title for learn-about sections for chitons in A SNAIL'S ODYSSEY
  Ecology
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  Orientation & homing
  Topics of interest in the ecology of chitons include orientation & homing, considered here, and ROLE AS HERBIVORES, GENETIC PATTERNS, and COMPETITIVE INTERACTIONS, considered in other sections.
 

photograph of a homing chiton Chiton tuberculatus in Jamaica

For almost a century scientists have been interested in whether chitons home, that is, return to the same spot on a rock after feeding excursions.  Some early studies report a “feeble kind of homing phenomenon” with reference to the tropical species Chiton tuberculatus, but responses to light or dark may have been involved.  However, later research on Australian, Indo-Pacific, and Caribbean species reveals clear evidence of homing, and the behaviour has been described in at least one west-coast species. Crozier 1921 Am Nat 55: 276.

 

 

 

 

A homing chiton Chiton tuberculatus in its "home" on fossilised
coral rock in Jamaica. Note how the chiton fits tightly into its defined
space on the rock. This requires correct anterior-posterior orientation
following return from feeding excursions. Occupation of a tightly fitting
homesite in this tropical species probably serves to conserve moisture 2X

 

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Before getting into the west-coast literature on the subject, here is a list of possible mechanisms used in homing by chitons.  All but one have been actually tested, although not on west-coast species.  Read through them and identify the idea NOT yet tested, then CLICK HERE for explanations.   Ideas mainly from work on 2 non-west-coast species: Thorne 1968 Austr J Mar Freshw Res 19: 151 on Acanthozostera gemmata in Australia and Chelazzi et al. 1986 Mar Biol 95: 539 on Acanthopleura gemmata in Somalia.

Celestial navigation. 

Homesite acts as visual beacon. 

Olfaction. 

Topographical recognition and memory. 

Trail-following. 

Ask a nearby winkle for directions. 

Geotaxis. 

Random wandering. 

 
Research study 1
 

drawing of purported homing tracks and ranges of chitons Mopalia muscosaIn the Pacific Grove area of California chitons Mopalia muscosa are active at night when covered by the tides, and inactive during the day and when exposed by low tides at night.  Individuals occupy home sites and have home ranges that rarely overlap.  For example, during 2wk observation of marked individuals only 2 of 63 journeys involved entry of an individual into another’s home range.  While foraging, a chiton crawls no more than 50cm along an established pathway, then returns along the same pathway.  The pathways are notable, not just that they exist, but that they do not always coincide with the contours of the substratum.  Home sites may be occupied for up to 10mo and are often located near small patches of sand.  The study is valuable in its apparent demonstration of homing in a west-coast chiton (but see the contrary opinion in the first NOTE below) and, were homing to be confirmed in the species, a more detailed analysis of its mechanism and adaptive value would be justified.  Smith 1975 Veliger 18(Suppl): 57.

photograph of chiton Mopalia muscosaNOTE a similar study to this one, done at Marin County, California on Mopalia muscosa and M. ciliata over a 3-mo period, finds no evidence of homing.  Mopalia muscosa does, however, have a “home range”, defined by the author as occupation of an area of 1-m radius.  Fitzgerald 1975 Veliger 18: 37.

NOTE the advantage of homeward trail-following is obvious, in that it permits return to the homesite, but what advantage is there in outward trail-following?   The answer must lie in its effectiveness in gaining access to proven-good feeding grounds, with savings in time, energy, and risk.  If this is true, what happens when a feeding ground becomes exhausted, either through over-feeding by the population or by seasonal change?

Mopalia muscosa: to home or not to home?

 
Research study 2
 

graph showing lab recordings of movements of chitons Mopalia muscosa in relation to an artificial tide cycleStudies on laboratory-held Mopalia muscosa in Pacific Grove, California show that locomotory movements are less at simulated low-tide periods. Westersund 1975 Veliger 18 (Suppl): 70.

NOTE  this particular set of experiments is run in the dark.  There appears to have been no attempt to synchronise the simulated tidal cycle in the lab with the natural tidal cycle, which would seem to be important.  Some of the chitons used are recently collected, while others have been in the lab for 1-2wk

 
Research study 3
 

photograph of chiton Cyanoplax hartwegii courtesy In contrast with the observed homing tendencies of Mopalia muscosa reported in Research Study 1 above, another chiton species studied in the Pacific Grove region, Cyanoplax hartwegii, appears to home only weakly, if at all.  In this study of 84 marked individuals monitored daily over a 2-wk period, 10 homed consistently and 22 homed partially.  However, when 11 of the best “homers” are displaced 10 cm from their home site, only 2 are back in their home sites after 24h, and only 1 is in its home site after 3wk.  Five weeks after the start of the study, 28 of the original 84 individuals are recovered.  Most have moved less than 1m from their original points of release.  This last observation supports the notion that chitons are highly sedentary whether they home or not.  Lyman 1975 Veliger 18(Suppl): 63. Photograph of Cyanoplax hartwegii courtesy Jim Watanabe, Hopkins Marine Station, Pacific Grove, California and calphotos.

Cyanoplax hartwegii 1X

 
Research study 4
 

Chitons have magnetite as a hardening agent in the cusps of their radulae, and there has been some research interest into its possible role in orientation. However, while magnetite has been implicated in orientation and migration in other organisms, including bacteria, birds, newts, salmon, and honeybees, similar research on chitons has been less than convincing.  For example, experiments involving attachment of magnets of different sizes and intensities to chitons Katharina tunicata, such that the radula is straddled by the two poles, produce no observable effect on orientation or direction of movement.  Unpublished experiments by Carefoot 1962 University of British Columbia.

NOTE in many of these organisms, experimental reversal of magnetic moments in magnetic particles, for example, in the heads of birds, leads to disruption or alteration of normal orientation.  Similar disrupting effects have also been induced in honeybees by attaching tiny magnets to their heads

 
Research study 5
 

schematic showing compass orientations of chitons Mopalia muscosa and Katharina tunicata in intertidal areas of central CaliforniaResults of later studies to determine if west-coast chitons orientate to the earth’s magnetic field are suggestive, but further work needs to be done.  Shown here are data for Mopalia muscosa and Katharina tunicata on various rocky shores in Central California.  The results for both species show statistically significant orientation to true north, but the biological significance of this is unclear.  Neither species is known to migrate (or even to move much at all), so an ability to orient to a certain compass direction is not likely to be useful for this.  Mopalia muscosa is shown in other studies to occupy home sites (see Research Study 1 above), but foraging expeditions are short, usually less than 0.5m, so it seems also unlikely that a magnetic compass would be used for this.  There also appear to be uncertainties about the data.  For example, no attempt appears to have been made to standardise the compass orientation of a beach. Thus, individuals aligned northwards could have been pointing down the slope of the beach, or up the slope of the beach, or at any conceivable crosswise angle.  It would have been more convincing were this to have been standardised.  The author notes another unusual feature of the data; namely, a seasonal difference in the strength of the behaviour: weaker in winter and stronger in summer.  Tomlinson 1980 Veliger 23: 167.

NOTE each line represents a compass direction for individuals found (each circle diameter = 5 individuals). Each set of data represents a different beach (3 for Mopalia and 2 for Katharina). Data for other experiments with chitons enclosed in aluminum, iron, and steel pans are not shown here
 
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