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

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 begin to grow within the larva.  In the embryological development of advanced molluscs such as limpets or other snails, 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 to more complex 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 0

drawings of egg-jelly string and an egg of gumboot chiton Cryptochiton stelleriIn the apparent absence of any detailed west-coast research on reproduction in gumboot chitons Cryptochiton stelleri we turn to work by a Japanese researcher at the Akkeshi Marine Biological Laboratory in Hokkaido. Spawning occurs over a 3wk period in May, with presence of sperm stimulating egg release. Eggs are laid in large (1m x 2cm dia) spiral strings of jelly, with simultaneous production from each genital pore. Eggs (250µm dia) are yolky and orange-brown in colour. Trochophores hatch after 3d (Fig 7 pre-hatching) and within a day break free of the egg mass (Fig8). The author describes a kind of “metamorphosis” that begins within a day of free-swimming life, occurs gradually over a 2d period, and culminates in loss of the apical tuft and prototroch (propulsive ciliary band; see figures below). Shell plates are added incrementally, with the 8th and last not appearing until 8d post-“metamorphosis” (Fig17). The juvenile is now crawling, albeit “sluggishly”, the mouth is open, but the shell plates remain naked. The author appears not to have kept the juveniles much longer than about a week, not long enough for the mantle to overgrow the shell plates. Okuda 1947 J Fac Sci Hokkaido Imper Univ Zool 9 (3):267.

NOTE this may be stimulated by eddies from the northward flowing warm Kuroshio Current that makes its appearance in northern Japan during springtime. The Current has marked climatic effects in Japan, even it is said to the timing of cherry-blossom appearance, that marches northwards synchronous with the flow

drawing of trochophore larva of gumboot chiton Cryptochiton stelleri drawing of 5d swimming larva of gumboot chiton Cryptochiton steller drawing of 6d larva of gumboot chiton Cryptochiton steller drawing of 7d metamorphosing larva of gumboot chiton Cryptochiton steller  
Trochophore larval stage 4d post- fertilisation; red eyespots visible 5d swimming larva, just released from egg string 6d larva with seven shell plates visible
0.5mm L
7d larva beginning metamorphosis
with apical tuft disappearing

Developmental stages of Cryptochiton stelleri from
trochophore larva to newly metamorphosed juvenile

Metamorphosis is gradual, starts about 7d post-fertilisation and
takes 1-2d

drawing of 1d  juvenile of gumboot chiton Cryptochiton steller drawing of 5d juvenile  of gumboot chiton Cryptochiton stelle  
  Approx. 8d juvenile with 8th shell plate & eyespots gone 5d juvenile with paired tegumental sense organs visible on shell plates  
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

Research study 5

photograph series showing development of chiton Mopalia kennerlei from egg to 72h larval stageResearchers at Friday Harbor Laboratories, Washington confirms that the hull configuration on eggs of chitons Mopalia kennerleyi functions as a buoyancy device. The hull itself is less dense than the egg, and eggs with hulls removed will sink about 3 times faster than ones with hulls intact. Without the hull the egg is negatively buoyant, but in addition to its buoyancy effect the shape of the hull increases its frictional resistance and slows sinking (“parachute” effect). The early developmental stages tend to swim upwards towards the surface using their circumferential ciliated band (see photos); later, within a few days in laboratory culture, the larvae swim more and more feebly and eventually settle to the bottom of their container. The culture appears not to have been maintained past about 3d, several days prior to metamorphosis. Rebolledo & Emlet 2015 Invert Biol 134 (1): 31. Photographs courtesy the authors.