Part of a space rocket has been recovered from the Atlantic
Ocean near the Scilly Isles. It was from a launch vehicle carrying cargo to the
International Space Station and large chunks fell to Earth after the rocket
exploded [1]. The pieces that were recovered had coverings of goose
barnacles (above and also below left) and those with a very fanciful imagination may conclude
that these were carried back to Earth from somewhere in the planet's atmosphere,
or beyond. However, the story of their colonisation is not the stuff of science
fiction, but nonetheless fascinating.
We are all familiar with acorn barnacles (above right) from
our walks along coasts. Millions of the crustaceans coat the surface of rocks,
often giving them a creamy-white appearance, but fewer people have seen their
relatives, the stalked goose barnacles, like those found on the pieces of space
rocket. They can be seen on many shores during low spring tides and they are familiar to
those who own boats and ships, where their presence causes a reduction in performance, so that hulls need to be cleaned of barnacles
from time to time. To prevent such colonisation, the hull may be treated with
anti-fouling paints containing metals that deter the attachment of barnacles,
or even have a covering of copper sheets that fulfils the same purpose. The
latter was a feature of the fastest tea clipper ships.
Barnacles do not colonise surfaces as adults, but as larvae.
The adults are hermaphrodite and shed both sperm and eggs into the sea where
fertilisation occurs. The first stage larva that hatches from the fertilised
egg is called a nauplius (A, above) and this transforms into a cypris larvae
(B, above). Both stages are free-swimming and are carried by currents, often very
far from the adults that produced them. The oceans contain enormous numbers of
larval stages of many familiar creatures, in addition to barnacle larvae, yet
most of us would be hard pressed to identify them as they are so unlike the
adults. Limbs are used both for locomotion and feeding and barnacle larvae
capture tiny particles of various kinds that are then passed to the mouthparts
and swallowed.
If cypris larvae are washed on to a shore, or to any other
solid surface, a remarkable series of events takes place. The first is that the
larva cements itself to the substratum (different types of adhesive being used
across the spectrum of barnacle taxa [2]) and calcareous plates are secreted to
begin the process of conversion to the familiar adult form. The process of
transformation has been described by Calman [3]:
..after a series of nauplius
stages the larva passes suddenly, at a single moult, into a stage in which the
body and limbs are enclosed in a bivalved shell.. ..this is known as the cypris
stage. Through the valves of the shell a large pair of compounds eyes can be
seen, as well as six pairs of two-branched swimming feet, while in front a pair
of antennules projects between the valves. On each antennule is a sucker-like
disc by means of which the larva, after swimming freely for some time, attaches
itself to a stone or some other object, where it remains fixed for the rest of
its life. A cementing substance produced by a gland at the base of the
antennules attaches the front part of the head firmly to the support; the
valves of the shell are cast off, and replaced by rudimentary valves of the
adult shell; the six pairs of swimming feet grow out into tendril-like cirri;
the compound eyes disappear, and the animal assumes the structure of the
adult.
Acorn barnacles and goose barnacles feed using the cirri on
their legs, these limbs no longer being required for swimming. Acorn barnacles
use the legs to sweep the cirri through an aperture between the shell plates
and captured particles of a wide variety are then transferred to the
mouthparts, in a similar way to that employed by the larvae. In goose
barnacles, the cirri are static, or nearly so, and feeding is from currents or,
in the case of moving objects, the water that passes over them. Unlike their
smaller relatives, goose barnacles also capture small planktonic animals and
the cirri close around these to prevent their escape. To enhance access to
currents, goose barnacles have developed a peduncle, or stalk, that is muscular
and covered with a strong cuticle. It grows intermittently [4] and those
familiar with the cuisine of Portugal will know the structure well after eating
a plateful of percebes (see recipe here [5]) and below a video from Gordon
Ramsay:
Goose barnacles are not able to exist for long out of water
but acorn barnacles certainly can, as they have calcareous plates that close
off the aperture of the "shell" when the cirri are withdrawn. That is
why we see them so readily on the shore when the tide is out. Some forms are so
high on the shore that they only become submerged at the highest tides, remaining
exposed to the air for days, showing how effective the closing plates are at preventing
water loss. Altogether, barnacles are most remarkable animals and it is little
wonder that they so intrigued Charles Darwin [6].
The basis for the structure and complex life history of these crustaceans is
encoded in the barnacle's genes. Repeating a question here that I often asked
students: "How do you think the structure and development evolved?" Anyone like
to venture a guess as to the stages in the evolution of the life history of barnacles
and their way of life?
[2] Jaimie-Leigh Jonker, Liam Morrison, Edward P. Lynch, Ingo
Grunwald, Janek von Byern and Anne Marie Power. (2015) The chemistry of stalked
barnacle adhesive (Lepas anatifera). Interface Focus 5:20140062.
[3] W. T. Calman (1911) The Life of Crustacea. London, Methuen & Co.
[4] John Chaffee and Cynthia Arey Lewis (1988) Pedunculate barnacle stalk growth. Journal of Experimental Marine Biology and Ecology 124:145-162
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