Researchers mapped their observations of hyperiid amphipods against depth, association behavior (associating in orange, free-swimming in purple), and eye structure (complex in blue, simple in yellow, and reduced in green). Illustration courtesy of Vanessa Stenvers

While water depth is an important factor in driving animal adaptations in the deep sea, scientists increasingly recognize that behavioral ecology may be just as important. Some hyperiid amphipods are symbiotic hitchhikers on gelatinous animals, while others have a free-swimming, predatory lifestyle. Combining a literature review, blackwater scuba photography, and submersible observations, the team has demonstrated that association behavior is linked to both eye diversity and camouflage.

Approximately one-third of hyperiid genera have adopted a free-swimming lifestyle, challenging the longstanding assumption that these amphipods only live on or in close association with gelatinous animals. The evolution of a free-swimming lifestyle correlates with body transparency and eye structure in hyperiids living in the upper waters of the ocean’s twilight, or mesopelagic, zone.