subtitle for learnabout section of A SNAIL'S ODYSSEY
  Feeding & growth
 

Littorines and their relatives feed on various plant matter, including diatoms, algal sporlings, microalgae and macroalgae, lichens, and detritus. Littoriina spp. are considered here, while other species BATILLARIA spp., EPITONIUM spp. & LACUNA spp. are dealt with in another section

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Littorina spp.

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Research study 1
 

photographs of littorinid snails Littorina keenae courtesy James Watanabe, Pacific Grove, CaliforniaWinkles feed mainly on diatoms and unicellular algae, and to a lesser extent on macroalgae and lichens.  Because much of the food is scraped directly from the substratum, especially when feeding on sandstone, erosive activities may be high.  Based on average content of sand in the gut of Littorina planaxis on the rocky shores of La Jolla, California it is estimated that 100 average-sized individuals will excavate about 860cc of sandstone substratum every decade.  North 1954 Biol Bull 106: 185. Photograph courtesy James Watanabe, Hopkins Marine Station, Pacific Grove, California SeaNet.

NOTE  this species is now keenae

NOTE  the calculation is based on 1.6mg sand for an average-sized 8mm length individual for a gut-passage time of 2-6h (at 17-30oC field temperature)

 

Littorina keenae with arrows showing
notable shell identification features 2X

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Research study 2
 

graph showing growth of 3 species of littorinid snails Littorina scutulata, L. plena, and L. keenae in Bodega Bay, Californiaphotographs of littorinid snails Littorina plena courtesy James Watanabe, Hopkins Marine Station, Pacific Grove, CaliforniaStudies at the Bodega Marine Laboratory, California show that littorines in Bodega Bay, California eat red and green microalgae, blue-green algae (Cyanophyta), diatoms, and sporelings of macroalgae.  The graph shows estimated rates of growth of Littorina scutulata, L. plena, and L. keenae assessed over a 29-mo study period.  Chow 1987 J Exp Mar Biol Ecol 110: 69. Photograph courtesy James Watanabe, Hopkins Marine Station, Pacific Grove, California SeaNet.

Littorina plena with arrow
showing identification
feature on aperture2X

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Research study 3
 

West-coast winkles feed on diatom films, lichen, and algae. Diets vary with location and species and, in particular, with radular morphology.  For example, chisel-shaped radular cusps of Littorina scutulata enable them to scrape black lichen from rocks in the upper-shore spray zone (see photo on Right).  In contrast, photographs of littorinid snail Littorina scutulata courtesy James Watanabe, Pacific Grove, Californiathe coarser shaped cusps of L. sitkana permit better exploitation of drift algae stranded in tidepools. photograph of a radula of an Alaskan species of littorinid snail Littorina naticoidesBehrens Yamada 1989 Mar Biol 103: 403. Photograph on Left courtesy James Watanabe, Hopkins Marine Station, Pacific Grove, California SeaNet.

NOTE the author does not include photos of radulae in her presentation; however, for interest's sake, on the Right is a radula of an Alaskan species Littorina naticoides, viewed lengthwise, but at an angle of 45o to show the shape of the scraping cusps.  Reid & Golikov 1991 The Nautilus 105: 7.


Littorina scutulata showing
opercular morphology 3X

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Research study 3.1
 

graph showing effect of increasing density of littorines Littorina sitkana and L. scutulata on growth of brown alga Fucus distichusAlthough generally considered to be eaters of microalgae, littorine snails Littorina spp. will readily consume surface tissues of macroalgae including those of the brown alga Fucus spp.  Research at Friday Harbor Laboratories, Washington shows that wounding by the snails can significantly affect growth and reproduction in the alga.  The researcher monitors 3 intertidal areas with different densities of Littorina sitkana and L. scutulata over a 5mo period and finds that both growth and reproduction of Fucus distichus are significantly decreased by degree of wounding caused  by the snails (see graph on Right for growth effects).  As for effects on reproduction of the alga, in all study areas only plants that receive little damage from the herbivores become reproductive.  The author notes that such non-lethal grazing is important in the evolution of algae in that it should lead to selection for grazer-resistant genotypes.  Van Alstyne 1990 J Phycol 26: 412.

NOTE  densites of both species combined differ significantly in the 3 areas, averaging about 180, 400, and 2300 individuals . m-2

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Research study 4
 

histogram showing preference of littorines Littorina scutulata for certain high-level algaeas foodIn areas of Barkley Sound, British Columbia the upper intertidal region is primarily populated with red algae Mazzaella (cornucopiae) parksii, along with fucoids Fucus distichus and Pelvetiopsis limitata.  The chief non-limpet grazers in this zone are littorines Littorina scutulata.  Counts show that densities of littorines may reach 70 . 100cm-2 in growths of M. cornucopiae and 15 . 100cm-2 in fucoids, suggesting a preference for the red-algae habitat.  Results of substratum-preference tests and feeding assays show that Mazzaella is preferred over the fucoids (see histogram). The authors propose that on a broader scale L. scutulata may indirectly affect competitive interactions between Mazzaella and the 2 fucoids in this high-level area.  Possible follow-up research to thisphotograph of red alga Mazzaella parksii and brown algae Fucus sp. and Pelvetiopsis sp. courtesy Mike Hawkes, Botany Department, UBCstudy would be to re-examine preferences of the littorine in relation to presence of potentially defensive secondary metabolites in the algae, most notably, tannins in the brown algae. Kim & DeWreede 1996 Bot Mar 39: 561. Photograph on Left courtesy Mike Hawkes, Botany Department, UBC.photograph of littornid snails Littorina scutulata feeding in amongst red alga

NOTE  the authors note that some of the smaller, similar-appearing species, L. plena may have gotten mixed in with L. scutulata

NOTE  these tests involve placing a number of snails in a container with equal amounts of the 3 algae and counting their disposition on the algal fronds 15min later. Note that this experiment does not assess whether the winkles are feeding on the test algae; however, other tests show that feeding does occur



Littorina scutulata amongst red
alga Gloiopeltis furcata. The
snails may be feeding on this
alga or on microalgae 1X

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Research study 5
 

photograph of littorinid snail Littorina scutulata feeding on the surface of a piece of kelpGut analyses of Littorina scutulata and L. sitkana collected from a variety of habitats on rocky shores around the southern end of Vancouver Island, British Columbia reveal that both species are opportunistc omnivores.  In locations where the species co-occur the diets are similar. However, L. scutulata differs in that it can subsist on sparse growths of microflora in the supralittoral area, whereas L. sitkana is restricted to microenvironments with photograph of a large aggregation of winkles Littorina sitkana in San Juan Island, Washington courtesy Dave Cowles, Walla Walla University, Washingtonabundant growths of microalgae and macroalgae. Protists and animal matter such as nematodes, copepods, annelids) are eaten by both species.  Seagrasses Phyllospadix and Zostera are also consumed, but only when present in decomposing litter.  These results differ from those of other investigators mainly in the extents to which macroalgae, animal matter, and decaying plant matter are present in the diets.  Voltolina & Sacchi 1990 Hydrobiologia 193: 147. Photograph below courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.


Not much algae is present with this large aggregation of winkles
Littoriina sitkana
in a small cove in San Juan Island, Washington

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Research study 6
 

map showing collection site for study of herbivory by littorinid snails Littorina spp. on the red alga Mazzaella parksiiIn Barkley Sound, British Columbia Littorina spp. snails are important grazers of high-level red algae Mazzaella parksii. During prolonged exposure to sunlight, especially during spring and summer spring tides, the distal parts of the fronds of the alga become bleached. These bleached parts stay on the plants for some time and are eaten by littorines. Interestingly, in paired-choice experiments conducted at the Bamfield Marine Sciences Centre, British Columbia using Littorina spp., these bleached portions are preferentially consumed over normal photosynthetic tissues. The authors suggest that the preferential consumption of the bleached portions, which are located distal to the more basally sited photosynthetic tissues, may actually be of advantage to Mazzaella in diverting grazing pressure away from its vital photosynthetic tissues. The authors suggest that softer texture of the bleached portions, perhaps combined with improved nutritional content, may be contributing factors to their greater palatability to the snails. Heaven & Scrosati 2004 Hydrobiologia 513: 239.

  Research study 7
 

histogram showing feeding preferences of the littorine Littorina sitkana for different algaephotograph of littorines Littorina sitkana and L. scutulata feedingCafeteria-style feeding-preference tests at Shannon Point Marine Center, Washington reveal that Littorina sitkana prefers1 green algae Ulva linza over 6 other algal species including a closely related membranous green alga U. lactuca (see histogram).  Both green-algal species contain high concentrations of a chemical DMSP2 known to deter feeding by sea urchins Strongylocentrotus spp.  Now, Ulva is known from several other nutritional studies of invertebrate herbivores to be less good in promoting growth than other algal species, even though it may be preferred.  So, the questions asked by the researchers are: 1) is L. sitkana insensitive to the potentially deterrent effects of the DMSP chemicals, and/or 2) is U. linza in some way nutritionally superior for Littorina than the other seaweeds tested? In answer to 1), the authors confirm the presence of DMS in feeding containers when either Ulva species is eaten, confirming cleavage of DMSP, and agar-based diets containing acrylic acid are less-well eaten than control diets, confirming deterrence by at least one of the products of DMSP cleavage.  So, U. linza is preferred as a food despite its content of potential deterrent chemicals.  The answer seems to lie in 2), that this alga is nutritionally superior to the others tested for this snail species.  Analyses by the researchers show that Ulva linza has a higher ratio of nitrogen to carbon than the other species, and agar-based diets enriched with nitrogen through inclusion of varying amounts of casein are preferentially eaten by the snails. Based on the results of the cafeteria-style feeding experiments the researchers state that if the carbon/nitrogen content of an alga reaches a value higher than 8 (that is, 8 times more carbon than nitrogen), then the snails avoid3 it. The authors conclude that L. sitkana “preferentially select foods with high nitrogen concentrations, even though these food contain chemical defenses”.  What would now be useful to know is whether U. linza can promote good growth and reproduction in L. sitkana.  Van Alstyne et al. 2009 J Exp Mar Biol Ecol 379: 85; for more on DMSP and its breakdown products see Lyons et al. 2007 Mar Biol 153: 179.

NOTE1  for some reason the authors throughout the study state that both Ulva species are “consumed…at a greater rate than 6 other algal species”, but as witness their own graphed data (histogram), this appears to be not true for U. lactuca.  It is surprising that no pair-wise statistical tests are performed on these data to test whether U. lactuca truly does differ significantly from U. linza as the authors contend; it seems unlikely

NOTE2 DMSP = dimethysulfoniopropionate.  When eaten by urchins, the chemical is cleaved into dimethylsulfonate and acrylic acid, and these are the actual deterrent agents as demonstrated for other invertebrate herbivores

NOTE3 this is a simplification, and the authors well know that there are many morphological and chemical features of a seaweed that play roles in its palatability to various invertebrate herbivores.  Nitrogen content will be only one of many such features

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