appears to correlate with function; ventral emission near the anal fin acts as camouflage, and lateral emission plays a role in communication Anglerfish or Ceratioids, occur in the deep, dark abyssal ocean as solitary, relatively rare fishes They are characterized by a prominent bioluminescent lure at the end of a modified dorsal spine that extends out over the head (Figure 6(f)) The superficial light organ is colonized by a strain of unculturable Vibrio-like species of symbiont The function of the lure, easily identified by the location, is used for prey attraction Symbiont transmission is unknown, but the rarity and solidarity of these fishes and the fact that their symbiont has not been cultured suggest a mechanism for vertical transmission These few examples highlight the major functions of luminescence in marine fishes but are not the only examples Similar phenomena are seen in more than 20 families of bony fishes and are present in all marine environments around the globe Squid Bioluminescence is very common in squids, with at least 70 luminous genera Most produce endogenous light, but only a few families are known to utilize bioluminescent bacteria: the Sepiolidae and Loliginidae One species, Euprymna scolopes, the Hawaiian bobtail squid, has become the model organism for studying light-producing symbioses, as well as a model for host symbiont–pathogen interactions and metazoan immunity in general (Mcfall-Ngai, 2008; Mcfall-Ngai et al., 2010) This particular association has become a well-studied scientific model because the organisms are easy to catch in the wild, the host can be reared from egg to adult in aquaria, and the V fischeri symbionts can be cultured independently of the host (Figure 6(c)) Hawaiian bobtail squid are small, nocturnal creatures, about cm in length, that burrow in the sand during the day and come out at night to hunt The light organ lies ventral to the ink sac, which reflects light emitting from the light organ This reflected light creates down-welling, counter illumination that masks any shadow caused by overhead moon or starlight The light organ consists of two enclosed sacs called crypts which are connected to the surrounding seawater through ducts that open up as pores in the mantel cavity (Figure 6(g)) In spite of the continuous connection with the environment, the light organ houses a dense monoculture of V fischeri, which are acquired from the environment by juvenile squids, typically within hours after hatching Adults regulate symbiont population by venting up to 95% of their symbionts into the environment at the end of every night Then they bury themselves in the sand for the day, allowing the bacterial culture to repopulate; by nightfall, the light organ is back to full capacity The initiation, recognition, and establishment stages of this symbiosis are very well understood They are presented in some depth here to illustrate the intricate regulation mechanisms involved in setting up a highly specific, horizontally transmitted symbiosis (Nyholm and Mcfall-Ngai, 2004) Most