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

A littorine snail living high in the intertidal zone incurs risks from drying, thermal stress, UV exposure, dislodgment by waves, and other environmental hazards. 

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  Desiccation
  This section on life on the high shore is divided into topics of desiccation, considered here, and THERMAL TOLERANCE, SALINITY TOLERANCE, WAVES & CURRENTS, and VISION & OTHER SENSORY INPUTS, considered elsewhere.
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Tides on our shores have a semidiurnal unequal pattern.  For a littorine living on the upper wave-splash area of the shore, this means that several days or even weeks can pass between tidal immersions.  During these critical periods many Littorina species will close up and glue their opercular region to the substratum to prevent water loss.  Some species, most notably, Littorina scutulata, attach the tips of their shells to the substratum and wait in this vertical position for the tide’s return. On re-immersion, they quickly pop out of their shells and begin crawling about. No research specifically on this topic seems to have been done on west-coast Littorina species.

NOTE  lit. “half daily” L., referring to 2 high and 2 low tides per day.  The two sets of tides are unequal in height, that is, there is a “high” high tide and a “low” high tide, and the same pattern for the 2 low tides.  Commonly, and depending upon moon and sun cycles and other factors, the heights of the 2 low tides may overlap with the lower of the 2 high tides

photograph of a littorinid snail Littorina scutulata glued to the substratumNOTE  although apparently not researched for west-coast species, a similar behaviour in the British periwinkle L. littorea has been described.  As the tide recedes and a snail begins to dry in the sun, it secretse a sticky film of mucus between the outer arc of the shell’s lip and the rock surface.  When the film dries, and becomes harder and more brittle, the animal releases its foothold, completely retracts within the shell, and closes and seals the operculum.  In this species the dried mucus film is so fragile that on a gusty day the occasional specimen may be blown off its perch.  On casual observation the fastening of L. scutulata seems stronger than this, but the entire behaviour is something that may repay a researcher’s time to investigate.  Wilson 1929 Nature 124: 443; see also Newell 1958 J Mar Biol Ass UK 37: 229.

 


A Littorina scutulata has attached itself to the upper intertidal
part of a dock with a gummy, mucilaginous substance. The
operculum seems also to be sealed with this material 4X

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Research study 1
 

graph showing degree of desiccatory water loss in littorinid snails Littorina scutulata over several hours air exposureDrying over even comparatively short periods of air-exposure during low tide in summer can be severe, especially for smaller-sized individuals.  This is shown in laboratory studies with Littorina scutulata at the Bodega Marine Laboratory, California in which large individuals lose water less quickly than smaller snails at 19oC (see graph).  In the Bodega Head region a vertical size-gradient of L. scutulata exists on the shore, with a slight tendency for larger-sized individuals to be found at the top of the shore, and smaller-sized ones lower down.  Chow 1975 Veliger 18: 69.photograph of littorinid snails Littorina scutulata attached by mucilaginous glue to shell bits during aerial exposure

 

 

 


 



All of these Littorina scutulata have glued
themselves to the substratum, but note
that attachment points are varied - some
are even attached by their back-sides 0.5X

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Research study 2
 

histograms showing desiccatory water loss in field littorinid snails Littorina sitkana in different microhabitats, comparing large and small individualsThat similarly large relative water losses can occur in the field are shown in collections of Littorina sitkana from different microhabitats in Barkley Sound, British Columbia. Collections are made 3h into a mid-day low tide and desiccation is estimated from the amount of water required to re-hydrate the snails over 16h of re-immersion in seawater. Three things are apparent from the data (see histograms).  First, littorines can incur 30-40% tissue water loss over short periods on even a moderately warm day (20oC air temperature). Second, small snails of 2mm shell width, with relatively greater surface area, can experience 50% greater water loss than large snails of 4mm shell width.  Finally, selection by the littorines of barnacle and algal microhabitats over crevices and bare rock greatly ameliorates the risk of lethal desiccation for a snail of either size.  At the onset of low tide, selection of protective microhabitats by the snails occurs quite quickly.  For example, snails released on a hot day into small fenced arenas on the shore containing equal proportions of barnacles, algae, crevices, and bare rock select the barnacle and alga microhabitats within 30min.  On a cool day, however, the microhabitat preference is reversed, with many more snails remaining out on bare rock and in crevices, and the snails are slower in making their choices.  The study shows that littorines decrease their foraging time and increase their degree of habitat selectivity as conditions become more stressful.  Jones & Boulding 1999 J Exp Mar Biol Ecol 242: 149.

photograph of littorinid snails Littorina sitkana with L. scutulataNOTE  lit. “small place in which to live”, but with implication of small-scale differences in temperature, humidity, ground cover, food availability, sun exposure. As shown here for littorines, different microhabitats may be occupied at different times depending upon age and other conditions.  Generally, "microhabitats" are defined on scales of centimeters but, as we learn more of how different factors influence distribution and abundances of intertidal organisms, these dimensions are sure to shrink

NOTE  earlier translocation experiments at Lilly Point, Washington show that desiccation on juveniles and wave impact on adults can select against L. sitkana and exclude it from some beaches.  Behrens 1972 Veliger 15: 129.


Littorina sitkana (L.) and L. scutulata
during air exposure 3.5X

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