Reproduction & development
  After a general introduction to life cycles of clams, the sections are arranged alphabetically by genus.
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Research study 1
 

diagram showing stages in development of a clam

Sexes are separate in most bivalves and paired gonads lie in the upper part of the foot region intermixed with elements of the digestive system. For this reason it is difficult to obtain reliable measuremens of gonad size; hence, gonad indices are unavailable. Paired gonoducts release gametes into the exhalent water flow just before it enters the exhalent siphon. Fertilisation leads to the development of a veliger larva that swims and feeds in the plankton for several weeks. This transforms into a bivalved form known as a pediveliger, that is the settling stage. All developmental times shown in the schematic are, of course, temperature-dependent. Abraham & Dillon 1986 Life histories & environmental requirements of coastal fishes & invertebrates. Biol Rept 82 SOFTSHELL CLAM U.S. Wildlife Serv, Natl Wetlands Res Center, Lafayette, LA.

NOTE  about 4% of bivalves are hermaphroditic

NOTE  a measure of size of gonad relative to body mass, changes in which allow seasonal cycles of gonadal growth and spawning to be monitored

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Clinocardium nuttallii
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graph showing annual reproductive cycle in cockles Clinocardium nuttallii in Netarts Bay, OregonReproductive events in intertidal and subtidal cockles Clinocardium nuttallii in Netarts Bay, Oregon are compared by a researcher from Oregon State University, Corvallis.  Results show that seasonal reproduction is synchronised in intertidal and subtidal populations in the same area, and that all individuals spawn in summer and gonads are renewed through late winter/spring (see graph on Left).  Individuals are protandric hermaphrodites, spawning initially more as males up to about 2yr of age and then later as both sexes (see graph below Right). 

graph showing spawning ages of male and female cockles Clinocardium nuttallii in OregonFecundity is greater in subtidal populations than intertidal ones, suggesting that this habitat may be a type of refuge for adult breeding populations.  Ratti 1978 MSc thesis, Oregon State University, 105pp.

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Research study 2
 
photograph of a cockle Clinocardium nuttallii

Cockles Clinocardium nuttallii are a notable exception to the general rule of separate sexes in bivalves.  In these hermaphrodites, sperm and eggs mature at the same time.  Adults are reproductive at about 5cm shell length, or 2y in age, and spawning in the Puget Sound area is April-November.  Some authors believe that this “bisexuality” is advantageous where populations are in low density, but there is no evidence that self-fertilisation occurs.  Gallucci & Gallucci 1982 Mar Ecol Progr Ser 7: 137.

 

Annual growth lines on this cockle Clinocardium nuttallii suggest
that it has just reached reproductive maturity at 2yr of age 0.8X

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Lasaea sp.
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Research study 1
 

map showing distribution of planktotrophic versus direct developing bivalve species of the genus LasaeaThe bivalve Lasaea is a cosmopolitan, intertidal, crevice-inhabiting genus.  All species are simultaneous hermaphrodites, with development being either by planktotrophic larvae or directly by “crawl-away” juveniles.  The mode employed, however, depends upon geographic location.  Both modes involve brooding.  Interestingly, the planktotrophic developers employ cross-fertilisation, while the direct developers appear incapable of cross-fertilisation, and employ self-fertilisation. Thus, direct developers can reproduce in isolation. Without other knowledge, developmental mode of a species can be determined from egg-mass morphology and from prodissoconch structure (see photograph below).

Interestingly, species of Lasaea with a direct, "crawl-away juvenile" mode of development have a much greater collective geographic range than ones with a planktotrophic larval mode. The present study, done on preserved museum specimens, shows that only 33 of 152 world-wide populations examined in this study have planktotrophic larvae, while the remainder brood to crawl-away juveniles. All west-coast populations employa direct mode of development (see map). Atlantic Ocean and Mediterranean Sea populations are almost universally direct developers. Thus, in a unique kind of  “reversal”, cross-fertilising species with planktotrophic development are much less successful than species with direct development.  The common consensus of researchers is that the main distributive mode of Lasaea is likely by rafting, although byssus photograph of larval shell, or prodissoconch of clam Lasaea australisdrifting and transport in ship’s ballast waters cannot be ruled out.  Ó Foighil 1989 Mar Biol 103: 349;  see also Ó Foighil & Eernisse 1988 Biol Bull 175: 218 for information on genetic variation in west-coast populations of Lasaea

NOTE  the first, or larval, shell appearing in development

 

The prodissoconch of Lasaea australis, a planktotrophic developer, is characterised
by a well-defined umbonate hingeline and a double-demarcated appearance,
distinct from the same features associated with species with direct-development mode

 

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Nutricola tantilla
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photograph of clam Nutricola tantilla showing egg mass being broodedgraph showing allometric relationship between brood number (fecundity) and shell length in the clam Nutricola tantillaNutricola1 tantilla is a small protandric brooding species that inhabits shallow surface sediments in locales such as False Bay, San Juan Islands, Washington.  Individuals are male up to about 3mm shell length, then transform to females. Final female size is about 7mm length.  Embryos are brooded year-round between the inner gills and the visceral mass (see photograph). The egg mass is not flat; rather, it has a globular shape.  The brood mass is added to sequentially, resulting in a temporal spacing of juvenile release.  Thus, younger embryos are located more dorsally, closer to the gonopores, while older, larger embryos are situated more ventrally.  Dorsally, the brood mass is 3-5 embryos thick, while ventrally it is only 1-2 embryos thick.  The author investigates possible allometry2 in numbers of embryos brooded, based on how much room is available between the gills and viseral mass.  If the embryos are simply brooded along the gills, in other words, based on the surface area available, then brood number will increase with the square3 of adult length.  If, however, the embryos are packed 3-dimensionally rather than linearly, then the number able to be accommodated will scale isometrically with adult volume, which itself scales as a function of the cube of adult length. 

Results show that the number of brooded embryos increases in proportion to the cube of adult length (slope4 of 3.02); hence, adult size does not act as a morphological constraint on reproductive output, as it would if the embros were to be restricted to the maternal gill-surface area. Kabat 1985 J Exp Mar Biol Ecol 91: 271.

NOTE1  formerly Transennella tantilla

NOTE2  gill surface area increases as a function of adult length squared

NOTE3  the author tests the so-called allometry hypothesis in invertebrate brooders, which states that larger adults can theoretically produce more young than they have space to brood, and consequently only small adults can brood all the embryos that they are capable of producing.  This idea is proposed for adults brooding on external surfaces, which would, indeed, become limiting with increase in adult size.  These allometric constraints would theoretically limit such brooders to smaller adult sizes than closely related freely spawning species.  While certainly true for external brooders, the present study shows that similar constraints may not apply for internal brooders like Nutricola

NOTE4 based on results shown in the graph, this equates to a 10mm long female producing about 1000 embryos.  The actual number differs from this based on the equation brood number = 1.26(shell length)3.02, which works out closer to 1300 embryos

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Nuttallia obscurata

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map of southern Vancouver Island, B.C. showing site at Saanich Inlet for study of varnish clams Nuttallia obscuratagraph showing relationship between mass of soft tissues and shell length in varnish clams Nuttallia obscurata in 2 different locations
Collections of varnish clams Nuttallia obscurata from Barkely Sound and Saanich Inlet, British Columbia show considerable geographic differences in sex ratios and condition indices (see graph on Right), and small but significant differences in fecundity (graph below Right). Spawning in both locations occurs approximately between June-September.  Pelagic duration of larvae cultured in the laboratory ranged from 3-8wk depending upon temperature and salinity.  Sexual maturity is reached comparatively early and this, combined with the lengthy planktonic phase, may explain the success of varnish clams at colonising west-coast shores so quickly following their introduction into British Columbia in the early 1990s.  Dudas & Dower 2006 Mar Ecol Progr Ser 320: 195.

graph showing relationship between fecundity and shell length in 2 areasNOTE  female:male:hermaphrodite proportions are 46:49:5 in Barkely Sound and 39:57:4 in Saanich Inlet.  The authors comment that such male-skewed sex ratios are generally uncommon in bivalves

NOTE  here expressed as dry mass of soft parts over shell length; in effect, a measure of the extent to which the clam fills its shell. The authors have plotted their data linearly, but the scaling relationship between mass and length would be curvilinear on an arithmetic plot, with a predicted slope of 0.67

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Panopea generosa
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Research study 1
 

histogram showing In subtidal sand-beds in southern British Columbia, geoducks Panopea abrupta of average live mass 0.8kg, equivalent to about 30yrs of age, spawn in June-July. The authors find ripe gonads in clams ranging in age from 7-107yr, indicating that Panope is capable of reproducing for over a century of life. In a total sample of several hundred individuals over 15 monthly collections, the authors could find no evidence of reproductive senility.  Sloan & Robinson 1984 J Shellf Res 4: 131.

NOTE  the species name abrupta has recently been replaced with generosa. However, until the change becomes commonly accepted, species names for geoducks in the ODYSSEY will correspond with what is used in the research publication

photograph of several geoduc bivalves Panope abrupta in a basketNOTE  determined from counts of growth lines in the shell using the acetate-peel method

 

 

 

 

A selection of geoducs Panopea abrupta ready for study or
perhaps for sale. Large clams often have their shell valves
bound with elastic. This reduces the metabolic cost of
holding shelll valves together by adductor-muscle contraction

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Research study 2
 

A recent study on larval dispersion of geoduck clams Panopea abrupta in a small bay off Puget Sound, Washington uses molecular-identification technology adapted from studies on marine zooplankton.  Results show that the larvae spend most of their several-week pelagic life at or near the ocean surface.  The capture of larger larvae in the inside and middle portions of the Harbor indicates at least some larval retention.  Two larval pulses are recorded, in March and in May/June.  The study is the first of its kind to be done on geoduck larval distribution and the first to use this marking technology on bivalve larvae.  Becker et al. 2012 J Shellf Res 31 (3): 711.

NOTE  the FISH-CS technique involves hybridization in situ with a fluorescently labeled species-specific DNA probe followed by sorting of the fluorescently labeled larvae in a large-particle cell sorter

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Protothaca staminea

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photograph showing native littleneck clam Protothaca stamineaIn Ladysmith Harbour, British Columbia littleneck clams Protothaca staminea spawn May-October.  Gonads can be seen in their earliest stage of development in individuals of 1mm shell length.  Sexes differentiate at 15-30mm or 2-3yr of age and animals are mature at 22-35mm.  Quayle 1943 J Fish Res Bd Can 6: 140.

 

 

Although imprecise, a count of the major growth
lines on this speciment gives an age of about 3yr

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Research study 2
 

Littleneck clams Protothaca staminea in Prince William Sound, Alaska spawn primarily in June, but there are sex-based differences during the remainder of the year.  Thus, females spawn from June-September, while males are reproductively active from June-January, with some additional spawning throughout other months.  Feder et al. 1979 Veliger 22: 182.

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Siliqua patula
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photograph of razor clam Siliqua patula courtesy Dave Cowles, Walla Walla University, WashingtonRazor clams Siliqua patula at Masat, Haida Gwai, British Columbia maure in late spring, with major spawning during July-Sept.  The juveniles grow about 16mm in shell length during the first year.  Bourne 1979 Proc Nat Shellfish Assoc 69: 21. Photograph courtesy Dave Cowles, Walla Walla University, Washington www.wallawalla.edu.

 

 

 

 

Razor clam in last stages of burial. One further
burial cycle will see the clam fully buried 3X

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Research study 2
 

graph showing seasonal appearance of mature ova in razor clams in Oregongraph showing larval growth in razor clams Siliqua patula
On beaches around Newport, Oregon, populations of razor clams Siliqua patula produce gonads from Jan-Jul, and spawn July through August (see graph on Left).  In the laboratory at 16.5oC larvae become competent to metamorphose after 20-25d (see graph on Right).  Although mortality is high during metamorphosis, the authors report comparatively little post-metamorphic deaths.  Breese & Robinson 1981 Aquaculture 22: 27.

NOTE  although there are many ways to induce spawning in bivalves, most traditional techniques are not fully effective for razor clams.   However, the authors find that immersion in seawater containing 2-2.5 million cells of the green alga Pseudoisochrysis paradoxa per ml works well for razor clams and also for other bivalves

NOTE  the authors choose to plot larval growth linearly, rather than curvilinearly, which would be the relationship expected.  Perhaps if they had started their plot from 0-0 points on the axes, the curve would have been apparent

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Research study 3
 

map showing collecting sites for genetic study of razor clams Siliqua patulaWith larval lifespans of bivalves ranging from 5-16wk, there is potential for genetic exchange over wide geographical distances.  This is tested in the Pacific razor clam Siliqua patula by electrophoretic analysis of allele frequencies in 49 enzymes in clams from 5 populations ranging from Alaska to Oregon (see map). Results show that clams from all locations exhibit high levels of within-population diversity.  The most genetically similar populations are the 2 from Washington and the authors think that a pathogen that hit these 2 populations hard, but not other populations, may have had a genetic basis.  Nevertheless, the finding of significant allelic composition among the 5 populations indicates restricted gene flow.  The authors suggest that the explanation may be in restricted larval movement across major freshwater plumes such as the Columbia River, and entrances to Grays Harbor, Washington and other bays.  LeClair & Phelps 1994 J Shellf Res 13: 207.

NOTE  known as NIX: a bacterium named Nuclear Inclusion X

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Teredo navalis/Bankia setacea

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Research study 1
 

Studies on shipworm reproduction in southern California show that Teredo navalis is protandric, with a functional male being present 4-6wk after completion of a free-swimming larval phase in summer at about 25mm in length, and becoming female at 8-10wk.  The larvae are retained in the maternal gill chambers for about half the larval period before being released. Overlapping settlement of larvae into a single piece of wood ensures that both sexual phases are represented.  Thus, most of the smaller, younger individuals function as males, while most of the larger, older individuals function as females.  photograph of burrows of shipworms Bankia setacea, close view

Bankia setacea differs in being oviparous, with eggs being discharged directly into the water where fertilisation occurs.  The veliger larvae spend about 4wk in the plankton before settling.  The author identifies 2 types of males in B. setacea: those that pass through the male phase quickly, and an equal portion that stay much longer in the male phase.  Some of these latter, termed “true males” by the author, retain the male state throughout life.  Most Bankia in southern California spawn during spring and autumn, and undertake resting periods in summer and winter.  Functional hermaphroditism is uncommon in both species.  Coe 1941 Biol Bull 81: 168; Coe 1933 Biol Bull 65: 283.

NOTE  termed “larviparous” by the author


Burrows of shipworm Bankia setacea 1.7X

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Tresus spp.
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photograph of clam Tresus capaxIn Humboldt Bay, California, gonads in fat gapers Tresus capax are ripe from November-January, and spawning is January-March.  Sex ratio is 1:1.  Machell & DeMartini 1971 Calif Fish Game 57: 274.

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Research study 2
  Spawning time for Tresus capax in Oregon is in April. Other clam species are variable depending upon species and ranges from late winter to mid-summer.  For example, butter clams Saxidomus gigantea spawn in June, littleneck clams Protothaca staminea in August, and cockles Clinocardium nuttallii in October. Robinson & Breese 1982 J Shellf Res 2: 55; photos of these species can be found in another section: TYPES OF WEST-COAST CLAMS.
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Venerupis philippinarum
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Research study 1
 

photograph of Manila clam Venerupis philippinarumManila clams Venerupis philippinarum in Hood Canal, Washington are reproductively mature in May-June, and spawn during July-October.  The authors find a few hermaphroditic specimens in their collections.  Holland & Chew 1974 Proc Nat Shellfish Assoc 64: 53; see also Nosho & Chew 1972 Proc Nat Shellfish Assoc 62: 50.

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Research study 2
  graph showing size at different ages for Manila clams Venerupis philippinarum in different locations in British ColumbiaManila clams Venerupis philippinarum were introduced into British Columbia into Ladysmith Harbour in 1936 along with oyster seed from Japan.  Since then they have become one of the dominant intertidal species of bivalves and are of great commercial importance to the province’s economy.  Spawning in the Strait of Georgia is from mid-late June, and larvae are in the plankton until September.  Age-estimates from counts of winter annuli (growth lines) are quite precise in Manila clams.  The graph, based on data collected from 8 sites around British Columbia, shows that about 15y of age is the maximum life span (6cm shell length).  Legal commercial size is at 38mm or about 4-5y of age.  Bourne 1982 J Shellf Res 2: 47.
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