Adaptations for living deep: a new bathypelagic doliolid, from the eastern North Pacific

Abstract

Adapting to the bathypelagic habitat imposes serious challenges for taxa that originate in shallower depths. We describe a new doliolid that has successfully made the transition into deep water. Nine specimens of Pseudusa bostigrinus have been found at depths between 1164 and 1890 m in three distinct regions of the eastern North Pacific. This new thaliacean has exchanged the typical doliolid body plan for one resembling a craspedote hydromedusa. This adaptation allows it to collect sinking particles by simply directing its large buccal opening upward. The development of a hydromedusa-like velum allows it to trap zooplankton prey and to propel itself with considerable force and control. While carnivory is not unknown in tunicates, this is the first report of a carnivorous doliolid. The endostyle of P. bostigrinus is greatly reduced, there are no ciliated bands, and there is no spiral gland; all evidence that mucus feeding filters have been abandoned by this species. Anatomy, diet, behaviour, and habitat distinguish this doliolid from all others described to date.