Hawaiian bobtail squid

What is a bobtail squid? Euprymna scolopes (Figure 1Figure 1) is a small sepiolid squid, or bobtail squid, occurring in the marine coastal environment of the Hawaiian archipelago. Whereas most other squid are always out and about, the bobtail squid buries itself in the sand during its daylight quiescent period. Then, at night, individuals of this species emerge from the sand to forage on prey, such as shrimp and worms, that are swimming in the water column. To camouflage themselves while swimming, these squid form a relationship with the luminous bacterium Vibrio fischeri. The light of their symbiotic microbial population is emitted out of the animal's ventral surface, and mimics down-welling moonlight and starlight, so that the squid does not cast a shadow. In this way, potential predators on the sea floor cannot see the bobtail squid as easily.Figure 1The Hawaiian bobtail squid Euprymna scolopes.Top: this fully adult animal is approximately 3 cm in total length. Bottom: a confocal micrograph of the surface of one side of the juvenile light organ. Appendages on the surface, such as the one in the upper left, secrete mucus in which the symbiont V. fischeri is harvested. After a couple of hours of gathering in the mucus, the symbiont cells enter host tissues through the pores, seen here as the typical triad of openings at the base of the appendage. For scale, the size of the smallest pore in this image is 10 microns across.View Large Image | View Hi-Res Image | Download PowerPoint SlideHow does the animal manage the light emission of the symbionts? The symbiotic microbial partner lives extracellularly along the apical surfaces of epithelia in a ‘light organ’, which is in the center of the squid host's body, or mantle, cavity. These bacteria have the potential to emit light constantly. The host squid controls the bacterial light in two ways. First, the epithelia that support the bacteria are embedded in a set of tissues, very similar to those that surround the retina of the eye. Whereas in light reception by the eye, the accessory tissues modify incoming light, in the bacterial light organ, they modify outgoing light. Specifically, the light-emitting tissues are surrounded by diverticula of the squid's ink sac. A reflective tissue lines the dorsal ink-sac diverticula, and together these layers ensure that light is directed out of the animal's ventral surface. The ventral ink-sac diverticula act as an iris, which contracts and expands to allow more or less of the bacterial light out into the environment. Finally, the entire ventral surface of the light organ is covered by a muscle-derived lens, which diffuses the bacterial luminescence. The other way that the animal controls bacterial luminescence is by withholding oxygen. V. fischeri luminescence is the result of the oxidation of substrates; by limiting oxygen to the culture of bacteria, the host squid effectively limits the amount of luminescence that the symbionts emit.Why is this symbiosis interesting? The squid–vibrio symbiosis is a naturally occurring symbiosis that can be studied under laboratory conditions. Most symbiotic associations are nutritionally based — the symbiont provides the host with some essential nutrient or nutrients, such as amino acids or vitamins. Because of this, it is often difficult to raise the host, and keep it healthy, without its symbionts. Also, it is common for the microbial partner in these associations to be unculturable outside of the symbiotic state. In the squid–vibrio symbiosis, the principal provision of the symbiont to the host is light used in the host's behavior and, under laboratory conditions, the host appears to be physiologically unaffected when the symbionts are not present. Furthermore, the symbiont is easily grown in culture, and molecular genetic techniques have been developed to analyse the role of specific genes in the dynamics of the symbiosis. Biologists have taken advantage of these features of the squid–vibrio system over the past twenty years to study how a symbiotic association between a bacterium and an animal is established and maintained.How do the partners manage to get together each generation? Like the associations that humans, and other vertebrates, have with their myriad bacterial partners, the squid–vibrio association is established anew each generation. The bacteria occur in the seawater at about 200–500 cells per milliliter. The newly hatched juvenile squid draws the symbiont-containing water into its mantle cavity as it ventilates its gills in respiration. The nascent light organ (Figure 1Figure 1), which is situated in the center of the mantle cavity, has special superficial epithelia that function in the harvesting of the symbiont. This epithelium secretes mucus in which the symbionts are concentrated. Once they have gathered for a while in this mucus matrix, the symbiont cells migrate into pores on the surface of the light organ, down ducts, and into blind-ended crypts, where they take up residence and multiply to fill the crypt spaces. From about eight hours after hatching and initial exposure to the symbionts, the host will have a population of the bacterial partner in its light organ for the rest of its life.Is access to the crypts exclusive to V. fischeri? Only V. fischeri is capable of populating the light organ of E. scolopes. The few hundred cells per milliliter of V. fischeri in the seawater surrounding the juvenile host occur within a mixed community of thousands of bacterial types that total about one million cells per milliliter of seawater. The selection for the specific partnering bacteria occurs early: when the mucus is first shed from the superficial epithelium, many other species of bacteria adhere to it, but within a couple of hours, V. fischeri cells occur as the dominant cell type. In the absence of V. fischeri — in seawater where V. fischeri populations do not occur — other bacteria will aggregate in host mucus. But only V. fischeri can take the next step — only the symbiont can enter the pores, negotiate the ducts and take up residence in the crypts.How do the partners influence one another's development? Full colonization of the crypts by the symbionts sends a developmental signal to the host that results in the regression of the superficial epithelium that facilitated the initial colonization process. This regression involves programmed cell death of these epithelial cells and their eventual sloughing. One of the remarkable features of this process is that the bacteria direct these events from the crypt spaces, several cell layers away from the affected tissues. The bacteria also cause changes in the crypt epithelial cells with which they directly interact; most notably, the microvilli of these cells increase in density so that host membrane eventually surrounds the surface of the bacterial cells. The bacteria also undergo development: once they achieve high densities, they become luminous and lose their flagella.How does the symbiosis remain stable? The bacteria continue to grow after they fill the crypt spaces. Thus, mechanisms must exist to control their populations. Studies of the association have revealed that the symbiosis has a profound diel rhythm. Each day at dawn, as it begins its quiescent period in the sand, the animal vents 70–90% of the symbiont population into the surrounding seawater. The bacteria that remain behind in the crypts grow to fill the organ, so that, by dusk, the light organ again has a full complement of symbionts. Associated with these diel changes in symbiont population are changes in the crypt epithelium. Over much of the day, the host crypt cells are highly polarized, with dense microvilli. In the hours before dawn, however, the microvilli and apical surfaces of the cells are blebbed into the crypt spaces. Analyses of the metabolism of the bacteria have suggested that the culture that grows up after the dawn venting uses these host membrane fractions as a nutrient source.What are the signals that mediate all of these various processes? Many of the above-mentioned effects of the symbionts on host tissues appear to be the result of signaling by molecules of the symbiont cell surfaces, particularly constituents of their outer membrane and cell wall. What is interesting about these findings is that these molecules have elsewhere been associated almost exclusively with mediating bacterial pathogenesis, as toxins that induce inflammatory responses. Although biologists have historically focused on bacteria as pathogens, they are now becoming aware that mutualistic animal–bacterial symbioses are more common than we could have imagined. As all bacteria have these surface molecules, perhaps they are the language of animal-bacteria interactions, and whether the association results in health or disease is a matter of when and where they are presented.