title for learn-about sections for chitons in A SNAIL'S ODYSSEY
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  Settlement & metamorphosis

Topics on chiton reproduction include settlement & metamorphosis, considered here, and GONADAL GROWTH & SPAWNING, MORPHOLOGY OF EGGS & LARVAE, and BROODING, considered in other sections.

Research study 1

photograph of chiton Mopalia ciliata courtesy Lovell & Libby Langstroth, CaliforniaLaboratory development of Mopalia ciliata in southern California follows the schedule:

0h               fertilisation
10-12h       gastrulation
24-48h       larvae break free of egg capsule and begin swimming
5-8d           end of free-swimming stage
8-16d         adult features begin to appear

The larvae exhibit photonegative behaviour when swimming. The author could not rear the larvae through metamorphosis.  Thorpe 1961 Veliger 4: 202. Photograph courtesy Lovell & Libby Langstroth, California and calphotos.




Chiton Mopalia ciliata 1.4X

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Research study 2

photograph of Linda Schroeder Northwest Shell Club, WashingtonStudies in the Pacific Grove area of California on Mopalia lignosa and M. muscosa show patterns of larval development similar to that described for M. ciliata above.   A swimming trochophore larva hatches within a day.  Dorsal ridges that are prelude to the shell plates and foot rudiment appear 4d after hatching.  The ciliated foot-flap is present at 5d, and settlement occurs within a day.  Metamorphosis, defined by appearance of shell plates and loss of the apical tuft, occurs at 7-8d.  The eyespots, so prominant in the larva, disappear completely about 8wk after fertilisation.  By 21d after metamorphosis the authors note radular movement, although feeding is likely to have commenced much earlier than this. Watanabe & Cox 1975 Veliger 18 (Suppl): 18. Photograph of Mopalia lignosa courtesy Linda Schroeder and Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE only the pattern for M. lignosa is shown here.  Mopalia muscosa differs in having a longer free-swimming period before settling and metamorphosing (13d s as compared with 7d for M. lignosa at 13-16oC)

Mopalia lignosa

  Developmental stages of Mopalia lignosa:
drawing of egg of chiton Mopalia lignosa
drawing showing fertilised egg of chiton Mopalia lignosa
Fertilised egg
drawing showing 1st cleavage in embryo of chiton Mopalia lignosa
1st cleavage
drawing showing 2nd cleavage in embryo of chiton Mopalia lignosa
2nd cleavage
drawing showing 3rd cleavage in embryo of chiton Mopalia lignosa
3rd cleavage
drawing of 3.5h stage of development of chiton Mopalia lignosa
Later cleavage 3.5h
early trochophore stage of chiton Mopalia lignosa
Early trochophore
drawing of hatching of trochophore larva in chiton Mopalia lignosa
Hatching at 19h
drawing of 1d trichophore larva of chiton Mopalia lignosa
1d trochophore larva
drawing of 4d trochophore larva of chiton Mopalia lignosa
2d trochophore with ocelli
drawing of 4.5d trochophore larva of chiton Mopalia lignosa
4d trochophore
drawing of 4.5d trochophore of chiton Mopalia lignosa
4.5d with mantle fold
drawing of early settlement stage of trochophore larva of chiton Mopalia lignosa
5d settlement commences
drawing of settling trochophore larva of chiton Mopalia lignosa showing foot flap5d showing foot flap
drawing of 5d juvenile of chiton Mopalia lignosa
5d juvenile, shell plates form
drawing of 7-8d juvenile of chiton Mopalia lignosa
7-8d shell plates
drawing of crawling juvenile chiton Mopalia lignosa
Crawling juvenile
drawing of side view of crawling juvenile chiton Mopalia lignosa
Same stage, side view
drawomg pf 24d juvenile chiton Mopalia lignosa
24d juvenile
drawing of ventral view of 24-d juvenile chiton Mopalia lignosa
24d juvenile, ventral view
Research study 3

photograph of chiton Mopalia muscosa courtesy Linda Schroeder, Pacific Northwest Shell Club, SeattleExperiments on temperature effects on the development of metamorphic competency  in chitons Mopalia muscosa at Friday Harbor Laboratories involve rearing of the lecithotrophic larvae at different temperatures.  Best metamorphic performance (50%) is attained with larvae reared for 17d at 11oC, then switched to 16oC.  Growth is most rapid at the warmer temperature, but survival is poor.  In fact, the author notes that most larvae in the experiments die before metamorphosing, so the results should be considered with this in mind.  Pechenik 1984 Int J Invert Repr Dev 7: 3. Photograph of Mopalia lignosa courtesy Linda Schroeder and Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE  metamorphosis, when it occurs in the experiments, is “spontaneous”, that is, it takes place in the absence of an inducer

Research study 3.1

drawing of larva of chiton Mopalia muscosadrawing of post-metamorphic chiton Mopalia muscosaA researcher at Friday Harbor Laboratories, Washington describes metamorphic changes in chitons Mopalia muscosa (see also Research Study 3 above).  In culture at 11oC, the trochophore larva settles after 9-10d (see figure on Left), and metamorphoses within 24h of settlement (see figure on Right).  Settlement is induced by exposing the competent larvae to rocks filmed with red alga Hildenbrandia sp.  Metamorphosis is characterised by loss of the locomotory prototroch and apical tuft, secretion of shell plates, appearance of spicules and chitinous hairs in the newly forming cuticle, and incorporation of lateral ciliated bands into the pallial grooves.  The author concludes that rather than being gradual, as described by some authors, metamorphosis can be clearly defined by these changes.  Leise 1984 Zoomorph 104 (6): 337.

Research study 4

graph showing recruitment of black leather chitons Katharina tunicata to beaches in northern Californiaphotograph of black leather chitons Katharina tunicata courtesy Russ Markel, BamfieldRecruitment of Katharina tunicata in areas around Trinidad in northern California is mainly through late summer/autumn, suggesting that larval settlement is in spring/summer.  Stebbins 1988 Veliger 30: 351. Photograph courtesy Russ Markel, Bamfield Marine Sciences Centre.





Black-leather chitons Katharina tunicata in
Barkley Sound, British Columbia 0.5X

Research study 5

graph showing growth of juvenile gumboot chitons Cryptochiton stelleri  in the laboratoryA research study at the Oregon Institute of Marine Biology, Charleston provides detail on larval growth and metamorphosis, and on post-settlement growth of gumboot chitons Cryptochiton stelleri.  Lecithotrophic larvae hatch at 2d post-fertilisation (11-12oC) and 3d later begin to develop shell plates (see photo series below).  The larvae are competent to metamorphose 3d from hatching (5d post-fertilisation), and can be induced to settle by exposure to aqueous extracts of coralline algae.  In the absence of an appropriate cue, metamorphosis can be delayed for at least 2mo in the laboratory. In culture the post-metamorphic juveniles eat diatoms and cyanobacteria off the bottoms of the culture vessels, as evidenced by grazing trails and presence of these food organisms in their fecal pellets. The author provides 8mo of growth data for 4 juveniles collected from red algae Cryptopleura sp. in the field, and fed in the laboratory on diets of Cryptopleura sp., green algae Ulva lactuca and, later at 1yr of age, on red algae Mazzaella splendens, a common food of the adults.  Lord 2011 J Molluscan Studies 77: 182.

NOTE  the species is not provided by the author 

photograph of 2d trochophore of gumboot chiton Cryptochiton stelleri photograph of 5d post-fertilisation juvenile of gumboot chiton Cryptochiton stelleri photograph of 4wk juvenile gumboot chiton Cryptochiton stelleri
Fertilised egg with outer hull, overall 600um diameter Early trochophore larva, 46h post-fertilisation, swimming 5d post-fertilisation, crawling, red eyespots, shell-plate rudiments visible 4wk crawling, feeding juvenile, shell plates have widened but still exposed