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Velella Velella (Linnaeus, 1758)


Jessica Nelms 2015

Summary
Velella velella are free-floating hydroid polyps that live on the surface of the open ocean. They are the only known species in the genus. V. velella are deep blue in colour and consist of a flat oval transparent float with a thin, erect sail set diagonally across the upper surface (Fig. 1). The distinctive sail of V. velella gives the species its scientific name, based on the Latin word velum, which translates to sail. It has also led to the species common name ‘by-the-wind sailor’. The species is known to be dimorphic, where the sails of individuals can be angled to the left or to the right. The sail orientation determines which way the V. velella will travel according to the wind. This feature was examined in this project through a field experiment. V. velella have a worldwide ocean distribution and are commonly associated with other pelagic drifters including the genera Porpita and Physalia. This has led to incorrect classification in the past. Typical of a hydrozoan, the life cycle of V. velella is polyp-medusa-egg-planula-polyp. V. velella are thought of as important pleutonic predators feeding on small surface prey and fish eggs. They also possess symbiotic zooanthellae that provide nutrients to the host. V. velella are also an important food source for pelagic snails, fish, crabs, as well as birds and sea turtles. V. velella contribute a significant amount of organic matter to coasts during mass strandings and are known to play a role in the storage and transfer of trace metals in the ocean.  V. velella are do not have a special conservation status and are not considered a threat to humans.
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Figure 1

Physical Description
Unlike other hydroid polyps, which settle on the ocean floor and become sessile, V. velella are described to have flipped upside down and instead settled on the surface of the ocean by growing a float (Fig. 2, Field & Mackie, 1970; Larson, 1980). Individuals can be simply characterised by their deep blue to purple colour, flat oval float 40-60mm across in size with short fringing tentacles underneath and erect, thin, transparent sail (Fig 3; Fig. 4; Ricketts et al. 1985; Russell-Hunter, 1979).

The species’ diagnostic description, proposed by Richmond (1997) is: “Resembles Porpita but with triangular sail on upper surface, to 4 cm. Colour blue. Habitat: floating on sea surface. Distribution: Pantropical.”
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Figure 2
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Figure 3
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Figure 4

Ecology
V. velella spend their adult lives floating at the air-water interface of northern-hemisphere and southern-hemisphere oceans feeding on crustaceans (mainly copepods), larval fish, and the eggs of various invertebrates caught immediately below the surface using its short peripheral tentacles armed with nematocysts (Corbera & Montferrer, 2014; Kirkpatrick & Pugh, 1984; Purcell, 1985; Purcell et al. 2012). They are considered important predators in the pleustonic community.

In addition to active predation, V. velella also possess symbiotic zooxanthellae within the tissues of both their hydranth and chrysomitra, which provides the host with an additional source of nutrition (Gast & Caron, 2001; Purcell et al. 2012).

V. velella are an important food source or pelagic snails, fish, crabs, as well as birds and sea turtles (Bieri, 1961; Emmett & Krutzkowsky, 2008; Purcell et al. 2007; Revelles et al. 2005; Wickham, 1979). They are the main prey item of the pelagic snail Ianthina, which secrete a bubble raft to keep themselves afloat until they encounter a V. velella, where it attaches itself to the underside of the organism and browses on its appendages (Fig 5; Kirkpatrick & Pugh, 1984; Wilson et al. 1956). V. velella have been found in the stomach contents of Pacific hake and jack mackerel (Emmett & Krutzkowsky, 2008) and also sighted being preyed on by Mola Mola (see Figure 6). They have also been recorded in the diets of green, hawksbill and loggerhead turtles (Purcell et al. 2007; Revelles et al. 2005).

Mass strandings of V. velella are known to provide brief but substantial concentration of organic matter in the beach and nearshore environments, benefitting macrofauna and meiofauna (Fig. 7; Kemp, 1986). They are also known to play a role in the storage and transfer of trace metals in the ocean, which marine organisms rely on for growth (Romeo et al. 1992).
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Figure 5
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Figure 6
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Figure 7

Life History and Behaviour
As a single hydroid, V. velella has a bipartite life cycle, where reproduction consists of an alternate generation between polyp and medusa stages. The life cycle of V. velella is polyp-medusa-egg-planula-polyp (Bayer & Harding 1968; Ricketts et al.1997). The gonozooids of the polyp produce 1mm high and wide medusae (Fig. 8), which break away from the hydroid and reproduce sexually. Thousands of medusa can be produced across several weeks. The medusae take three weeks to reach sexual maturity, where they then free-spawn eggs and sperm, giving rise to planula larva (Brinckmann-Voss, 1970).

During its extended development it is assumed that the V. velella medusa sink through the water column to depths of 600-1000m (Kirkpatrick & Pugh, 1984; Larson, 1980). Thus, after external fertilisation of the eggs, the early developmental stages are thought to take place at this depth, which aids in the dispersion of the species (Kirkpatrick & Pugh, 1984; Savilov, 1969). The first larva identified is conaria, which have a pair of shore, aboral tentacles, rudimentary float and conspicuous crimson cone. The cone secretes oil droplets, making the larva positively buoyant, allowing it to ascend to the sea surface. It is there, the conaria transforms into a rataria larva. The hollow, adult tentacles appear with a cruciform symmetry and the float becomes enlarged and the sail forms. From there, the hydranth gradually increases in size (Kirkpatrick & Pugh, 1984; Kirkpatrick & Pugh, 1984; Larson, 1980).
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Figure 8

Anatomy and Physiology

Detailed Anatomy Description
A more detailed description of V. velella (Fig. 9; Rupert et al. 2004) describes the species as a zooxanthellate, intensely blue, raftlike, highly modified, Tubularia-like gastrozooid (axial polyp) bearing gonozooids. The body of an individual consists of an upper float, middle central mass (coenosarc), and undersurface with zooids suspended mouth-down. It has a discoid float of concentric, chiton-lined, air-filled chambers for buoyancy, which develops as the pedal inpocket of the axial polyp. The float opens to air at small pores on the upper-surface and blind-ending tracheae extend from air chambers into the central mass. The undersurface has a large central tentacle-free gastrozooid, marginal tentacles, and gonozooids bearing medusa buds between the gastrozooid and tentacles. The zooids are interjoined by solenia in the central mass and periphery of the float and the medusa buds release thimble-shaped, free-swimming, zooxanthellate medusae with manubrial gonads and two capitate tentacles (Rupert et al. 2004, Russell, F.S. 1939). V. velella, as shown in anatomical drawing in Figure 5, have a parallelogram body shape with triangular sail at an angle to long axis of float. This sail can either be angled to the right or left.
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Figure 9

Asymmetry Field Experiment
Introduction
Velella velella have been long known to be dimorphic (Bieri, 1959; Francis, 1991; Zenkevich, 1970). The species can occur in two forms with opposite, mirrored asymmetries in relation to their sail orientation. The ‘left-handed’ form has a sail going from upper-left to lower-right along its long body axis, whilst the ‘right-handed’ form has a sail going from upper-right to lower-left (Bieri, 1959). The two forms are thought to exist, so they are not all blown ashore together. This dimorphism has led to speculation that there is a difference in preferred sailing direction in the northern and southern hemispheres, or on the eastern and western shores of oceans. According to records, the left-handed forms occur predominantly in the southern hemisphere (Edwards, 1966). To test this theory, the following experiment examined the sail orientation of a number of V. velella specimens collected from a Sunshine Coast beach (see Appendix 1, Figure 7).

Materials and methods
Along the open beaches of the Sunshine Coast, Australia, prevailing southeasterly winds bring fresh specimens to shore fairly predictably. On 18th May, 2015, 18 freshly stranded specimens were collected at Castaways Beach, placed together in a small container with fresh seawater and taken to the laboratory for analysis. Testing was done as soon as possible, within 6 hour after the animals had been collected as the animals are known to deteriorate quite quickly, even when collected from the sea rather then from the beach (Francis, 1991). Individuals were removed from the container and placed on a transparent plastic sheet protector. To determine the sail orientation, as well as give an indication of individual size, the sheet protector contained a sheet of 2mm graph paper inside of it. Each individual’s sail was then examined to determine whether it was for left-handed or right-handed.

Results & Discussion
All 18 individuals examined were left-handed, with the sail going from upper-left to lower-right of the long body axis. This is shown in Figure 10. These results are concurrent with previous studies that have looked at the distribution of V. velella with special reference to the occurrence of the two oppositely sailing isomorphs (Bieri, 1959; Edwards, 1966; Savilov, 1961). This study further supports the theory that the left-handed V. velella form occurs predominantly in the Southern Hemisphere.
 

Appendix 1: Velella vellela Occurrence Record
Dataset
- Collection location: Castaways Beach, Queensland (Distance: 1.8km).
- Collection time: 11:30-1:30PM (mid-tide, outgoing), MON 18th May,2015.
- Individual count: 18

Geospatial
Figure 11 illustrates the geospatial record of the Velella velella occurrence. The image was retrieved from Google Earth, 2015.

Additional Properties
- Nearest BOM weather station: Tewantin Station
- BOM Monday Weather Conditions for Sunshine Coast area: Partly cloudy. High (70%) chance of showers. Winds south to southeasterly 15 to 20 km/h becoming light in the late evening.
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Figure 10
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Figure 11

Evolution and Systematics
V. velella are a species member of the Porpitidae family of the Hydrozoa, which contains three genera of hydroids, including Velella, Porpita and Porpema. The classification of the species Velella velella according to the Integrated Taxonomic Information System (ITIS, 2015) can be viewed in Table 1. According to the World Register of Marine Species (WoRMS, 2015), synonyms for the species include Velella lata (Chamisso & Eysenhardt, 1821) and its common names include By-the-wind sailor, Jack sail-by-the-wind, little sail, purple sail, sea raft, Velella.

The systematic position of this species has long been a topic of discussion among taxonomists. Older zoological opinions classified the V. velella as a smaller species member of the order Siphonophore, along with the Portuguese man-of-war and other colonial creatures. However, recent taxonomist now classify the species as a Chondrophore and propose V. velella is a highly modified individual hydroid polyp, and not a colonial hydrozoa (Russell-Hunter, 1979; Ricketts et al. 1997).

Table 1: Velella velella Taxonomic Hierachy (ITIS, 2015)

Kingdom                       

Animalia  – Animal, animaux, animals

   Subkingdom

Radiata

      Phylum

Cnidaria Hatschek, 1888 – cnidarians, coelenterates, cnidaires, coelentérés, água viva, anêmona, caravela, cnidario, coral, hidra

         Subphylum

Medusozoa 

            Class

Hydrozoa Owen, 1843 – hydralike animals, hydroids, hydrozoans, hydraires, hydrozoaires, água viva, hidra, hidrozoa, hidrozoário, pólipo

               Subclass

Hydroidolina

                  Order

Anthoathecatae – athecate hydroids, hydromedusae

                     Suborder

Capitata Kühn, 1913

                        Family

Porpitidae Goldfuss, 1818

                           Genus

Velella Lamarck, 1801

                              Species

Velella velella (Linnaeus, 1758) – by-the-wind sailor, by-the-wind sailor
 

Biogeographic Distribution
V. velella are pelagic pleustons that live at the air-water interface of the oceans surface, with their float above the water line, and polyps handing down in the water column. Their distribution is enabled by their erect sail, which catches the wind and propels them along the ocean surface. V. velella have a worldwide ocean distribution (Fig. 12) in temperate and tropical seas, more commonly found in warmer waters (Kirkpatrick & Pugh, 1984; Ricketts et al. 1997). V. velella normally occur offshore, however, strong onshore winds can result in mass strandings (Kemp, 1986; McGrath, 1985; McGrath, 1992; Turk, 1982). Figure 13 shows recorded Velella Velella occurrences in Australia.
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Figure 12
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Figure 13

Conservation and Threats
V. velella currently do not have a special conservation status. Protection of the species is not a priority, as they are endemic to virtually all the seas and oceans of the world.

In terms of threats to humans, the stinging nematocysts of V. velella are essentially harmless to humans. According to experts, they might cause mild skin irritation if handled carelessly (Fig. 14; Lee, 2015).

It has been suggested massive blooms of V. velella reported from time to time may have some influence on atmosphere chemistry and changes in cloud composition, with a possible negative feedback mechanism to global warming (Sibley, 2007).
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Figure 14

References

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