subtitle for learnabout section of A SNAIL'S ODYSSEY
  Role as herbivores
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Research study 1

graph showing grazing effects of littorinid snails Littorina scutulata on diatom abundanceStudies near Coos Bay, Oregon on grazing effects of Littorina scutulata on high-intertidal-level diatom populations show that as few as 3 littorines per dm2 is sufficient to keep an area of sandstone free of diatoms over a 2-wk period.  Numbers fewer than this allow diatom patches to develop.  The study, which employs artificial tidepools carved into sandstone with screen enclosures to control numbers of littorines in different tests, stems from an observation that diatoms are virtually absent from the mid-high intertidal regions in summertime.  Castenholz 1961 Ecology 42: 783.

NOTE  species listed as dominant are Navicula (Schizonema) ramosissima, N. grevillei, and Licmophora abbreviata.

NOTE  a square decimeter is equivalent to 0.01 of a square meter.  Not: on the graph a value of 0.2 cu cm/sq dec is equivalent to approximately 3 littorine individuals

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Research study 2

drawing showing parts of an experimentally damaged fucoid plant Fucus distichusphotograph of fucoid alga Fucus sp.Predator-induced defenses are well known in terrestrial vascular plants, but not so much for non-vascular plants such as seaweeds.  Investigations on the brown alga Fucus distichus at Tatoosh Island, Washington show that when the fronds are damaged by clipping (see drawing on Right) a plant increases its concentrations of polyphenolics by more than 20% within 2wk. The increases occur in the parts experimentally damaged and within adjacent undamaged branches. Similar increases are recorded in naturally grazed plants, for example, 12% dry-mass phenolics measured in grazed plants vs. 9% in ungrazed ones. 

In experiments where experimentally damaged plants along with undamaged control plants are translocated from areas of low herbivore density to areas of high herbivore density, grazing snails Littorina sitkana initially show a preference for the experimentally damaged plants, but over a 2-wk period shift their preference towards uninjured control plants.  Over 2-wk feeding tests the injured plants lose about 50% less surface area to grazing L. sitkana as compared with uninjured algae.  The study is a nice one, especially in that all experiments are done in the field.  Van Alstyne 1988 Ecology 69: 665.

NOTE  the clipping is intended to mimic injury by herbivores. Phenolic levels are then measured in the remaining part of the branch, and in proximal and distal branches (see drawing)

NOTE  primarily polymers of phloroglucinol linked by aryl-aryl, ether-ether, or aryl-ether bonds and bound to the plant proteins, known in other studies to reduce feeding by sea urchins, snails, and other marine invertebrates

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Research study 3

drawing of Fucus plant showing appearance of short and long adventitious brancheshistogram comparing consumption of normal meristematic tissue by littorinid snails Littorina sitkana versus short, medium, and long adventitious branchesA possible example of predator-induced defenses in an alga is provided by the same author, also involving Tatoosh Island-collected specimens of brown alga Fucus distichus and the littorines Littorina sitkana and L. scutulata.  The author provides no direct cause and effect evidence, but notes that the incidence of appearance of adventitious branches on the alga (see drawing on Left) is positively correlated with density of littorines around the plants.  In 4-d laboratory feeding assays done at Friday Harbor Laboratories, the snails prefer to eat normal meristematic growth over these adventitious branches (see histogram, data for L. sitkana only). The author reports that concentration of polyphenolic compounds is 10-70% higher in the adventitious branches than in the normal meristematic growth. Although interesting, it is not clear how the presence of these adventitious branches will do much more than direct the attentions of the consumers to more palatable parts of the same plants. Van Alstyne 1989 Mar Ecol Progr Ser 56: 169.

NOTE  these apparent defensive structures grow out of the midrib, and can be of varying lengths

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Research study 4

At Bird Rock in Pacific Grove, California the perennial green alga Prasiola meridionalis forms a distinct band about 0.8m in height in the high intertidal region (+5m above MLLW level).  Dominant grazers on the shady site of boulders are limpets and several species of littorines, L. keenae and L. scutulata/plena.  If these herbivores are excluded for a 6-mo period from vertical experimental plots using a combination of fencing and marine antifouling paint, Prasiola grows further downward in the plots.  In this area, grazers appear to set the lower limit of Prasiola; however, in other sites with greater sun exposure and where photograph of green alga Prasiola meridionalisgastropod grazers are rare, the alga occurs only seasonally, arriving in early spring and dying back in early summer.  Several days of warm weather are enough to cause Prasiola to turn black, become brittle, and die.  Thus, the effect of the herbivores is relative, depending upon degree of insolation and other factors including, interestingly, mites.  Anderson & Foster 1999 Phycologia 38: 349.

NOTE  numbers of limpets and littorines in nearby control plots are about 110 and 195 . m-2, respectively, while numbers in the exclusion plots are only 1 and 2 . m-2, respectively.  Grazers entering the exclosure plots are removed monthly

NOTE  the authors note that their data do not allow separation of effects of several factors associated with increased insolation, such as desiccation, UV irradiation, elevated tissue temperatures, and so on

Green alga Prasiola meridionalis 0.3X

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