Chitons 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 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)

Researchers 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 findings 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

Chitons 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.

Close 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 these 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 27-mo experimental period.  Additonally, 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. 

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 (Hsp70) 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 alga³ 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 2 warm summers included in the 27-mo 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  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