title for learn-about sections for chitons in A SNAIL'S ODYSSEY
  Feeding, growth, & shell repair
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  Topics in this section of the ODYSSEY include growth, considered here, and RADULA & FEEDING, DIETS, and SHELL REPAIR considered in other sections.
Research study 1

photograph of gumboot chiton Cryptochiton stelleri, ventral viewFew growth studies of west-coast chitons have been done.  In one of the earliest, in San Juan Island, Washington, researchers count 18-20 growth lines in the shell plates of gumboot chitons Cryptochiton stelleri, and estimate the age of a 13-cm individual at 20yr. MacGinitie & MacGinitie 1968 Veliger 11: 59.

NOTE  this is the largest chiton species in the world, reaching over 30cm in body length

photograph of the shell plates of a gumboot chiton Cryptochiton stelleri

Ventral view of gumboot chiton
Cryptochiton stelleri



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Research study 2

graph showing growth of gumboot chitons Cryptochiton stelleriIn a later study on growth on Cryptochiton stelleri at Cape Arago, Oregon, researchers are unable to verify the presence of growth lines in the shell plates.  Instead, they tag individual chitons and use them in a mark-recapture study.  The authors use live mass as a measure of size, rather than length, which they note is unreliable.  Out of all chitons marked and released, 228 are recaptured 398 times.  Seventy-one of these are recaptured after 1yr, 9 after 2yr, and 3 after 3yr.  The majority of recaptures are within 20m of the point of release, even after 2yr, confirming that gumboot chitons don’t move around much.  From their growth data, the authors estimate that a 1200g live mass individual is around 16yr of age. Palmer & Frank 1974 Veliger 16: 301.

NOTE  the tags are monfilament line bearing coloured beads in a numerical code, sewn through one side of the girdle

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Research study 3

photograph of shell plate of Cryptochiton stelleri showing annual growth linesgraph comparing size and age in chitons Katharina tunicata and Cryptochiton stelleriA nicely illustrated account of growth rates and aging in black-leather chitons Katharina tunicata and gumboot chitons Cryptochiton stelleri is provided by a researcher at the University of Connecticut using specimens collected from Oregon.  Simple counts of growth lines on the shell plates suffice for age estimates of K. tunicata, but not for C. stelleri (see Research Study 2 above).  For this species the researcher is not able to discern growth lines and uses instead the acetate-peel technique (see illustration on Left).  Major growth lines are formed during winter when food algae are less abundant, but in both species these major lines are interspersed with minor ones of uncertain causes.  The author is unable to discern latitudinal differences in growth rates in C. stelleri (California, Oregon, Washington: 1600km).  Growth in terms of body length and, more accurately, shell width, seems to be steady throughout the lifetimes of both species (see graph on Right).  Maximum age of K. tunicata is 17yr, while that of C. stelleri is 40yr.  Lord 2012 Malacologia 55 (1): 43.

NOTE  briefly, this requires that the shell plate be sliced lengthwise, the cut edge polished, and a piece of acetate applied.  When stripped away, the acetate retains an imprint of the growth lines on the shell and can be examined microscopically

NOTE  no similar comparison is made for K. tunicata owing to insufficient sample numbers

photographs showing shell shape in gumboot chiton Cryptochiton stelleri photographs showing shell shape in gumboot chiton Katharina tunicata
KATHARINA TUNICATA: shell shape & dimensions used in study CRYPTOCHITON STELLERI: shell shape & dimensions used in study