subtitle for learnabout section of A SNAIL'S ODYSSEY
  Reproduction & development
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  Patterns of development
 

This section on reproduction is divided into topics of patterns of development, considered here, and MATE SELECTION & COPULATION, DISPERSAL, and GENETICS, considered elsewhere.

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Planktotrophic larvae

 

West-coast littorinids either spawn egg capsules freely into the ocean or they attach the capsules to the substratum.  If the former, the eggs hatch to planktotrophic larvae, considered in this section; if the latter, they eggs hatch to CRAWL-AWAY JUVENILES, considered in another section along with protandric development.

The present section deals with the genera Littorina and Lacuna, with a final subsection describing the less closely related Trichotropis cancellata.

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Research study 1
 

drawings of egg capsules of littorinids Littorina sitkana and Littorina scutulataOf the several littorine species common on west-coast shores, 2 species Littorina scutulata and L. plena release planktonic egg capsules and 2 species L. sitkana and L. subrotundata lay benthic egg masses that hatch to crawl-away juveniles.  Buckland-Nicks et al. 1973 Can J Zool 51: 359; Behrens Yamada 1989 Mar Biol 103: 403.

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Research study 2
 

drawings of egg capsules of littorinids Littorina scutulata and L. plenaphotograph of littorinid Littorina scutulataThe 2 sympatric and highly similar species Littorina scutulata and L. plena have similar developmental patterns, but somewhat different capsule morphologies.  Actually, 2 types of capsule are produced by each species with one of the types being common to both species.  Littorina scutulata’s unique capsule has rims of unequal diameter, while L. plena’s unique capsule has rims of equal diameter.  The common capsule, produced by some females of each species, has only one rim and encloses about 5 eggs (range 2-11).  Littorina plena’s unique capsule is larger than the other types and has a mean of 19 eggs (range 6-47).  A large female L. plena produces about 1500 eggs in a season.  Murray 1979 Veliger 21: 469; Mastro et al. 1982 Veliger 24: 239; Hohenlohe 2002 Invert Biol 121: 25. Drawings are taken from the last reference cited.

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Research study 3
 

histograms showing spawning periods for littorinids Littorina keenae, L. scutulata, and L. plena in Bodega Bay, CaliforniaStudies at Bodega Marine Laboratory, California show that despite occupying markedly different heights on the shore, local littorinids Littorina keenae (= supratidal), L. scutulata (= high intertidal), and L. plena (= high intertidal) spawn during roughly the same period Mar-Sept/Oct. Chow 1987 J Exp Mar Biol Ecol 110: 69.

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Research study 4
 

photograph of Lacuna vincta with bull kelp Nereocytis luetkeana courtesy Linda Schroeder, PNWSCIn the Barkley Sound area of British Columbia herbivorous snails Lacuna vincta deposit eggs in winter/ spring and these hatch to planktotrophic veligers 2-4wk later.  Free-living planktonic life lasts for 7-9wk and settlement is mainly to the canopy fronds of large kelps Nereocystis luetkeana and Macrocystis integrifolia.  A large settlement occurs in April/May and a smaller one in summer/autumn.  After a brief period on the kelp the juveniles migrate down the stalks to the under-canopy area and into the intertidal zone.  Martel & Chia 1991 Mar Biol 110: 237. Photograph of Lacuna vincta courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

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Research study 5
 

photograph of an egg capsule of the littorinid Littorina scutulatagraph comparing length of cilia in the vela of larvae of planktonic-developing littorinids Littorin scutulata, L. plena, and L. keenae with those of a direct-developing littorinid Littorina sitkanaPlanktotrophic species such as Littorina scutulata, L. plena, and L. keenae, actually have a “mixed” development where the early developmental stages are contained within complex pelagic capsules, each in their own egg envelope (see photo of L. scutulata on Left). The larvae emerge from their envelopes, then break free of the capsule and begin swimming and feeding in the plankton. 

A question with respect to species such as Littorina sitkana that have benthic, encapsulated development is the means by which the intracapsular albumen is consumed.  A survey of several species collected around Charleston, Oregon and Monterey, California including L. scutulata, L. plena, L. keenae,and L. sitkana shows that the velar cilia are much longer in the first 3 planktonic-developing species than in the last benthic-developing species (see graph on Right). This makes sense because after the planktotrophic species hatch the velum is used for locomotion and for collecting phytoplankton food.  Its retention and use by non-planktonic encapsulated species such as L. sitkana is less clear, but possibilities include rotating the larva to enhance oxygen diffusion, feeding on nurse eggs, or stirring the intracapsular albumen-containing fluids to enhance consumption.  In the confined, more viscous environment of L. sitkana’s egg capsule, short cilia may be more effective than long cilia.  Through use of fluorescent labeled bovine-serum albumen and of ferritin, the author describes an additional function for the velum, that is, taking up protein by the larva endocytotically in its cilitated cells as well as in cells of the foot tissue.  Thus, retention of a relatively large-sized velum in L. stikana, although smaller than in planktotrophic species, may be favoured because of its role in uptake of intracapsular proteins. Moran 1999 Biol Bull 196: 229.

NOTE an iron-containing serum protein in vertebrates, representing one of the chief forms in which iron is stored.  Uptake of bovine-serum albumen is monitored with a fluorescencce microscope.  Ferritin is an electron-dense protein and can be identified in tissues using transmission electron microscopy

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Research study 6
 

graph showing relationship between number of eggs produced seasonally with shell length in the littorinid Littorina scutulataA female Littorina scutulata encloses 2-4 eggs within capsules during Mar-Oct and releases them into the plankton.  Studies at Friday Harbor Laboratories, WA show that a female of "graph showing length of planktonic life to metamorphosis in the littorinid Littorina scutulata8mm shell length may produce 1000 eggs in a season, while one of 10mm may produce 10,000 eggs (see graph on Left).  In laboratory culture, the eggs hatch to veliger larvae within 7-9d and the larvae swim freely for several weeks before settling and metamorphosing at about 28d post-hatching (see graph on Right).  The study is the first to document metamorphosis of Littorina in the laboratory.  Life span of the adult is about 7yr.  Hohenlohe 2002 Invert Biol 121: 25.

NOTE the larvae are fed on mixed rations of microalgae Isochrysis galbana and Rhodomonas lens, at 12-14oC. Four rations are tested, but only the one yielding best growth and metamorphosis is shown here: 105 cells . ml-1

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Trichotropis cancellata

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Research study 1
 

drawing of snail Trichotropis cancellat to show "hairy" spinesdrawing of male  Trichotropis cancellata showing water flow through mantle cavity and location of penisA related family to the littorinids is F. Capulidae, with several unusual members including the hairy snail Trichotropis cancellata. An investigation at Friday Harbor Laboratories, Washington discloses that it is a protandric hermaphrodite, with all individuals starting off as male, then transforming to female at about 2yr of age.  Interestingly, during the process of transformation and even when functioning as a female, a penis is present and is retained until death.  During the change, the testes transform into ovaries and the ductings leading from them to the testes atrophies; however, the author notes the presence of sperm in some individuals that are preparing to lay eggs, so further work is needed to resolve whether self-fertilisation may occur.  The author does not witness copulation in any of these laboratory specimens.  In Feb-May eggs are deposited in capsules, each containing up to 100 eggs.  Development is not followed by the author after this time.  Based on survival of adults in the laboratory, the life span is 2-3yr.  Yonge 1962 Biol Bull 122 (1): 160.

NOTE  as an adult Trichotropis cancellata is both a suspension-feeder, filtering planktonic food from the water being pumped through its mantle cavity and ctenidium (see drawing above Right), but is also a kleptoparasite, or food thief, of several species of tubeworms. As a juvenile, it mainly feeds by kleptoparasitism. A description of this behaviour can be found elsewhere in the ODYSSEY: LEARN ABOUT TUBEWORMS: FOOD & FEEDING: THEFT OF FOOD BY SNAILS

NOTE  lit. “first” “male” G.  Protandry is much more common in invertebrates than protogyny and the explanation is obvious.  Small males need produce only a small amount of sperm that goes a long way.  Transformation to a female at a larger size allows for the production of eggs that are large and full of yolk to provide energy for development of the embryo, in the case of Trichotropis, through to hatching to a feeding veliger larva

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Research study 2
 

A more recent study of development of Trichotropis cancellata at Friday Harbor Laboratories provides useful information on larval growth and metamorphosis, and includes several excellent scanning e-micrographical views of the processes.  Eggs are deposited on the sea bottom within capsules that at 10oC hatch after 8wk to free-swimming veliger larvae.  The larvae feed on  plankton for about 5wk before settling and metamorphosing.  Metamorphosis occurs in response to inducer chemicals emanating from different tubeworm species; in the absence of an inducer species, metamorphosis is delayed.  Loss of the velum at metamorphosis, used by the researchers as one indicator that metamorphosis has taken place, interrupts feeding only briefly.  This is because the mechanism for parasitism of food from a tubeworm host is already present in the larva (see Middle photo below).  The mechanism employed in food-stealing is a greatly extensible ciliated lower lip known as a pseudoproboscis that can be inserted into the mouth of a host tubeworm to divert its food stream into its own mouth. The authors’ data show that the pseudoproboscis begins to form in the larva about halfway through its development (see Left photo below).  Within only a few hours or a day following metamorphosis the tiny juvenile  Trichotropis is capable of stealing food from a host worm (see Right photo below), and the authors suggest that at this early stage of life it is likely an obligate behaviour.  Later, at a larger size, the snail becomes capable of suspension-feeding, using its single ctenidium as a straining device.  At such time food-stealing is relegated to a facultative but nonetheless nutritionally important behaviour.  The authors discuss possible “which came first” scenarios, and conclude that induction of metamorphosis by tubeworms may have been key to evolutionary selection for formation of the pseudoproboscis prior to larval metamorphosis (see 1st and 2nd photos below).  Parries & Page 2003 Can J Zool 81: 1650.

NOTE  these include tubeworms Serpula columbiana, Schizobranchia insignis, and Spirorbis sp.  Substrata tested that fail to induce settlement include organically filmed pebbles, coralline alga Bossiella sp., and juvenile barnacles Balanus glandula attached to shell fragments

 
photograph of veliger larva of snail Trichotropis cancellata with close view of pseudoproboscis photograph of 9h post-metamorphic juvenile snail Trichotropis cancellata showing details of head photograph of newly metamorphosed snail Trichotropis cncellata feeding
Metamorphically competent veliger larva showing details of pseudoproboscis with its extra-long cilia for transporting food particles to the mouth Detail of head of 9h post-metamorphic juvenile showing relationship of pseudoproboscis and mouth. The arrows indicate previous location of velum Newly metamorphosed juvenile using its pseudoproboscis to feed from a tubeworm. Note phytoplankton particles in the food stream and inside the snail's gut
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