Tropical butterflies flying over the canyon

 

This remarkable animal is a sea butterfly, Hyalocylis striata.  Kate and Emily discovered it when sorting the contents of a deep-water plankton tow from >200 meters depth during an ACCESS (Applied California Current Ecosystem Studies) cruise over Bodega Canyon on 24 September 2014.

A sea butterfly is a type of pelagic snail called a pteropod.  It spends its entire life at sea.  Note several special adaptations, highlighted in the photo above and the diagram below.

• The foot has been modified into two, large wings for swimming.  The wings flap together, and slowly, hence the name sea butterfly.  

• The shell is transparent, and very thin and fragile, traits associated with an open ocean existence.

 

Modified from Invertebrates (Second edition) by Brusca and Brusca (2003)

Although other pteropods have been documented in this area, Hyalocylis striata is unusual.  Typically it is a warm-water species; most records appear to be tropical and subtropical.  We're actually trying to figure out if this might be the first definite record for California.  

I've done a very basic literature search, as well as a search of online museum collections.  So far I can't find any records along the West Coast north of Baja California, Mexico.  In 1967, McGowan published a Distributional Atlas of Pelagic Molluscs in the California Current Region.  The map below is the only cruise for which he reported finding Hyalocylis striata.  Each dot on the map is a sampling station.  Can you locate the station where Hyalocylis was found?

 

Modified from McGowan, J.A. 1967. Distributional Atlas of Pelagic Molluscs in the California Current Region. CALCOFI Atlas 6: 1-218.  Hyalocylis illustration from Tesch, J.J. 1913. Mollusca, Pteropoda. In: F.E. Schulze. Das Tierreich. Eine Zusammenstellung und Kennzeichnung der rezenten Tierformen 36. Berlin (Friedberger and Sohn).

I'm guessing you found the one station off the west coast of Baja where Hyalocylis was discovered, just south of 25°N latitude. 

This atlas, along with several other sources, makes it seem like Hyalocylis is generally not expected to be at 38°N, the latitude of Bodega Bay.

It may be that Hyalocylis is another species brought north with the warmer-than-usual water this summer.  For example, warmer ocean temperatures near shore during the mid-late summer are usually in the range of 13-15°C (55-58°F); this summer they have been around 16-18°C (60-64°F).  When I was near Bodega Canyon/Cordell Bank on 19 September, it was 19°C (66°F)!

Although these sea butterflies are small (up to 8 mm long), it's fun to imagine them flying over Bodega Canyon!

P.S.  Many thanks to Roger Seapy for helpful discussions about Hyalocylis striata.

P.P.S.  If you want to see the location of Bodega Canyon, click here.

P.P.P.S.  The Applied California Current Ecosystem Studies (ACCESS) Partnership is an ongoing collaboration between Point Blue Conservation Science, Cordell Bank and Gulf of the Farallones National Marine Sanctuaries, to support marine wildlife conservation and healthy ecosystems in northern and central California.  For more information about ACCESS, visit www.accessoceans.org, and for new collaboration opportunities contact Jaime Jahncke at jjahncke@pointblue.org.