Euprymna berryi is a species of bobtail squid, which are part of a group of cephalopods closely related to cuttlefish. Bobtail squid tend to have a rounder mantle than cuttlefish and have no cuttlebone. They have eight suckered arms and two tentacles and are generally quite small.
Bobtail squid have a symbiotic relationship with bioluminescent bacteria (Vibrio fischeri), which inhabit a special light organ in the squid’s mantle. The bacteria are fed a sugar and amino acid solution by the squid and in return hide the squid’s silhouette when viewed from below by matching the amount of light hitting the top of the mantle.