Painted shells

In nature shells provide varying forms of protection, from snails that use them to shelter their soft bodies from predation and drying out, to bird’s eggs that defend the growing embryo. This week I picked up a discarded egg shell of a Song Thrush (Turdus philomelos), characteristically a stunning summer sky-blue with sparing dark speckles. A couple of weeks ago I was dazzled by brightly coloured Painted Topshells (Calliostoma zizyphinum) dotted about on low tide rocks near Torquay. This led me to wonder at the biological function of some species having such visually distinctive shells that might make them more prone to predation.

Considering the Song Thrush there is some consistency with other birds that make their nests in darkened habitats and pale blue eggs. The Song Thrush chooses either a thick hedgerow, hollow in a creeper, or rarely if on the ground in a shady well-hidden site. It was believed that the distinctive colour may help the adults locate them more easily (Harrison, 1975). Many alternative theories of why thrushes lay blue eggs was summarised by Gotmark (1991): they may be cryptic and provide camouflage by imitating spots of light on green leaves; egg parasites such as cuckoos might have exerted selection pressures historically; the pigmentation of eggs enhances the strength of the egg shell and therefore its resistance to cracking; egg coloration influences egg temperature helping with its regulation. More recent research has tried to test whether the blue-green egg colouration of many bird species provides a health indicator of the female to allow males to modulate their parental investment. However it would appear that researchers are still really none the wiser and so we can only assume that the cost of producing blue pigment is outweighed by some unclear benefit that may no longer be relevant, or that it is simply maladaptive.

The Painted Topshell is the most colourful example of these intertidal molluscs in the UK. The colour varies from yellow or brown to pink or purple and is overlaid with irregular dark purple or red markings. The purplish blotches spiralling around the almost perfect conical shell give the illusion of a spinning top. There is also a white ‘morph’ form which is considered by some to be a separate species. This Topshell is also notable for its frequent shell-wiping behaviour, which would appear to both prevent surface fouling and top up its food intake by as much as a fifth (Holmes, 2001). Despite this interesting colour variation I can find no explanation to its cause. So whilst scientists continue to hypothesise I humbly suggest that we simply sit back and admire nature showing off its surprising beauty.

Gotmark, F. (1991) Blue eggs do not reduce nest predation in the song thrush, Turdus philomelos. Behav Ecol Sociobiol,  30:245-252

Harrison, C (1975) A Field Guide to the Nests, eggs and Nestlings of British & European Birds. London: Collins.

Holmes, S.P., Sturgess, C.J. Cherrill, A. Davies, M.S (2001) Shell wiping in Calliostoma zizyphinum: the use of pedal mucus as a provendering agent and its contribution to daily energetic requirements. Marine Ecology Progress Series, 212:171-181

Spurge-laurel - a healthy pungent purge or scourge?

Very few plants put energy into flowering in the winter due to the cold and lack of sunshine and pollinators. There are some obvious exceptions such as Snowdrops and Winter Aconites. Less obvious is the evergreen shrub, Spurge-laurel (Daphne laureola) which I spotted inadvertently last week in one of our local hedgerows. The origin of the Latin name Daphne apparently comes from the water nymph whom the Gods turned into a laurel-bush to hide from the advances of Apollo (SAPS, 2012). Well in keeping with concealment you would be forgiven for not spotting its flowers, with petals absent and small camouflaged yellow-green sepals. The plant flowers appear from January, a welcome source of nectar for early emergent moths and bees attracted by their strong scent rather than visual cues. But don’t let the sweet fragrance fool you – all parts of a Spurge-laurel plant are poisonous to humans, particularly the bark and berries. It can cause stomach pain, nausea, vomiting, bloody diarrhoea, delirium, collapse and death; even external contact between plant and skin can cause blistering. (Eland, 2012). Some tribes in northern Africa still use a bark extract to poison fish (Eland, 2012). This all serves a useful service for the Spurge-Laurel, as its production of powerful toxic compounds such as coumarin results in a “well defended plant” with very few animals able to eat it (Alonso, 2009).

Interestingly it is not universally toxic to animals; for instance the black berries are eaten by birds like greenfinches without any harm (SAPS, 2012), and in the past used as a veterinary cure for horses (Eland, 2012). It has also been used occasionally in the past for medicinal purposes, such as folk treatments for cancer, toothache and rheumatism (Eland, 2012). Johnson (1856) refers to a “decoction of the root and bark being recommended for children afflicted with worms”, but goes on to warn that “it should never be used, being one of those remedies that can only be employed at the risk of life”. He continues with a more strident note –

“The Lady Bountiful of the village would do more good by confining her benevolent practice to the issue of nourishing food and warm clothing, where necessary, than by trying to combat disease by remedies of which the action is equivocal, if not, as in this instance, dangerous in the highest degree.”

In modern times opinion of Spurge-laurel very much leans towards Mr Johnson’s – well at least botanically rather than his gender insinuations.

Alonso, C., García, I.M, Zapata, N. and Pérez, R.(2009) Variability in the behavioural responses of three generalist herbivores to the most abundant coumarin in Daphne laureola leaves. Entomologia Experimentalis et applicata, 32(1):76-83

Eland, S. (2012) [online] Plant Biographies http://www.plantlives.com/docs/D/Daphne_laureola.pdf  [Accessed 7/03/12]
Johnson, C. (1856) British Poisonous Plants. London: Taylor & Francis.
Science and Plants for Schools (SAPS) (2012) [online] http://www-saps.plantsci.cam.ac.uk/trees/laurels.htm [Accessed 7/03/12]