subtitle for learnabout section of A SNAIL'S ODYSSEY
  Life on the high shore
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Thermal tolerance

  This section on life on the high shore is divided into topics of thermal tolerance, considered here, and DESICCATION, SALINITY TOLERANCE, WAVES & CURRENTS, and VISION & OTHER SENSORY INPUTS, considered elsewhere.
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Research study 1

map showing collection sites for littorinid snails Littorina keenae for study of thermal tolerance and heat-shock proteinsgraph showing thermal tolerance of littorinid snails Littorina keenae in relation to latitudinal distributionphotograph of Littorina keenae courtesy James Watanabe, Pacifi Grove, CaliforniaLittorina keenae inhabits high intertidal levels from mid-Oregon to southern Baja California and as a population is therefore subject to widely varying temperature regimes.  It has planktonic development with the potential for strong genetic homogeneity.  Researchers at the University of Guelph, Ontario investigate thermal tolerance, body size, and allele frequencies of heat-shock proteins HSC70 in collections of L. keenae at 9 sites along almost its entire geographical range (see map).  Results show only weak evidence for a latitudinal cline in thermal tolerance (see graph) and no evidence for a cline in allele frequencies at HSC70 (data not shown here).  The researchers do report a significant latitudinal size variation, with larger individuals being found further north, but with no significant interrelation with thermal tolerance.  Finally, HSC70 shows no genetic differentiation among the populations, supporting the authors’ predictions of high gene flow during the species’ free-swimming larval stage.  In view of the extent of this mixing, the authors conclude that the latitudinal size differences are likely phenotypic responses to local thermal environments. Lee & Boulding 2010 J Linnean Soc 100: 494. Photograph courtesy James Watanabe, Hopkins Marine Station, Pacific Grove, California SeaNet.

NOTE  defined as the temperature at which normal nervous function is lost, manifested in cessation of movement and loss of attachment to the substratum.  This is done with snails in test-tubes, heated in a water bath at 1oC per min until they display the heat-coma symptoms just described.  The water in the tubes is then decanted and the snails allowed to cool.  All snails survive the treatment

NOTE  the heat-shock cognate 70 gene (HSC70) is related to the family of heat-shock protein 70 (HSP70) genes known as molecular chaperones.  The function of the expressesd proteins is to bind to proteins damaged, for example, by thermal stress, and to help repair them.  There has been widespread interest in HSPs in west-coast invertebrates as, for example, in mussels: LEARN ABOUT MUSSELS:HEAT-SHOCK PROTEINS

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Research study 2

photograph of several species of littorinid snails courtesy Miller & Denny 2011 Biol Bull 220: 209histograms comparing different-coloured littorine models for extent of thermal buffering

There are many references in the literature to motile intertidal organisms regulating their body temperature by behavioral means, but none is as complete as a study on littorinids at the Hopkins Marine Station, Pacific Grove, California.  The authors use a mechanistic heat-budget model to compare predicted body temperatures in 5 littorinids under natural weather conditions (see photos on Left).  Both behavioral and morphological traits are used as parameters in the model.  Results show that while morphological features such as shell colour or sculpturing contribute only slightly to temperature regulation (0.2-2oC; see histogram for colour data), behavioral traits photographs showing how littorinids glue themselves to the substratum under stressful thermal conditions, courtesy Miller & Denny 2011 Biol Bull 220: 209can lower body temperature by as much as 3-5oC. Such behavioral features include removing the shell from the substratum (see photos lower Left) and re-orienting the shell in relation to angle of the sun.  The former strategy, that of standing the shell on edge, done by all of the species used here, can maximally lower body temperature by 3.5 oC.  Cooling effects from this particular strategy come from a combination of exposing greater surface area for convective heat exchange and raising the shell into faster-flowing air over the rock.  The functional significance of these different behaviours is not known, but the authors think that under conditions of extreme high temperatures they could be critically important.  Warming or cooling the body by crawling into the sun, or into water or crevices, as done by other animals such as insects and reptiles, might be a nice follow-up behavioral study to do on live snails.  Miller & Denny 2011 Biol Bull 220: 209. Photographs courtesy the authors.

NOTE  the model used is adapted from that employed by the same research group for study of heat flux in limpets Lottia gigantea: see LEARN ABOUT LIMPETS & RELATIVES: LIFE IN THE INTERTIDAL ZONE: TEMPERATURE STRESS.  The main modification to the original model is the addition of behavioral components relating to change in area of conduction, surface area for convection, and projected area facing the sun depending on the modeled shell orientation and foot position.  Models are silver-epoxy-filled casts of each species, calibrated against thermocouple-implanted live snails both in the field and in the laboratory.  Correspondence between measured and predicted temperatures for model shells set out in the field is 0.2oC for a crawling snail and 0.02oC for one resting on the substratum with foot withdrawn.  Colours tested are black, green, brown, and white

NOTE  the species are Littorina scutulata, L. plena, L. keenae, L. sitkana, all west-coast species.  A 5th west-coast species, L. subrotundata, is not included for reasons of its morphological similarity to L. sitkana.  An additional species used in the study, Echinolittorina natalensis, inhabits the Natal region on the east coast of South Africa.  The researchers include it to provide an ornamented shell for comparison with the smooth-shelled L. scutulata and L. plena

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Research study 3

In areas of southeastern Alaska, intertidal winkles Littorina sitkana are seasonally subject to freezing temperatures, raising the question of whether a mechanism such as increased free amino-acid pool to increase their freeze tolerance has evolved. This issue is investigated1 by researchers from Louisiana State University and Alaska Fisheries Science Center who expose winter and summer winkles to temperatures2 down to -12oC for 5h or 10h, record mortality, then assay for free amino acids3. Results show that winkles are more tolerant of freezing temperatures in winter than in summer, as the authors predict, and the free amino-acid pool is significantly more concentrated in winter than in summer (by about 30%, including taurine and glycine). Stickle et al. 2015 J Exp Mar Biol Ecol 463: 17.

NOTE1  the authors also include 2 whelk species Nucella lima and N. lamellosa in the comparison. These live in the mid- to low-intertidal zone, down to shallow subtidal areas, but are only distantly related to winkles. Perhaps more relevant would have been to compare Alaska winkles with lower-latitude conspecifics, for example, L. sitkana in Washington or Oregon. Even more interesting might have been to compare high- versus low-shore L. sitkana at the Alaska site, as recent studies have suggested that vertically separated populations of L. sitkana may be genetically distinct ecotypes. As it is, the study appears to be of an “apples and oranges” type

NOTE2  temperatures as low as -13oC have been recorded in February in mid-intertidal areas at the study site in Alaska

NOTE3  these colligative osmolytes effectively lower the freezing point of the hemolymph and tissue fluids

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