title for learn-about sections for chitons in A SNAIL'S ODYSSEY
  Reproduction
 

Trochophore larvae of chitons are primitive in comparison with the veliger larvae possessed by advanced molluscs such as bivalves and snails.  The trochophore lacks a head with associated tentacles, lacks a velum for feeding and locomotion, lacks a shell, does not undergo torsion, and has a gradual kind of metamorphosis in which shell plates grow within the larva.  In the embryological development of advanced molluscs such as neogastropods, a trochophore form is present, but only as an encapsulated transitional stage leading to the free-living veliger tage.  The implication of this is that correlative with evolution of more advanced adult form in molluscs, there has been an evolution of larval form.

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  Morphology of eggs & larvae
 

Topics on chiton reproduction include morphology of eggs & larvae, considered here, and GONADAL GROWTH & SPAWNING, BROODING, and SETTLEMENT & METAMORPHOSIS considered in other sections.

 
Research study 1
 

photograph of lined chiton Tonicella lineataphotographs showing early development of a chiton Tonicella lineata


















Experiments at Friday Harbor Laboratories, Washington show that after 4-5d in laboratory culture at 12°C the larvae of Tonicella lineata become competent and begin to seek out a place to settle.  As in many molluscs and echinoderms, the urge to settle may be the product of diminishing energy reserves and actual physical mass of developing shell plates/ossicles that the larva has to carry.  In the laboratory a sure inducement for larvae of Tonicella lineata to settle is contact with coralline algae such as Lithothamnion and Lithophyllum, or with bits of ceramic roofing tile soaked in extracts of these algae.  Contact induces a swimming/crawling behaviour in the larvae and, within 6h at 12°C, the larvae are attached and metamorphosing.  Shortly after, shell plates become visible.  Within 1d the juveniles are crawling about looking for diatom and bacterial scums on which to feed.  Barnes & Gonor 1973 Mar Biol 20: 259.



 

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Research study 2
 

photographs of developing stages of a black leather chiton Katharina tunicataBlack-leather chitons Katharina tunicata on the shores of central California appear to have a development similar to that described in the above Research Study.  After about 6d in laboratory culture the trochophores, obtained from May/June spawnings, settle.  Metamorphosis is complete within 1d.  Of a variety of substrates tested in the laboratory, only the crustose coralline alga Lithothamnion induces high percentages of settlement and metamorphosis.  GABA is highly effective in inducing settlement, just as it is for abalones.  Rumrill & Cameron 1983 Mar Biol 72: 243.

NOTE  settlement-inducing effects of the neurotransmitter substance Gamma-Amino-Butyric Acid are considered elsewhere in the ODYSSEY: LEARN ABOUT ABALONES & RELATIVES: REPRODUCTION

 
Research study 3
 

drawing of trochophore larva of a chiton Katharina tunicatadrawing of ocellus of a chiton Katharina tunicata showing details of pigment and sensory cellsStudies at Friday Harbor Laboratories, Washington show that the trochophore larva of Katharina tunicata is featureless save for an apical tuft of sensory cilia, an equatorial band of cilia known as the prototroch and involved in locomotion, and a pair of ocelli (see drawings).

Each ocellus is ovoid in shape, 18-26µm in size, red in colour, and consists of 8 pigment cells and 1 sensory cell (see drawing on Left).  At their outer ends the pigment cells form a cup lined with microvilli. Within the cup is contained a single sensory cell, also with microvilli, and also bearing a single cilium with a 9x2 arrangement of microtubules.  Based on its structure, that is, a sensory cell screened by pigment cells, the author suggests that the ocelli are likely photoreceptive; however, the presence of abundant microvilli and a cilium suggests additional functions, perhaps mechanoreception and/or chemoreception.  The bilateral position of the ocelli might permit a degree of directional perception.  Rosen et al. 1979 Veliger 22: 173.photograph of chiton Katharina tunicata

 

 

 

 

 

 

Black leather chiton Katharina tunicata 1.2X

 
Research study 4
 

photographs of cupule structures on the eggs of chitons Lepidochitona fernaldi and Mopalia lignosa, as well as a photograph of Lepidochitona dentiensphotograph of egg of a chiton Mopalia ciliataphotographs of eggs of chitons showing cupule structure Most chiton eggs are covered with tiny external projections called cupules (see photo upper Left).  The function of the cupules has long been debated, but studies on Mopalia spp. at Friday Harbor Laboratories, Washington indicate that one consequence of the cupules is that sperm are directed to specific regions of the eggs during fertilisation.  The cupules in M. lignosa, a free-spawning species, are open to the outside.  Each has 7 channels through which the sperm penetrate (see photo far Right).  In comparison, the eggs of Lepidochitona fernaldi and L. thomasi, both brooding species, have cupules that are closed (see photo on Right).  Sperm in these species enter the egg via micropores in the egg covering or hull. Another function of the cupules relates to buoyancy.  A comparison of sinking rates of chiton eggs show that the eggs of free-spawning Mopalia species, with open cupules, sink 6 times more slowly than the eggs of the brooding species Lepidochitona fernaldi, which has closed cupules. Egg densities of the 2 species are about the same. Buckland-Nicks 1993 Biol Bull 184: 269; Buckland-Nicks 2008 Am Malacol Bull 25: 97. Photograph of Leptidochitona dentiens courtesy Rian Dickson and the Royal British Columbia Museum, Victoria.

NOTE  lit. “cup or cask” L.

 

Eggs of brooding chitons, such as Cyanoplax fernaldi (bottom
Left), have closed cupules and the egg surface appears flat

 
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