Evolution of ammoniacal buoyancy within oegopsid squids and its relationship with depth

ABSTRACT Ammonium sequestration is a method of achieving neutral buoyancy in the squid clade Oegopsida. Oegopsids are oceanic, with many deep-sea species, so maintaining neutral buoyancy is crucial for energy conservation. Previous studies on ammoniacal buoyancy in squids have suggested a single origin of this trait, but others have refuted this claim; however, no study has utilized molecular data to trace the ancestral states of this character. This study employs four genes from 165 oegopsids and 5 outgroups to build a maximum likelihood tree with divergence times. Ancestral state reconstruction of this buoyancy mechanism was conducted on the finished tree. In order to test for correlation between buoyancy and depth, we also conducted ancestral state reconstruction of the depth range of the species included. We found that ammoniacal buoyancy has arisen nine times across Oegopsida, with an unknown ion in Bathyteuthis spp. having a single, separate origin. Depth distribution is significantly correlated with buoyancy mechanism.