The Thickness of the Flat Periwinkle Shell

Flat Periwinkle, Littorina obtusata

The flat periwinkle Littorina obtusata is a very common grazer of seaweeds on the middle shore around Ireland (1) and is a large constituent of the diet of the common shore crab Carcinus maenas (2). C. maenus feeds on the shellfish by crushing the shell and extracting the periwinkle. The evolution of the shell is though to be driven mainly by the selection pressures associated with such shell crushing predators (3). However Geoffery Trussell proposes that morphological co-evolution between predators and their gastropod prey may be driven by natural selection on reaction norms rather than genetically fixed phenotypes (4). In his study of geographical variation in L. obtusata in the Gulf of Maine, he found that periwinkles in areas with more C. maenas had thicker shells. This result was underlined in a subsequent laboratory experiment when L. obtusata raised in the presence of C. maenas again had much thicker shells. The trade off for better protection was smaller individuals with an accompanied reduction in body growth.

Shore Crab, Carcinus maenas

References:

1. Chinery 1987, Field Guide to the Wildlife of Britain and Europe p. 200
2. Ropes 1968, Fishery Bulletin 67 pp. 183-203
3. Clements et al. 2008, Biology Letters 4 pp. 179–182
4. Trussell 2000, Evolutionary Ecology Research 2 pp. 803-822

How to Eat an Insect

A common sight on bogs and heaths throughout west Cork and Kerry, the Large Flowered Butterwort (Pinguicula grandiflora) is one of a number of carnivorous plants native to Ireland (1). On these poorer, wetter soils, carnivory offers an advantage to plants (2) and makes them indicative of such wet soils. Due to the low pH of peaty soils, nutrients become unavailable to plants as they become bound in salts. While it was previously held that the main reason for insectivorous behaviour in plants was to obtain mainly nitrogen (3), it has been shown that in a Pinguicula species, only phosphorus absorbed through the leaves leads to a significant increase in plant biomass (4).

Large Flowered Butterwort, Pinguicula grandiflora

 
So how does P. grandiflora capture insects to eat? The leaves of the plant carry a number of cells that are devoted to this task: secretory head cells, endodermal intervening cells and basal reservoir cells (5). The head cells hold drops of mucilaginous secretions which attract and then trap the insect prey. Further incapacitation is achieved by release of more mucilage from the reservoir cells. Digestion can then begin by the release of enzymes from the endodermal-like cells.

References:

1. Phillips, 1977 Wild Flowers of Britain p. 46
2. Brewer et al., 2010 Aquatic Biology In Press, Corrected Proof
3. Thompson, 1981 Biological Journal of the Linnean Society 16 pp. 147-155
4. Karlsson and Carlsson, 1984 New Phytologist 97 pp. 25-30
5. Heslop-Harrison, 1981 Annals of Botany 47 pp. 293-319

The Brittle Star Link

Two Small Common Brittle Stars, Ophiothrix fragilis

The Common Brittle Star (Ophiothrix fragilis) is a widespread invertebrate of European waters. It has a body comprising of a central, flattened disc from which five narrow arms radiate (1). Dense beds of O. fragilis are quite common in European waters, often of up to 2000 individuals m^-2 (2). Their suspension feeding action means that they remove and recycle organic material in the water, and as such provide an important link between benthic and pelagic ecosystems (3).

References:

1. Sterry, 2004 Collins Complete Guide to Irish Wildilfe p. 176
2. Broom, 1975 Journal of the Marine Biological Association of the UK 55 pp. 191-197
3. Allen, 1998 Marine Biology 132 pp. 383-390