An article on the Sky News web site had the headline: Does Humble Limpet Hold Key To Future F1
Cars? It referred to the findings of Asa Barber and colleagues on the
strength of limpet teeth (there were many other reports of this work in the
Media, most with catchy headlines). Intrigued, I read the original publication describing the detailed studies and the comparisons made by the authors between the strength of limpet teeth and
strong, Man-made materials. In the paper [1], there was none of the colourful reporting
that I had seen in newspapers and on the web.
The teeth of limpets are set on the radula (see above), a
rasping tongue, used to scrape rock surfaces and thus acquire food from
attached algal and microbial biofilms. In moving over stone surfaces, the teeth
“need to be extremely strong and hard to avoid catastrophic failure” although
the scraping action does cause them to become worn [1]. Strength comes from a
mineral, goethite (iron oxy hydroxide), that is formed in filaments within a
matrix of protein with the “mineral nanofibres typically many micrometres in
length but only a few tens of nanometers in diameter” [1]. In non-technical
language, the strengthening filaments are long and very thin and this provides
the optimum arrangement for resilience.
Limpets are very common around shores and are certainly
successful snails (members of the Class Gastropoda). As I was brought up by the
sea, I became fascinated by marine organisms and their habits and, like many
children, was intrigued by the ability of limpets to cling on to rocks and thus
resist any amount of kicking by me. I learned that this attachment was made possible
by the muscular foot and was not related to suction as I had thought. The foot
has numerous minute extensions over its base and these interface with the (microscopically)
rough surface to which the limpet is attached, a thin film of mucus ensuring good
contact. This tight attachment serves not only to prevent limpets from being
removed, but also allows a near perfect match between the shell and the rock,
minimising water loss when the tide is out. The attachment is so close that the
rock can become eroded into “home scars” and the margin of the shell follows
contours, like those created by the barnacles in the illustration below.
Everything changes when the tide comes in. The tight hold is
relaxed and the limpets now use their foot, and secreted mucus, to glide over
the surface, just like other snails. It is then that they feed by movements of
the radula and this leaves a characteristic trail of removed biofilm (see below
- and for further, excellent pictures see https://natureinfocus.wordpress.com/category/seashore-creatures/limpets-seashore-creatures/page/2/).
As some limpets are territorial, it is thought that local areas of biofilm is
then “farmed” by individuals and their excretions may well act as fertiliser,
as it is known that biofilm components are highly adsorptive [2]. Algae also
grow on the home scars [3] and these will again be fertilised by the limpet
during attachment; exposure to light allowing the algae to photosynthesise when
the limpets forage away from the home scar.
The findings of Barber et
al., and our knowledge of the Natural History of limpets, bring to mind
many questions: “How did the structure of limpet teeth evolve?”; “How did
mineral fibres become embedded in a protein matrix?”, “How did teeth become
arranged on a radula?”; “How was a radula formed?”; “How did the structure of
the foot develop?”; “How did limpets become territorial and develop the habit
of gardening?”; etc. These questions have no easy answers.
We tend to regard animals like limpets as being mundane and
lacking in interest, although they are valued as food, especially in Madeira, where an annual festival is held in their honour.
This doesn’t extend to considering the welfare of the limpets, especially as
they are fried alive in butter, before the addition of a little garlic and
lemon juice makes them a tasty treat. Their use as food, and as examples of useful
bioengineering, emphasises our anthropocentric approach and we rightly continue
to have this attitude. However, we should also step back and admire how
evolution has resulted in such an impressive array of adaptations to life on
the rocky shore. The child in me has a sense of awe at the selection of
mutations that allowed all the changes that have occurred through time to
produce such wonderful creatures. I’m not suggesting that limpets should be
regarded as honorary humans, in the way that we regard our pets, but these
splendid snails are not just around for our purposes, whatever the sentiments
of the Book of Genesis, or the impressions provided by headlines in the Media. All
the adaptations referred to in the questions above were in place long before
humans appeared and limpets deserve our respect.
[1] Asa H. Barber, Dun Lu and Nicola M Pugno (2015) Extreme
strength observed in limpet teeth. Journal
of the Royal Society Interface 12: 20141326.
[2] Roger S Wotton (2004) The essential role of exopolymers
(EPS) in aquatic systems. Oceanography
and Marine Biology, An Annual Review 42: 57-94.
[3] Gray A Williams and Colin Little (2001) Preliminary
observations on algal growth under limpet (Patella
vulgata) home scars. Journal of the
Marine Biological Association of the United Kingdom 81: 175-176.
No comments:
Post a Comment