Light organ development was studied in the sepiolid squid Sepiola robusta Naef and compared to that of Euprymna scolopes Berry. Both species form obligate mutualisms with luminous bacteria from the environment. The embryonic period of S. robusta, which is at least
double to that of E. scolopes at similar temperatures, produced additional features not reported to occur in the E. scolopes light organ, including an extended ciliated ridge at the base of the light organ, as well as additional crypt spaces to house symbiotic bacteria. Accessory
structures, which are used to modify the bacteria-produced light, are not under the induction of symbiotic bacteria and appear in S. robusta before hatching, whereas in E. scolopes these tissues form post-hatching. At hatching the light organs of both species respond to symbiotically
competent bacteria and undergo similar developmental remodeling. Specifically, the ciliated epithelial fields on the surface of the light organ undergo a program of cell death and regression that spans 4 to 5 d in E. scolopes, and over 9 d in S. robusta. Although the timing of
embryogenesis differs between these two species, the results of these studies demonstrate that the influence of Vibrio fischeri, the specific symbiont, appears to be restricted to the initial stages of post-hatching development of both light organs.
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