subtitle for learnabout section of A SNAIL'S ODYSSEY
  Life on the high shore
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Vision & other sensory inputs


This section on life on the high shore is divided into topics of vision & other sensory inputs, considered here, and DESICCATION, THERMAL TOLERANCE, SALINITY TOLERANCE, and WAVES & CURRENTS, considered elsewhere.

The first two Research Studies deal with vision in Atlantic-coast species, iincluded for generating ideasfor research on west-coast forms.

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Research study 1

drawing of eye of Littorina littorea, an Atlantic-coast speciesLittorina spp. have well-defined eyes and the question has arisen many times as to whether they are capable of resolving images.  Close view of the structure of the eye of L. littorea reveals a lens, a kind of cornea made of transparent collagen fibres, a non-adjustable pupil, a non-reflective pigment layer, and a retina composed of 2 types of sensory cells.  Based on the physical properties of the lens the author suggests that sharp images can be formed in air, and possibly also in water.  As for accommodation or focussing, the author suggests that this may also be possible through muscular distortion or movement of the lens, although it is not clear where these muscles are or how they might operate.  In other publications the author has proposed that the eye may be able to resolve topographical features enabling shoreward or seaward orientation, but later studies suggest that movements up and down the shore photograph of Littorina scutulata close view showing eyes and tentaclesby winkles are more likely a response to general dark and light perception rather than to actual image resolution. Squint your eyes on any beach and note how light the seaward direction appears in comparison with the shoreward direction. No research appears to have been done on vision in west-coast littorines. Newell 1965 Proc Zool Soc Lond 144: 75. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE  this is an Atlantic species, much larger than west-coast littorines, but morphologically and ecologically similar, except that it occupies the lower part of the shore rather than the upper part and is active mainly when immersed

Littorina scutulata with Right eye visible

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Research study 2

model of eye of littorinid snail Littorina littoreaInterestingly, research in Sweden on the eye of Littorina littorea based on accurate geometric modelling of the eye and examination of images formed by isolated lenses suggests that the eye can theoretically produce a crisp and apparently aberration-free image, but only in water and only during the day.  In air the images are focused distal to the retina, and a muscle-based system would be necessary to move or distort the lens for focussing.  The author shows, in fact, that the optical image focused by the lens is much finer, down to 1.4µm in diameter, than the retinal sensory cells are able to sample. Receptor-cell diameters are much larger at 4µm.  Consequently, the eye is unable to use all the information produced by the optical components, and the image presumably is slightly blurry, that is, as a human eye would see it.  In practical terms, the eye would allow L. littorea to resolve an object of its own size of 2cm at a distance of about 70cm.  The author suggests that the eyes are “good enough” to find food, withdraw from predators, and locate mates, but how this might work is unclear.  Chemotactile information sensed by the tentacles would doubtless be more important for perception in all these regards.  Seyer 1992 J Exp Biol 170: 57.

NOTE  another Atantic-coast species L. irrorata found in the southern United States is shown to have eyes designed for vision in air.  Unlike L. littorea, this species inhabits the high part of the shore and actively grazes during low-tide periods when emersed.  Hamilton et al. 1983 J Comp Physiol 152: 435

NOTE  littorines also perceive and orientate to polarised light, but how this might be used by them is not known

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Research study 3
  photograph of Littorina keenae courtesy James Watanabe, Hopkins Marine Station, Pacific Grove, CaliforniaAn early study in the Pacific Grove region of California on the function of the cephalic tentacles in Littorina keenae suggests that they are used for righting, sex recognition, and following mucous trails.  The author uses tentacle extirpation to assess function and, although the operations are noted as leading to good recovery and resumption of normal behaviour, there remains the question of possible associated effects of anaesthesia and trauma.  No nerve cutting, sham operations, or other controls are included.  Unilateral tentacle-removal results in the test subjects crawling in circles.  Peters 1964 The Veliger 7: 143. Photographs courtesy James Watanabe, Hopkins Marine Station, Pacific Grove, California SeaNet.



Littorina keenae, photographed at
Pacific Grove, California 2X

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