chapter  6
14 Pages

Is the Vibrio fi scheri–Euprymna scolopes Symbiosis a Defensive Mutualism?

ByEric V. Stabb, Deborah S. Millikan

Experimental Studies of Early Infection ............................................................................ 86 6.2.2 Ecology and Behavior of V. fi scheri ................................................................................... 87 6.2.3 Ecology and Behavior of E. scolopes ................................................................................. 88 6.2.4 Evidence of Mutualism in this Symbiosis .......................................................................... 88 6.3 Counterillumination Model ............................................................................................................ 89 6.4 Weighing the Evidence for Counterillumination and Other Models .............................................. 90 6.4.1 Does Light-Organ Anatomy Suggest Counterillumination? .............................................. 90 6.4.2 Does Host Behavior Fit the Counterillumination Hypothesis? .......................................... 91 6.4.3 How Conclusive Are Attempts to Measure Counterillumination

in E. scolopes? .................................................................................................................... 92 6.4.4 Counterillumination Is Not Necessarily a “Defensive” Camou age ................................. 93 6.4.5 Alternatives (or Amendments) to the Counterillumination

Camou age Hypothesis ...................................................................................................... 93 6.5 Future Directions ............................................................................................................................ 94 6.5.1 The Life and Times of E. scolopes ..................................................................................... 94 6.5.2 What about the Flashes? ..................................................................................................... 94 6.5.3 More Tests of Counterillumination ..................................................................................... 95 Acknowledgments .................................................................................................................................... 95 References ................................................................................................................................................ 96

The light-organ symbiosis between the bioluminescent bacterium Vibrio fi scheri and the Hawaiian bobtail squid Euprymna scolopes is a fascinating association rife with intriguing biology. This sepiolid squid is a small nocturnal inhabitant of shallow sandy reefs in the Hawaiian archipelago that allows V. fi scheri, and only this bacterium, to colonize epithelium-lined crypts of a specialized light-emitting organ. This “light organ” is located just ventral to the squid’s ink sac in the mantle cavity. Once they have colonized the light organ, the bacterial symbionts emit a bluish light, and adaptations by the host allow it to direct and modulate the emitted light. Below, we will discuss how the light organ’s architecture suggests that it functions in the camou aging behavior referred to as counterillumination (Figure 6.1), wherein marine animals emit light downward, roughly matching the downwelling light from above to obscure their silhouette from animals beneath them in the water column (Clarke, 1963; Dahlgren, 1916; Harper and Case, 1999; Latz, 1996; McFall-Ngai and Morin, 1991; Warner et al., 1979).