I grew up by the sea and always enjoyed walking on local beaches
looking for shells washed up by the tide. There were many shell fragments, especially
of cockles that must have grown in their millions just offshore, but also a
wide variety of whole shells, especially those of snails. I continued to enjoy
beachcombing and, while on a family holiday in Jersey, found an excellent
beach, dominated by shells of the flat periwinkles Littorina obtusata and L.
mariae. [1] Some were bright yellow, others orange, white, or of a reddish hue.
We collected as many of the shells as we could and, more than twenty years
later, they are still exhibited in a glass jar in our bathroom.
Occasionally, violet shells from the snail Janthina are washed up on beaches, especially
after long periods of strong winds. They are similar in form to those I had collected in Jersey, but they have less strengthening than the Littorina shells, which need to be
strong to withstand the erosive action of the water, and suspended mineral
particles, over the shores on which the snails live. Can we assume, therefore, that
Janthina exists in a less erosive
habitat? Indeed it does - the snails live at the surface of oceans, attached to a
float of bubbles.
In July and August 1954, there were sustained, strong
westerly winds in Great Britain and Ireland and several people reported finding
large numbers of Janthina shells on exposed beaches This prompted Dr Douglas P. Wilson of the Marine Biological
Association to write a letter to The
Times to ask readers for more information about sightings. Reports came
in from many locations and, among all the shells, there were a few living specimens.
These offered the scope for investigations of the formation of the float,
adding to the information acquired by earlier investigators.
Among the first to make observations was Reynell Coates, who,
in 1825, published a description of the float. Coates qualified as a medical
doctor in Philadelphia and then set sail as a ship’s surgeon on a voyage to the
East Indies. He was very interested in Natural History and took samples of
organisms from the surrounding water (presumably when becalmed, or unable to
continue the passage - the journey terminated at Kolkata after the outbreak of
the Burmese War [2]). This is what he wrote about some specimens of Janthina:
Individuals being placed in a
tumbler of brine, and a portion of the float being removed by the scissors, the
animal very soon commenced supplying the deficiency in the following manner:
the foot was advanced upon the remaining vesicles, until about two-thirds of
the member rose above the surface of the water; it was then expanded to the
uttermost, and thrown back upon the water.. ..it was contracted at the edges,
and formed into the shape of a hood, enclosing a globule of air, which was
slowly applied to the extremity of the float. A vibratory movement could now be
perceived throughout the foot, and when it was again thrown back to renew the
process, the globule was found in its newly constructed envelope. [3]
Wilson’s description
of float formation in Janthina included
these observations:
Sometimes the new bubble fails to
be attached and floats away as a tiny glassy sphere.. ..The completed float is firm
between the fingers, springy and dry - it is not in any sense sticky. [4]
Although the bubbles are surrounded by mucus secreted from
glands in the snail’s foot, there are clearly components of this secretion
(proteins?) that, after dehydration of the mucus mass, form a solid coating for
the trapped bubbles. This ensures that the float is near-permanent, although
older pieces break off and need to be replaced. [4] To say that the float is
important for the snail is an understatement as, specimens of Janthina that sink into the water column
cannot regain the surface. [4]
Janthina is a
predator and feeds on other members of the floating community at the air-water
interface. It is especially associated with Velella
(the “by-the-wind sailor”) [3], and both are blown ashore in masses. Velella is a colonial relative of jellyfish,
with polyps attached to a secreted, flattened float that has a sail rising from
it (see below), allowing the colony to be transported by wind. The polyps use
stinging cells to capture small creatures from within the water column, such as
invertebrates and small fish. Wilson quotes Mr Peter David in describing the
manner in which Velella is preyed
upon by Janthina, the snail cutting out
semicircular pieces of the float and attached polyps “in much the same way as a
caterpillar does on the edge of a leaf.” [4]
Gastropod snails, like Littorina
and Janthina, have very similar body
plans and it is relatively easy to see how the latter evolved from a shore-dwelling
ancestor, but how did it develop an ocean-going existence? Observation of pond snails shows that some individuals move across
the underside of the water surface while the foot is held in the surface film and, to these
snails, the interface is like a solid surface. [5] They move here as they do
over the substratum, using muscular contractions of the foot, and feed as they
go. A characteristic of pond snails is their relatively thin shell, as strengthening
calcium salts are not as available in fresh waters as they are in the sea. It
is likely that the ancestral Janthina
did not strengthen the shell in the way that Littorina does and that it moved under surface films as well as
over the substratum, just like some pond snails. In time, the mucus used for
lubrication and attachment during crawling was used to coat bubbles formed by
the foot and this resulted in the development of the float. They then lost the
power of locomotion, or it was highly reduced, and their feeding changed from
scraping materials from surfaces to the removal of sections of prey such as Velella, into which they had drifted, or
which had been blown towards them.
How Velella
evolved its current form is a mystery. There are various theories on how
gastropod molluscs evolved from their ancestral molluscan form, but how did the colony of polyps develop, complete with a float to which
they were attached? Velella has many sedentary colonial relatives
(members of the Cnidaria), but how did life at the ocean surface
begin - and what were the origins of the sail?
[1] Gray A. Williams (1990) The comparative ecology of the
flat periwinkles, Littorina obtusata
(L.) and L. mariae Sacchi et
Rastelli. Field Studies 7: 469-482.
[2] W.J.Snape (1968) Reynell Coates (1802-1886): politician,
poet, editor, naturalist, lecturer and physician. Transactions and Studies of the College of Physicians of Philadelphia
35: 112-118.
[3] Reynell Coates (1825) Remarks on the floating apparatus,
and other peculiarities, of the genus JANTHINA. Journal of the Academy of Natural Sciences of Philadelphia 4:
356-360.
[4] Douglas P. Wilson and M. Alison Wilson (1956) A
contribution to the biology of Ianthina janthina
(L.). Journal of the Marine Biological
Association of the United Kingdom 35: 291-305.
[5] Roger S. Wotton and Terence M. Preston (2005) Surface
films: areas of water bodies that are often overlooked. BioScience 55: 137-145.
