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

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.  The 'bluebottle' found for example in Australia is a similar color of blue but has an inflated float like a Portuguese man-of-war.

Geographical Range:  The species is pelagic and usually offshore (oceanic), 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 2000 m 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. 

Jones et al. (2021), see also the Washington Sea Grant article, report that Velella velella experiences mass strandings all along the US Pacific coast when winds switch to onshore in the spring, especially after warm winters.

References:

General References:
  Barnes, 1980
  Brusca and Brusca, 1990
  Kozloff, 1993
  Morris et al., 1980
 
 Washington Sea Grant news article 2021

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 

Jones, Timothy, Jilia K. Parrish, and Hillary K. Burgess, 2021. Long-term patterns of mass stranding of the colonial cnidarian Velella vella: Influence of environmental forcing. Marine Ecology Progress Series 662 pp. 69-83. DOI: https://doi.org/10.3354/meps13644

In the summer of 2014 a huge number of by-the-wind sailors were washed up all up and down the Oregon and Washington open coast.  Here is a collection of several at the high tide wrack line at Rialto Beach.
Photo by Dave Cowles, August 2014

This is one of a small flotilla washed ashore near Cape Flattery in 2015.  Photo by Dave Cowles, July 2015

Since this is an oceanic species one would generally expect to find it only on exposed beaches of the open coast, if ashore at all. However, in spring 2018 a flotilla of thousands of individuals of this species arrived at the westward-facing beach of Rosario Bay, about 100 miles up the Strait of Juan de Fuca from the open ocean. Nearly all the individuals were faded or clear, indicating that they had died before reaching the beach. The photos below were taken by Kirt Onthank at the beach of Rosario Beach Marine Laboratory. The first individuals were seen on April 4, 2018 and the peak washup was on April 17.

Velella velella drifted onto the beach at Rosario Beach Marine Laboratory, April 2018. Photo by Kirt Onthank

Velella velella drifted onto the beach at Rosario Beach Marine Laboratory, April 2018. Photo by Kirt Onthank

A child holding Velella velella drifted onto the beach at Rosario Beach Marine Laboratory, April 2018. Photo by Kirt Onthank