subtitle for learnabout section of A SNAIL'S ODYSSEY
  Population & community dynamics
  This topic is divided into a larger section on Littorina and a smaller section below entitled other genera.
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Littorina

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Research study 1
 

photograph of high-density population of littorinid snails Littorina scutulataIn areas of California 2 species of winkles, Littorina keenae and L. scutulata, live more or less vertically separated in the highest regions of the intertidal zone.  Littorina keenae inhabits the highest rocks in the splash zone and eats mainly microalgae, including diatoms.  Littorina scutulata lives lower on the shore among macroalgae or around the edges of tidepools, and eats mainly macroalgae including the greens Cladophora and Ulva, and brown Pelvetiopsis, but also consumes diatoms and some blue-green algae.  North 1954 Biol Bull 106: 185; Dahl 1964 Veliger 7: 139.





Aggregation of Littorina scutulata on an upper-level
shoreline in Barkley Sound, British Columbia 0.3X

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Research study 2
 

photograph of brown alga Pelvetiopsis sp. growing on sandstoneWhat regulates the distribution of the 2 species?  Physiological tolerances? Biotic factors, such as food, predation, and competition?  Not surprisingly, it seems that several factors interact to segregate the species, including behavioral responses to gravity.  Studies at Bodega Marine Laboratory, California show first, that L. keenae is more tolerant of drying and exposure to freshwater than is L. scutulata, and can live for at least 64d without being wetted.  These factors combined with lack of suitable foods could set the upper limits of distribution for both species.  Lower limits for could be set by predation and possibly by gas-exchange inefficiencies during submersion and, for the higher-dwelling species L. keenae, by possible competition with L. scutulata .  Both species are negatively geotactic, meaning that they tend to crawl upwards.  In Littorina scutulata this is inhibited by contact with certain algae, such as Pelvetiopsis, which tends to hold the species within the upward distribution of this fleshy, moisture-retaining alga.  In contrast, negatively geotactic crawling of photograph of littorinid snail Littorina scutulata glued to substratumL. planaxis is not suppressed by contact with macroscopic algae, and it crawls to higher levels where microscopic, encrusting algae abound.  This species tends not to crawl on dry rock, and limits its activity to periods when the rocks are wet from wave splash.  Bock & Johnson 1967 Veliger 10: 42.

NOTE  the authors note only that only L. keenae secretse a mucous seal between its shell and the substratum to minimise drying, but it is known that L. scutulata is equally able to do this

Brown alga Pelvetiopsis sp. growing on
sandstone offers a humidity-rich micro-
habitat to water-"challenged" littorines



Littorina scutulata
attached
by mucilaginous glue to a
wooden dock substratum 3X

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Research study 3
 

schematic showing dietary compositions of juvenile black turban shells Chlorostoma funebralis and Littorina scutulataIn mid-intertidal areas of rocky shores in Pacific Grove, California juvenile Littorina scutulata of <4mm shell length may potentially compete with juvenile Chlorostoma (Tegula) funebralis for food and space resources.  Gut analyses show that while there are some small differences in dietary preferences, both species as juveniles are non-selective detritivores. Competition for detritus, if it were to occur, would be minimised by the 2 species feeding in different microhabitats.  Jensen 1981 Veliger 23: 333.

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Research study 4
 

graph showing effect of density on body mass in littorinid snails Littorina plenaphotograph of Littorina plena courtesy James Watanabe, Hopkins Marine Station, Pacifi Grove, CaliforniaPopulation densities of littorines are often high and the question arises as to how population numbers are regulated.  Experimental manipulation of field densities of Littorina plena in Bodega Bay, California indicate that intraspecific competition for food may be the primary mechanism involved in the regulation of population numbers. The experiments employ small cage enclosures with differing densities of littorines, but based on natural field densities, replicated over 2yr (only 1981 data shown here).

Results after 90-d experimental periods reveal that body masses significantly decline in the higher-density enclosures, accompanied by increasing mortality (see graph). Measurements of chlorophyll contents of scrapings within the enclosures show that higher densities are correlated with elimination of algal cover. Thus, within the range of natural abundances of L. plena, mortality rates increase and individual body masses decline with increasing population densities.  The author adds that physical processes that control rates of shore mortality and larval recruitment will likely alter the intensity of the competitive interactions.  Chow 1989 J Exp Mar Biol Ecol 130: 147. Photographs courtesy James Watanabe, Stanford University, California and SeaNet.

NOTE  10,000 Littorina plena individuals . m-2 commonly occur in zones of abundance in Bodega Bay, with >40,000 individuals . m-2 being present in some areas of maximal abundances

NOTE food consists of algal films made up of red and green microalgae, blue-green algae, diatoms, and macroalgal sporelings

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Research study 5
 

schematic showing shell measurements used in study of density effects on littorinid snails Littorina sp.Experiments at Friday Harbor Laboratories, Washington on Littorina sp. also show that density has a marked effect on growth rate, shell mass, and shell morphometry.  The researchers divide each of 288 broods into 2 treatments: low and high density, and culture these in the laboratory.  Snails in the low-density treatment grow faster and have lighter shells, with narrower whorls and narrower apertures, than their siblings in the high-density treatment. In addition to this environmental plasticity in shell morphometry there is also significant additive genetic effect.  This is manifested in a significant and large negative correlation between spire height and shell mass. Thus, shorter shells are thicker and heavier.  Based on their data the authors are able to predict the rate of response to selection for predator-resistant morphology, such as a lower spire and heavier shell, such as would occur with invasion of predatory crabs.  Boulding & Hay 1993 Evolution 47: 576.

NOTE  taxonomic uncertainty at the time of this publication has since been resolved by the author and her research group. The species appears to have been Littorina subrotundata

NOTE  the parental stock for these broods are collected when immature, grown in the lab, then mated with single, known males

 

 

Shell measurements
used in study

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Research study 6
 

photograph of littorinid snail Littorina sitkanaAt least 2 species of littorines live on the shores of Tatoosh Island, Washington.  One, Littorina sitkana, lives in protected habitats and has a shell that is twice as thick as the shell of the other species, L. subrotundata, that lives in wave-exposed habitats.  Translocation experiments are done to determine why each species does not live in the other’s habitat, but the results are equivocable.  When translocated to the wave-exposed habitat, L. sitkana exhibits only slightly lower survival than L. subrotundata, so it is not clear why it cannot live on wave-exposed shores naturally.  However, when moved to protected habitats, the thin-shelled L. subrotundata exhibits poor survival, possibly because of predation by shore crabs.  In separate laboratory studies, the authors find that shore crabs Hemigrapsus spp. prefer the thin-shelled exposed species over the thicker-shelled protected species.  Boulding & Van Alstyne 1993 J Exp Mar Biol Ecol 169: 139.  Photograph of L. subrotundata courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE at the time of publication the authors do not actually list a species name, but note that it has characteristics similar to L. subrotundata.  For convenience it will be referred to here as this species

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Research study 7
 

photograph of Littorina subrotundata, "barnacle ecotype", courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, WashingtonThe winkle Littorina subrotundata occurs in different geographic regions along the west coast.  However, in any given region, morphologies differ depending on specific habitat occupied.  Thus, individuals living on wave-exposed rocky shores, the so-called "barnacle ecotype", are low-spired, have a large circular aperture, have radulae with pointed cusps, and are black or yellowish with white stripes in colour (see photos on Left).  Those living in salt marshes, the "marsh ecotype", are high-spired, have radulae with blunt cusps, and are coloured differently.  Do the 2 types represent different species, clades, or ecotypes?  This question is addressed in a study using molecular-genetics analyses on specimens collected from salt-marsh and wave-exposed sites from Oregon to the Aleutian Islands, Alaska.  Results are consistent with the notion that the 2 forms are ecotypes and not separate species or clades as some have thought.  The results also suggest that each ecotype has evolved independently in different geographic areas and that the morphological similarity of individuals from the same habitat-type in these different areas most likely results from parallel evolution.  Kyle & Boulding 1998 Proc R Soc Lond B 265: 303. Photograph of L. subrotundata courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

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Research study 8
  An idea that number of littorines in an area may be correlated with micro-topographical variations in barnacle distribution, is tested in Barkley Sound, British Columbia.  The drawing shows the 2 major classifications used in the study. The first represents a densely-packed, even-aged distribution of barnacles Balanus glandula that provides few refuges for littorines.  The second has small and large gaps created by missing barnacles, and other dead barnacles with wall plates intact that provide refuges for littorines. Results show that the number of extra-small pockets alone provides the best predictor of littorine abundance.  These pockets may increase survival of littorines by providing refuge from wave action, temperature stress, and desiccation.  The authors suggest that manipulative experiments should now be done to test whether more barnacles or more pockets will lead to increases in littorine abundance.  Boulding & Harper 1998 Hydrobiologia 378: 105.  NOTE  species included in the study are Littorina subrotundata, L. scutulata, L. plena, and a few L. sitkana  NOTE the authors use image analysis to quantify micro-topography of barnacles within 100cm2 quadrats using as variables: number of live barnacles, number of dead empty barnacles, nearest-neighbour distances, total area covered by barnacles, and size of barnacle-free pocketsAn idea that number of littorines in an area may be correlated with micro-topographical variations in barnacle distribution is tested at the Bamfield Marine Sciences Centre, British Columbia.  The drawing shows the 2 major habitat classifications used in the study. The first represents a densely-packed, even-aged distribution of barnacles Balanus glandula that provides few refuges for littorines.  The second has small and large gaps created by missing barnacles, and other dead barnacles with wall plates intact, that provide refuges for littorines. Results show that the number of extra-small pockets alone provides the best predictor of littorine abundance.  These pockets may enhance littorine survival by providing refuge from wave action, thermal stress, and desiccation.  The authors suggest that manipulative experiments should now be done to test whether more dead barnacles or more micro-pockets will lead to increases in littorine abundance.  Boulding & Harper 1998 Hydrobiologia 378: 105.

NOTE  species used are Littorina subrotundata, L. scutulata, L. plena, and a few L. sitkana

NOTE the authors use image analysis to quantify micro-topography of barnacles within 100cm2 quadrats in relation to number of live and dead barnacles, nearest-neighbour distances, total area covered by barnacles, and size of barnacle-free pockets
 

photograph of barnacles Balanus glandula with littorines Littorina scutulata adults, and possibly many new recruits of the same or different species
Littorinid snails, adult and juvenile, scattered on
and within a cluster of barnacles Balanus glandula.
The llittorines appear mostly to be adult L. scutulata,
but the juveniles may be of mixed species 0.75X

 

 

photograph of littorines sheltering within spaces and dead barnacles in a barnacle bed
Littorina scutulata sheltering in spaces within a
cluster of barnacles Balanus glandula. There are
at least 11 littorines visible in the photo 1X
 
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Research study 9
 

photograph of several littorinid snails Littorina sitkanahistogram comparing survival of Littorina sitkana transplanted from high- and low-shore habitats to low- and high-shore habitats, respectivelyOn wave-sheltered shores the winkle Littorina sitkana exhibits a size-gradient in distribution, with larger individuals being found higher on the shore.  A popular explanation for this pattern is that crab and fish predators preferentially eat the larger individuals in the low intertidal areas.  In a study at the Bamfield Marine Sciences Centre, British Columbia, researchers additionally hypothesise that predation contributes to the shore-level size gradient, but indirectly in inducing earlier sexual maturation and/or reduced somatic growth in the low-shore snails. Field experiments involving mark-recapture and translocation to and from different intertidal levels, show that adults do, indeed, survive better at higher levels (see histogram).  Note that regardless of origin, individuals released onto the high shore survive better than individuals released onto the low shore. 

In other experiments, individuals taken from high levels and released there for 32d tend to stay, while individuals taken from low levels and released at high levels tend to move down the shore. The opposite is true for the reciprocal experiments.  Caging experiments over 54-d periods show that juveniles originating from a high-shore level do, indeed, grow faster than ones originating from a low-shore level, supporting the idea that low-origin snails start investing in reproduction at a smaller size than high-origin ones.  This idea is corroborated by evidence that reproductive structures such as penis and oviduct become apparent at a smaller size in snails collected from the low-intertidal region than ones collected from the high-intertidal area.  The authors conclude that L. sitkana responds to predation pressure by allocating resources differently to somatic growth and reproduction, thus contributing to their shore-level size gradient.  Rochette et al. 2003 J Sea Res 49: 119.

NOTE  the authors glue numbers to the shells and additionally mark them with green and orange fluorescent paint

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Research study 10
 

histogram comparing densities of littorinid snails Littorina sitkana and L. subrotundata at different heights on the shoreIn a related study by the same research group at the Bamfield Marine Sciences Centre, British Columbia, resource partitioning between 2 winkle species with overlapping distributions, Littorina sitkana and L. subrotundata is investigated.  Traditional studies on competition1 predict that species with similar needs will coexist through differences in competitive abilities or predation susceptibilities. Some species, however, appear to coexist in the absence of histograms comparing mortalities of tethered littorinid snails Littorina subrotundata and L. sitkana at high- and low-intertidal heightsapparent differences in these parameters. The premise in the present study is that species with similar needs will segregate spatially based on interspecific trade-offs between resource availability and mortality risk.  Through quadrat sampling the researchers first show that the distributions2 of the 2 species are not random; rather, L. sitkana predominates in the lower portion of the intertidal zone and L. subrotundata at the higher portion (see histogram upper Left).

Second, by tethering3  snails of both species at high and low intertidal heights, predation is shown to be almost non-existent at high levels but quite intense at low levels (see histogram on Right).

histogram comparing growth of littorinid species Littorina sitkana and L. subrotundata at different tidal heightsThird, “profitability” of habitats is determined by survivorship and growth of snails in cages set out in the field at high- and low-intertidal levels for a period of 3wk.  Both species survive well in the high-level cages, but L. sitkana survives better than L. subrotundata in the low-level ones.  More importantly, L. sitkana exhibits non-significant differences in survival at different tidal heights, while L. subrotundata survives significantly better in high-level cages than in low-level ones. Growth, as expected, is greater for both species at low-intertidal levels, but there are interspecific differences in rates of growth. Specifically, L. sitkana grows significantly more than L. subrotundata in the low-intertidal region but not in the high-intertidal region.

Thus, the species that experiences higher mortality across habitats, in other words, subrotundata, is more prevalent in the safer, high-intertidal habitat.  Overall, the authors conclude that their data provide support for an additional hypothesis, that spatial segregation and potential competitive coexistence “can occur in the absence4 of interspecific trade-offs in resource-acquisition ability or vulnerability to predation”.  The authors are careful to note that the actual mechanism producing the pattern of segregation seen may be competitive exclusion or a difference in behavioural preferences, or a combination of both.  Rochette & Grand 2004 Oikos 105: 512.

NOTE1 the present study tests 5 traditional hypotheses on mechanisms of spatial segregation: 4 based on differences in resource availability or differences in risk of mortality, and one based on a combination of resource availability and risk of mortality, with the latter habitat being most "profitable" in terms of better survival and growth

NOTE2  experiments are replicated at 2 sites, but only data for a single site are shown here

NOTE3 a 10-cm piece of monofilament line glued to the shell apex with the free end tied to a transect rope that is fastened to the shore in vertical orientation.  Snails are spaced at 30cm intervals along the rope to eliminate any interaction 

NOTE4 this work is an excellent example of how “integrative thinking” is increasingly being employed in contemporary studies of marine invertebrates, in this case, a relatively “simple” ecological pairing of species being used to test a set of 5 complex hypotheses.  To a naive reader, however, the final conclusion as quoted above seems at odds with the earlier defined research hypothesis found in the paper's Abstract: “that intraspecific trade-offs between resource availability and mortality risk can lead to spatial segregation of competing species”.  Is the interchanging of “intra” and “inter” in these 2 statements critical to our understanding, or just a misprint?

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Other genera

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Research study 1/2
 

Are introduced species of snails necessarily bad for a community?  No, not according to a study done by researchers from Bodega Marine Laboratory, California on 5 invader species and 2 native species on a mudflat in northern Puget Sound.  The chief invading species, the Asian hornsnail Batillaria attramentaria, is presently so abundant (densities greater than 1400 . m-2) that it creates a noticeable pavement of hard substratum on the mud surface that is inhabited by 2 other non-native species, the Atlantic slipper-limpet Crepidula convexa and the Asian anemone Diadumene lineata.  Both species are rarely, if ever, found free-living on the mud surface.  Batillaria’s shells, moreover, are used extensively as domiciles by 2 native hermit crabs Pagurus hirsutiusculus and P. granosimanus.  Finally, through use of exclusion cages, Batillaria’s presence is shown to facilitate the survival and abundance of 2 other non-native species, the mudsnail Nassarius fraterculus and the eelgrass Zostera japonica.  The first facilitation is thought to owe to selective grazing of certain diatoms by Batillaria, leaving other species to be eaten by Nassarius, while the second facilitation may result from bioturbation (“ecosystem engineering”) by Batillaria that enhances substratum conditions for growth of Zostera. Wonham et al. 2005 Mar Ecol Progr Ser 289: 109. Photographs courtesy Linda Schroeder, G. Holm, and Pacifc Northwest Shell Club, Seattle, Washington PNWSC.

NOTE  Batillaria attramentaria and the other 4 non-native species are thought to have arrived in the area along with oyster shipments from Asia in the late 1800s to early 1900s

NOTE  in absolute terms, the shell “pavement” represents a small proportion of available substratum, just 600cm2 . m-2 (10,000cm2) or 6%, and of this only 6% of the Batillaria shells is occupied by slipper-limpet and/or sea-anemone epibionts, but in ecological terms the effects could be significant

photograph of Asian hornsnail Batillaria attramentaria courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington
Batillaria attramentaria
photograph of dense gathering of Asian hornsnails Batillaria attramentaria, courtesy G. Holm, Pacific Northwest Shell Club, Seattle, Washington
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