Velella velella (Linnaeus, 1758) 

Common name(s) By-the-wind sailor
Synonyms:  Velella lata
Top view of Velella velella.  Note blue disk and clear, angled chitinous float.
Phylum Cnidaria
 Class Hydrozoa
  Order Chondrophora (previously Siphonophora)
   Family Velellidae

Underside of Velella velella, showing short tentacles.
Photos by: Dave Cowles at San Simeon, CA
Description:  A small blue float made of concentric circles of gas-filled tubes.  Up to 8 cm in diameter (usually 4 to 6), with a clear chitinous semicircular sail above and small tentacles below.  The sail is angled left or right from the long axis of the float.  Floats far offshore, but may be blown onshore in large numbers by some winds, especially in spring and early summer.  Found worldwide in the temperate and tropical oceanic zone.

How to Distinguish from Similar Species: There are no similar species.  The attached colonial hydrozoan Tubularia has a similarly structured (though much smaller) polyp but would not be mistaken for this species.

Geographical Range:  The species is pelagic and usually offshore, though thousands may be blown ashore by strong onshore winds (especially during El Nino), mostly during late spring and early summer.  It occurs worldwide in temperate and tropical seas.

Depth Range:  Float on surface (pleuston).

Habitat:  Worldwide in temperate and tropical seas.  Oceanic

Biology/Natural History:  This species is a puzzling one.  It has long been regarded by many as a type of siphonophore; a pelagic colony of hydrozoan polyps similar to Physalia, the Portuguese man-of-war.  Recent study suggests that, instead, it is a single very large hydrozoan polyp (Order Chondrophora), floating mouth downward and with a chitinous float and sail instead of a column.  If so, it is an extremely large polyp for a hydrozoan.  At any rate, the underside also includes many small polyps that bud off small medusae.  The medusae (up to 1.5 mm tall) sink to as far as 2000m depth and produce gametes.  The developing embryos develop floats and rise back to the surface.  This species is oceanic, being usually found far offshore.  The angled sail makes it sail at 45 degrees from the prevailing wind.  Some have a sail angled to the left, others to the right.   Off California the right-angled form prevails, and these remain offshore in the prevailing northerly winds.  Strong southerly or westerly winds, however, may bring huge aggregations ashore.  Velella have symbiotic algae (zooxanthellae) in their tissues, and also feed on zooplankton.  They are eaten by pelagic gastropods such as some nudibranchs and bubble-rafting snails.  The pelagic gooseneck barnacle Lepas anatifera occasionally attaches to the dead chitinous floats.  This species has many nematocysts and a few people have reported feeling a sting, but I have handled many and have never been stung even slightly.  The species feeds on fish eggs and crustacean larvae.


 
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References:

Dichotomous keys:
  Kozloff, 1987, 1996
  Smith and Carlton, 1975

General References:
  Barnes, 1980
  Brusca and Brusca, 1990
  Kozloff, 1993
  Morris et al., 1980
 
 

Scientific Articles:
Francis, Lisbeth, 1985.  Design of a small cantilevered sheet: the sail of Velella velella.  Pacific Science 39: 1 pp. 1-15

Francis, Lisbeth, 1991.  Sailing downwind: Aerodynamic performance of the Velella sail.  Journal of Experimental Biology 158: 117-132


General Notes and Observations:  Locations, abundances, unusual behaviors, etc.:
 


Authors and Editors of Page:
Dave Cowles (2004):  Created original page