title for learn-about sections for chitons in A SNAIL'S ODYSSEY
  Adaptations to intertidal life
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  Light, desiccation, & temperature stress
  This hodge-podge selection of topics relating to intertidal life in chitons includes light, desiccation, & temperature stress, considered here in separate sub-sections, and TENACITY IN WAVES, TOLERANCE OF FRESHWATER, and GAS EXCHANGE, dealt with in other sections.
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Light

 
Research study 1
 

photograph of lined chiton Tonicella lineataChitons are generally photonegative and tend to hide under seaweeds and overhangs of rocks during daylight.  The photoreceptors, or aesthetes, are embedded in the shell valves and have tubular connections to the shell surface.  Studies on species of chitons Mopalia spp. and Tonicella lineata collected from Monterey Bay, California show that the aesthetes are of 2 sizes, with micro-aesthetes branching from mega-aesthetes.  Tests on Mopalia lignosa using a small light shining on different parts of the upper surface show clamping down responses or movement away from the light depending on the direction of illumination.  Other tests on function of the aesthetes in which the shell valves and girdle are mechanically vibrated or exposed to different chemical stimulation from algae and extracts from sea stars Pisaster ochraceus are inconclusive. The author concludes that the aesthetes are probably not drawing showing experimental devices used in assessing chemical sensitivity in chitons Mopalia sp.acting as either mechanoreceptors or chemoreceptors.  Omelich 1967 Veliger 10: 77.


NOTE
one clue as to their function comes from the observation that old chitons with worn and encrusted valves tend to be indifferent to light.  There are anecdotal reports of chitons orienting to moonlight, which if true, suggests that the aesthetes are quite sensitive

NOTE the author uses a clever design in which open-ended glass cylinders are glued symmetrically onto either side of the upper surface of a chiton, with seawater being added to one and seaweed or sea-star extracts added to the other, and then observing any change in direction of movement. The cylinders are of such a height that they extend above the water line and fluid is added just to the water level (to maintain even hydrostatic pressure)

 
Research study 1.1
 

graph showing body temperatures of chitons Katharina tunicata in open sun and shaded locationsIn San Juan Island, Washington the chiton Katharina tunicata  associates closely with the canopy-forming brown alga Hedophyllum sessile1.  In this region the alga’s canopy cover changes from about 60% in winter to 90% in summer.  Experiments2 involving addition of bird-exclusion and shading screens3 show that the association owes to selection of shaded microhabitats by the chiton, and not to protection from predators, such as seagulls, or presence of a potential food.  The author notes that sea stars Pisaster ochraceus represent the chief predators of Katharina in cageless control plots and in shade-treatment cages, but not in the enclosed bird-exclusion cages, which are inaccessible to thesea stars.  In summer, 33% of Pisaster’s diet is made up of Katharina and, in winter, 17%.  Data on seasonal changes in levels of heat-shock proteins (Hsp70) indicate that the chitons experience sublethal temperature stresses during spring and summer (more on this subject can be found in the section on Temperature Stress below).  During these stressful periods Katharina’s intertidal distribution appears to be limited by the availability of shady refugia.  Note in the body-temperature graph (above Left) the marked effect that shade, whether naturally created by Hedophyllum or within the “artificial"-shade cages, has on mitigating temperature effects in Katharina. Burnaford 2004 Ecology 85: 2837.

NOTE1 this canopy-forming perennial kelp delineates a specific level in the low intertidal zone of rocky beaches.  So important is it in community dynamics of the shore that it has been termed a “foundation species” and an “ecologically dominant species” by various authors

NOTE2 the data presentation given in the paper on numbers of chitons within each treatment over the nearly 3yr study period is complex and is not presented here

NOTE3 these are upside-down vinyl-coated wire-mesh baskets, fastened to the rock surface.  Other top-less baskets, presumably accessible to birds, serve as control treatments.  Baskets with vexar-mesh tops represent shade treatments to mimic the natural canopy-shading of Hedophyllum.  Holes are cut in the sides of all cages at rock level to allow the chitons ingress and egres

 
Research study 2
 

scanning electron-micrograph of the aesthete-canal structure in the shell valve of a chiton Mopalia muscosaResearchers from UC Los Angeles use thin sections of epoxy-embedded shell plates1 to  provide a detailed comparison of the aesthete canal system2 in 12 species of California chitons.  Their stylised view of shell plate of a chiton with internal aesthete-canal structure revealedfindings indicate that the canal system in a shell plate is highly interconnected, and that the system is not found in all species.  The large and small aesthete-canal systems are clearly visible in the shell in SEM view (Scanning Electron-Microscope) shown here for Mopalia muscosa on the Right. In that the aesthetes have associated refractor lenses and sensory cells, features described over a century ago, the authors think that they likely act as photoreceptors3; however, other functions such as mechanoreception, chemoreception, and periostracum secretion have also been proposed.  Fernandez et al. 2006 Veliger 49 (2): 51.


NOTE1
  the plates are first cleaned, boiled, bleached, and sonicated to remove organic matter within, then embedded in epoxy resin, dried, and thinly sliced.  The calcium carbonate is removed by immersion in 10% hydrochloric acid to produce the SEM image shown here

NOTE2  the canals contain the tiny photoreceptor organs, or aesthetes, described in the following NOTE

views of shell plates of a Florida chiton Acanthophora granulata showing ocelliNOTE3 recent studies on aesthetes in Florida-collected chitons Acanthophora granulata using X-ray microanalysis and X-ray diffraction techniques reveal that the ocelli actually have image-focussing lenses uniquely constructed of aragonite, a common form of calcium carbonate in molluscs.  The eyes are about 70µm in diameter with pupal diameters of 40µm.  The lenses project an image onto a retina containing about 180 photoreceptors with an angular resolution of 9-12o.  The eyes have 2 refractive indices, one of which appears to function in air; the other, in water.  A chiton will respond to the sudden overhead appearance of a black disc with a typical shadow response, in which the mantle girdle drops suddenly onto the substratum.  The authors remark that if an image is resolved within each ocellus, then spatial information of a a multiple-eye format as found in scallops is likely provided.  Alternatively, if the ocelli are integrated in the way that the ommatidia of a compound eye of an arthropod are, then a composite image could be formed between schematic drawing of a chiton ocellus with its parts identifiedocelli.  However, until more is known about the neural arrangement of the ocelli, the authors suggest the many ocelli in a chiton should be considered just be a highly redundant “alarm system” for detecting passing objects.  Speiser et al. 2011 Current Biol 21 (8): 665.
 
Research study 3
 

drawings of aesthete/canal systems in 7 genera of west-coast chitonsIn a follow-up study the above researchers provide additional data on aesthete morphology in 14 California chiton species and speculate on their use in chiton phylogeny. All 14 are nominally in the Family Mapaliidae and, while aesthete and canal structure in them appear to be somewhat diverse (see drawings for 8 genera; aesthetes, when shown, are represented as dots), the authors identify several homologous features that justify the inclusion of most species in this family.  At the same time, at least one species Plaxiphora aurata is sufficiently different in aesthete morphology to suggest that it be moved to a different family, and the species Tonicella marmorea and Dendrochiton lirulatus (not always thought to be mopaliids) are sufficiently similar to suggest that they too be reclassified. The authors provide many fine e-micrograph views of aesthete/canal systems in the shells of west-coast chitons and also include a good review of recent research done on world species of chitons.  Vendrasco et al. 2008 Amer Malacol Bull 25: 51.

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Desiccation

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Research study 1
 

photograph of chiton Mopalia muscosaChitons Mopalia muscosa on the shores of Marin County, California are often found with black-turban shells Chlorostoma funebralis clustering around them.  The author of the study suggests that the mutual benefits of conserving moisture may allow the chiton, in particular, to occupy somewhat higher intertidal positions that it could otherwise.  Fitzgerald 1975 Veliger 18: 37.

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Temperature stress

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Research study 1
 

diagram of the 5 treatment plots used in study of desiccation effects on chitons Katharina tunicatagraph showing effect of different shade/no shade treatments on body temperatures in black leather-chitons Katharina tunicataClose association of black leather chitons Katharina tunicata with the kelp Hedophyllum sessile may provide more than just a source of food.  Possibilities based on other herbivore/algae relaltionships include refuge from predators (“enemy-free space”) or harsh environmental conditions, or both.  In fact, studies at Friday Harbor Laboratories, Washington show that the main benefit for the chiton living in association with Hedophyllum may be protection from seasonal temperature stresses, even though they are sublethal.  The author employs a variety of experimental canopy structures, some with screen tops to exclude bird predators1, some with open tops to allow access to birds but open to the sun, and others with opaque roofs to provide shade but restrict birds.  A plot without canopies acts as a cage control, and an untouched Hedophyllum plot as a natural control. Numbers of Katharina in each treatment plot are recorded periodically over a 27mo experimental period.  Additionally, body2 temperatures of Katharina are measured under different conditions to assess the role of shade, whether beneath natural Hedophyllum or plastic-screen canopies, in reducing thermal stresses.  graph showing temperature effects on levels of heat-shock proteins in chitons Katharina tunicata

The results of this last experiment show, as expected, a significant reduction in body temperature under the 2 shade treatments (see graph upper Right).  That Katharina  physiologically responds to thermal stress is shown by increased tissue concentrations of heat-shock proteins (Hsp703) during low-tide conditions in spring/summer (see graph lower Right). The first rise in Hsp70 levels in springtime (April 2000) actually coincides with the first daytime low-tide series of the year. 

Results from the cage-canopy experiments are highly complex (not shown here), but show that neither birds nor Hedophyllum have a significant effect on chiton abundance.  This means that in the absence of the alga4 the chitons are finding sufficient food, likely in the form of diatoms or other encrusting algae.  Chiton numbers are as high or higher under the plastic shade canopies as under natural Hedophyllum canopies, indicating that refuge from abiotic stress (sunlight) is critical to survival of Katharina.  The statistical strength of the interaction is more evident in the two warm summers included in the 27mo study than in the one cooler summer (data not shown).  The study is interesting and emphasises the value of taking a fresh look at even the most traditionally “well-understood” intertidal association.  Burnaford 2004 Ecology 85: 2837. 

NOTE1  the emphasis on birds, such as seagulls, as predators is explained by the focus on environmental conditions when the chitons are in air.  Of course, when immersed, temperature and shade are not an issue, but aquatic predators including sea stars, crabs, octopuses, and perhaps fishes may be at work.  The author is able to identify and account for the extent of sea-star Pisaster ochraceus predation versus predatory birds through characteristic differences in the chiton carcasses scattered around the plots.  Interestingly, sea stars are found to play a greater role in chiton mortality than birds, yet the chitons will still cluster in areas of high abundance of Pisaster as long as Hedophyllum shelter is available

NOTE2  a micro-thermometer is inserted into the mantle cavity to record body temperature

NOTE3 the expression of Heat Shock Proteins (HSPs) is commonly used to assess stress in animals resulting from physical factors such as temperature, UV light, and other stresses.  HSP70 is actually a complex of these proteins but, for convenience, is expressed collectively as “Hsp70” by the authors.  More on HSPs can be found at LEARN ABOUT ABALONES & RELATIVES: PHYSIOLOGICAL ECOLOGY: HEAT-SHOCK PROTEINS, and LEARN ABOUT MUSSELS: LIFE IN THE INTERTIDAL ZONE: HEAT-SHOCK PROTEINS

NOTE4 Hedophyllum recruits are removed every 2-4wk from the treatment plots.  However, even if some are missed and eaten by the chitons it doesn’t detract from the author’s point that the role of mature Hedophyllum plants is primarily in providing shade, not in providing food

 
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