Anyone searching for techniques to become invisible should study this heteropod.  The transparent body is very difficult to see, revealed only by the opaque gut and pair of eyes.  The body is elongate and cylindrical in cross section, with a length up to 30 cm but usually far less.  A significant portion of the length is made up of an elongate filament extending from the tail.  A shell is totally lacking, unlike the more commonly encountered Carinaria.  The compact, narrow visceral mass (situated posterior to the ventral swimming fin) and both cylindrical eyes are covered externally by a silvery reflective layer. At least 4 species are found in the eastern Pacific, with P. scutata and P. coronata being the only ones likely to be found in California waters.  A gelatinous structure called a "bib" extends from the ventral swimming fin to the mouth - it is significantly wider on P. scutata.  A sculling motion of the fin, which is typically held in an upward position, propels the heteropod.  When disturbed, rapid undulation of the body enables a quick downward escape response.  Pterotrachea has a habit of remaining curled up and motionless when not pursuing prey.  It is often seen in the presence of swarms of siphonophores, which may be the preferred prey.  Copepods and polychaetes are also eaten.  Pterotrachea favors warm oceanic waters, reaching nearshore central California only during periods of surface warming in fall months.

All photographs © David Wrobel and may not be used or copied without permission!