Reproduction
 

photograph of oyster shells Crassostrea gigas courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, WashingtonBecause of the commercial interest in oyster culture, there is a large and comprehensive literature on almost every aspect of their reproduction and larval biology, only a fraction of which can be included in the ODYSSEY. Oysters have separate sexes and gametes are released into the seawater.  Fertilisation leads to a swimming, feeding veliger larva that remains in the plankton for several weeks before settling and metamorphosing. Topics on reproduction include spawning & larval life, settlement & metamorphosis, and recruitment. Photograph courtsey Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

 


Oyster shells Crassostrea gigas. Note the smooth
nacreous shell lining on the lower valve, and the
prominent scar of the closing muscle

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  Gonadal growth & spawning
  Topics on reproduction of oysters considered in this section are gonadal growth & spawning and larval life, while SETTLEMENT & METAMORPHOSIS and RECRUITMENT are presented elsewhere.
 
Research study 1
 

drawing of sperm ball of Ostrea conchaphilaphotograph of shell valves of Ostrea conchaphila courtesy Linda Schroeder, Pacific Northwest Shell Club, SeattleEarly research at the Scripps Institution of Oceanography, La Jolla describes the life cycle of the Olymbia oyster Ostrea conchaphila and confirms that it, along with its European and other congenors, is a protandic hermaphroditic.  Regular collections of settlers on blocks suspended from the Institution’s pier and examination of  adults show that reproduction takes place in southern California during at least 7mo of the year, or longer where temperatures are above a critical minimum of 16oC.  Gonads begin to grow at 8wk of age and, by 14wk spermatogonia and oogonia are visible.  By 5mo of age, sperm are ready to discharge, and growth of oogonia is well underway.  At this time the sperm, cemented together in spherical masses called sperm balls (each with up to 2000 spermatozoa; see drawing), are released.  On contact with seawater the sperm balls spin rapidly owing to the tail-lashing of the contained spermatozoa.  The cementing substance soon dissolves and the sperm are free to swim about.  At about 6mo of age the eggs are released into the mantle cavity, and held there through fertilisation and early development. Although eggs and sperm balls may commingle in the mantle cavity, self-fertilisation apparently does not occur.  At this time, a second phase of sperm development occurs, much larger than the first.  After about 12d of incubation, the now bivalved larvae (straight-hinge) swim out of the mantle cavity. The oyster now is energy-depleted, and its body is soft, flabby, and translucent.  After recuperation, the cycle is repeated, and this may go on regularly throughout an individual’s life providing temperature and other conditions are suitable. Overall, hundreds of thousands of sperm balls may be formed during a single season.  Coe 1931 Science 74 (1914): 247;  for details of spermatogenesis see Coe 1931 Biol Bull 61: 309. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle.

NOTE  lit. “first male” + “Hermaphroditus”, a Greek deity.  The author adds the term “viviparous” to this description, referring to the retention of larvae in the mantle cavity, but this seems an inappropriate use of the term

NOTE  the author uses the terms “ovocytes” and “ovogonia” rather than the more commonly employed “oocytes” and “oogonia”

 
Research study 2
 

Later studies on reproduction in Olympia oysters Ostrea conchaphila in Puget Sound, Washington by a U.S. Bureau of Fisheries researcher confirms its long spawning period (6mo), with the production of multiple broods.  Brooding commences when the seawater temperature reaches about 13-14oC.  Broods of up to 300,000 eggs (100µm diameter) are released into, and held in, the mantle cavity for about 10d.  In order for this to happen the eggs, instead of going the usual bivalvian route out the exhalent siphon, back up into the gill chambers and eventually squeeze out of the gill slits into the mantle cavity.  The gill slits are too small normally to accommodate passage of the eggs, but the sheer numbers and volume of the eggs stretch and enlarge the slits, with some splitting, allowing the eggs to pass through.  The cloacal chamber appears to remain closed during this time, perhaps increasing the “back-pressure” to force the gametes through the ctenidial  slits. Fertilisation occurs within the mantle cavity, presumably by sperm drawn in with the water flow. A maternal mantle chamber or “brood sac” is described, but its location and structure are unclear.  It may be simply the area around the palps and anterior ends of the gills where the larvae tend to accumulate, especially towards the end of their developmental term.  Sperm, in the form of sperm balls (see Research Study 1 above), are thought by the author to exit the body via the exhalent siphon.  Development proceeds as follows: gastrula 2d, trochophore 3d, valves appear 4d, valves complete to straight-hinge stage 5d, larva 180µm and ready to be discharged 10d (see below for drawings}.  Release of the straight-hinge larvae may be accomplished by valvular clapping, as proposed for related species, but exactly how this occurs is not known.  Hopkins 1936 Ecology 17 (4): 551; Hopkins 1937 Bull (#23) US Bureau Fisheries 48: 438.

NOTE  this route is from the gonopores, then via the suprabranchial chambers and out the exhalent siphon in the exhalent water discharge that also bears excretory and fecal waste. Sperm, bound up in the form of balls, are released via this route

illustration showing reproductive cycle of Olympia oysters in Puget Sound, WashingtonNOTE  other reports suggest that shell-opening may decrease pressure in the mantle cavity, such that the eggs are sucked from the gills into the mantle cavity

 

 

 

 

Details of reproductive cycle of Ostrea conchaphila in
Puget Sound during 1932. The histograms show
percentage gravidity, both of early developmental
stages (solid orange) and straight-hinge larval stages (hatched). Average seawater temperatures are shown
in red, while the small open circles indicate minimum
temperatures. The cyclical dotted lines indicate tidal
heights for the 2 daily low tides: high-low and low-low

 

Some developmental stages are shown here:

 
drawing of sperm ball and egg of Olympia oystr Ostrea conchaphila drawing of blastula of Olympia oyster Ostrea conchaphila drawing of veliger larva of Olympia oyster Ostrea conchaphila drawing of straight-hinge veliger stage of Olympia oyster Ostrea conchaphila drawing of young spat of Olympia oyster Ostrea conchaphila
Eggs at discharge into the mantle cavity are 100um in diameter A gastrula stage is reached at 2d post-fertilisation A swimming, non-shelled veliger stage is reached at 5d Age to straight-hinge larva is about 10d This 3d-old spat is about 0.6mm in widest dimension
 
Research study 3
 

graph showing reproductive cycles of oysters Crassostrea gigas (var. kumamoto) in OregonJapanese oysters Crassostrea gigas have separate sexes and gametes are released into the plankton.  A study at the Hatfield Marine Science Center, Oregon shows that gonads in C. gigas (var. kumamoto) ripen through the summer and spawning occurs in Sept-Oct (see graph).  Robinson 1992 Aquaculture 106: 89.

 
Research study 4
 

Field populations of many oyster species release their gametes in mass-spawning events involving both sexes.  Although a spawn-sychronising pheromone has yet to be identified in any bivalve, studies in Galveston, Texas suggest that a membrane-bound protein in the sheath surrounding the sperm may have pheromonal properties.  The reason for investigating sperm characteristics, in particular, is that while the presence of sperm in suspension will stimulate both sexes to spawn, presence of eggs will only stimulate males to spawn.  In fact, it is common practise in oyster hatcheries to throw a spawning male into a tank containing both males and females to induce natural spawning.  Rice et al. 2002 J Shellf Res 21: 715.

NOTE  there are many artificial inducers for spawning in commercially cultured invertebrates, including various chemicals, temperature shock, and so on

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If addition of a spawning male induces spawning in both sexes, why is there a need for a spawn-inducing pheromone? Consider the answers provided, then CLICK HERE for explanations.

Chemicals are cheaper and more reliable. 

Males and female oysters can be induced to spawn separately. 

Allows induction of spawning without risk of introducing pathogens. 

Allows for simplification and standardisation of oyster-hatchery operation. 

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  Larval life
 
Research study 1
 

photograph of oysters Crassostrea gigas
graph showing larval development in oysters Crassostrea gigasA few days after fertilisation a veliger larva forms.  It feeds on phytoplankton for about 4-5wk until settling and metamorphosing. The velum performs double duty, propelling the larva through the water and filtering algal cells for consumption.  Larval stages include the so-called D-shaped, Umbo, and Pediveliger, the last being the settling and metamorphic stage.  The graph on the right is a summary from many research papers, and shows the fraction of developmental time spent in each stage by larvae of Crassostrea gigas. The variability owes to the effect of temperature, salinity, and food conditions on developmenal rate.  The authors present a complex biochemical model for the development of C. gigas (not included here) and the reader is directed to this paper as a good reference source for research done on this important commercial species.  Bochenek et al. 2001 J Shellf Res 20: 243; larval photos courtesy Wallace et al. 2008 Oyster hatchery techniques. SRAC Publ No. 4302, USDA.

NOTE  this literature base is huge and, because it is mostly mariculture-oriented, is not included in the ODYSSEY

The 2 curves bracket the range of sizes reported for larvae of Crassostrea
gigas.
D-shaped larvae occupy about 10% of the total larval developmental
time, while pediveligers occupy about 20% of the developmental time

 
Research study 2
 

While most of the food of the larvae is phytoplankton of plant origin, studies in Oregon using 14C-labelled bacteria (Strain CA2) reveal that larvae of oysters Crassostrea gigas are also capable of feeding and digesting food of bacterial origin.  Apparently, bacterial carbon obtained in this way can satisfy up to 1.5 times the metabolic carbon needs in early larvae (100µm in shell length), but this decreases to about 0.4 times in older, larger larvae (280µm in shell length).  Douillet 1993 Mar Ecol Progr Ser 98: 123; Douillet 1993 Mar Ecol Progr Ser 102: 303.

 
Research study 3
 

graph showing growth of oyster larvae Crassostrea gigas in relation to food abundanceStudies on veliger larvae of oysters Crassostrea gigas at Friday Harbor Laboratories, Washington show that when food is scarce the velum and cilia actually increase in size, presumably at the expense of other soft tissues of the body, thus increasing the larva’s capacity to capture food.  The authors note that this developmental plasticity enables veligers to feed more in times of food scarcity. Moreover, in times of food abundance, even though the velum is lost at metamorphosis, it enables the larvae to devote more energy to growth of structures retained into the juvenile at metamorphosis.  Strathmann et al. 1993 Biol Bull 185: 232.

NOTE  the studies were done in the lab at food concentrations of 30,000 algal cells . ml-1 for the HIGH food-level treatment, and 3,000 algal cells . ml-1 for the LOW food-level treatment

NOTE  for a description of the same phenomenon in sea-urchin larvae go to LEARN ABOUT SAND DOLLARS: LARVAL FEEDING

 
Research study 4
 

Studies on oysters Crassostrea gigas in culture facilities in Wales show that the larvae can take up 14C-labelled glycine and alanine from seawater.  Rate of uptake is about 10 times greater in the larvae than in adult bivalves per unit mass, in part reflecting the greater absorptive surface area-to-volume ratio of the larva.  The author notes that the ability to take up dissolved organic matter (DOM) may be of vital importance to the larvae when particulate food is scarce, because the larvae are often provided with minimal food reserves by the parent.  Manahan 1983 Biol Bull 164: 236.

 
Research study 5
 

photograph of cemened oyster Crassostrea gigas, courtesy Dave Cowles, Walla Walla University, Washingtongraph showing larval filtration rates in oysters Crassostrea gigasFeeding of Crassostrea gigas larvae peaks just prior to metamorphosis.  Within a day or 2 after metamorphosing, the juvenile oyster commences feeding.  Gerdes 1983 Aquaculture 31: 195. Photo courtesy Dave Cowles, Walla Walla University, Washington www.wallawalla.edu.

NOTE  diet consists of unicellular algae Isochrysis galbana and Chaetoceros calcitrans each at 50 x 106 cells . ml-1

Larvae of oysters Crassostrea gigas tend to settle
gregariously, resulting in extensive "oyster reefs 0.8X

 
Research study 6
 

graph showing percentage survival of oyster larvae Crassostrea gigas in the absence of foodFor any planktotrophic marine-invertebrate larva there is a point of no return, that is, a time at which the larva must have fed enough in order to survive and grow.  The fact that many invertebrate species with planktotrophic larvae synchronise their spawning to coincide with plankton blooms underscores the importance of food availability during early development.  Surprisingly, then, studies on Pacific oysters Crassostrea gigas in Santa Catalina Island, California show that the larvae can survive in the absence of phytoplankton food for several weeks (at 23oC). Note in the graph on the Left that histogram of survival of oyster larvae Crassostrea gigas in relation to different periods of starvationsurvival holds steady for about 2wks, then gradually decreases.ß 

A larva’s ability to “bounce back” after a period of food deprivation depends on the length of time involved.  If larvae are starved for varying lengths of time (2-17d), then fed for 8d (see histogram on Right), survival is not significantly affected over the first 14d (remains at roughly 60%).  Only after 17d of starvation does survivorship decrease significantly.  

Interestingly, based on known daily rates of oxygen consumption of a larva, all of its endogenous reserve of energy (protein, lipid, and carbohydrate represented by yolk) would be used up after only 8d.  Yet, the larva lives on for at least another 16d (see graph on Left).  By 24d it will have had to burn an additional 780µJ over the 485µJ it was endowed with as a 1-day-old larva. Where does this extra energy come from? The authors speculate that the energy shortfall is made up of a combination of feeding on detritus and utilisation of dissolved organic carbon.  Moran & Manahan 2004 J Exp Mar Biol Ecol 306: 17.

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