Monday, February 7, 2011

Dog Lichen Out Muscles the Grass

Dog Lichen, Peltigera canina
Lichens are excellent primary collonisers. All they need is sunlight, something to anchor to and, given enough moisture, they will grow. Most species are to seen on bare rock, on the bark of trees or the peaty soils of moors. Here their main competitors for light resources are bryophytes which rarely outgrow them. There are some exceptions, however, a notable one being the Dog Lichen, Peltigera canina. While it can be found on walls and rocks and other dry habitats, this marcolichen with Nostoc cyanobacteria as its photobiont (1), is most commonly seen in Ireland growing among grasses in woods and lawns (2). The large lobed thalli can form patches 10 cm across, which helps when competing with grasses for resources. Furthermore, the lichen has been shown to produce compounds with inhibitory effects on the germination of grass seeds and the subsequent growth of seedlings (3). The presence of cyanobacteria as the photobiont also gives P. canina the competitive advantage of being able to fix its own nitrogen (4).
Thalli of the Dog Lichen, Peltigera canina
The thalli of P. canina are thick and downy and when wet are dull brown, but pale grey when dry and produce distinctive reddish brown spore producing structures on the edge of the lobes (2) (the sample pictured was observed in January). The underside is covered with well developed rhizinae.
Fruiting Structures of the Dog Lichen, Peltigera canina
The ability of P. canina to survive in a number of habitats lies in the fact that its is not a single species but a species complex (5). Twenty five taxa are known in the complex, however only seventeen of these are recognised, the other eight being under-described.
Rhizinae of the Dog Lichen, Peltigera canina
  1. Miadlikowska and Lutzoni 2000, International Journal of Plant Science 161 pp. 925–958
  2. Phillips 1980, Grasses, Ferns, Mosses and Lichens of Great Britain and Ireland p. 176
  3. Pyatt 1967, The Bryologist 70 pp. 326-329
  4. Lockhart et al. 1978, FEMS Microbiology Letters 3 pp. 127–130
  5. Miadlikowska et al. 2003, Mycologia 95 pp. 1181–1203

Thursday, February 3, 2011

Who Needs a Shell?

Bar cephalopods, most marine molluscs move slowly, if at all. While this leaves them open to predation, tough calcium based shells provide excellent protection. The nudibranchs buck this trend. Over time, they have lost their protective armour, yet in spite of their apparent vulnerability they are rarely the victims of predators (1). The loss of shell in nudibranchs appears to be associated with the development of chemical defense mechanisms (2). In most cases these are derived from the diet of the nudibranch, however there are some species of synthesising their own chemical defenses (1).  Aeolid nudibranchs procure stinging cells from their cnidarian prey (3) and ascoglossan and aplysiid nudibranchs obtain toxic secondary metabolites as they graze on algae (4). Others obtain their chemical defenses from their diets of sponges, bryozoans and tunicates. Hypserlodoris webbi selects sponges with high levels of secondary metabolites for predation, which it then sequesters in dorsal glands for pretection (2). Similarly Hexabranchus sanguineus obtains its defense chemicals from Halichondria spp. sponges but also passes these compounds to its egg ribbons, which are otherwise defenseless (5).

  1. Fontana et al. 1994, Cellular and Molecular Life Sciences 50 pp. 510-516
  2. Pawlik et al. 1988, Journal of Experimental Marine Biology and Ecology 119 pp. 99-109
  3. Thompson 1976, Biology of Opisthobranch Molluscs p. 207
  4. Lewin 1970, Pacific Scientific 24 pp. 356-358
  5. Avila and Paul 1997, Marine Ecology Progress Series 150 pp. 171-180