Sure enough, it is too late to edit my comment but the Wikipedia page says: > A. fischeri have been most studied as symbionts of marine animals, including squi

Sure enough, it is too late to edit my comment but the Wikipedia page says: > A. fischeri have been most studied as symbionts of marine animals, including squids in the genus Euprymna and Sepiola, where A. fischeri can be found in the squids' light organs. Clicking through the genus Euprymna and Sepiola links, it looks like these are all Bobtail-Squid-like. I shouldn't have used the word "only" but that wasn't the source of my confusion. My issue was the word-picture of a school of reef squid, found in the upper water column, using bioluminescence to overcome moonlight shadows. It didn't make sense. Bobtail Squid, on the other hand, are tiny bottom feeding animals. They are not what I think of when you say "squid" without qualification. However, the images of Bobtail Squid on Wikipedia do not make their half-your-thumb size obvious and the silhouette image [1] makes it look like an upper water column phenomena. When I first realized we were talking about Bobtail Squid, I thought of the blue lights some people put under their cars. [1] https://en.wikipedia.org/wiki/Bobtail_squid#/media/File:Bobtail_squid_light_organ_silhouette.svg https://en.wikipedia.org/wiki/Bobtail_squid#/media/File:Bobt...