Mark W. Luckenbach 1 , Elizabeth North 2 , M. Lisa Kellogg 3 , Roger Mann 4 , Steve M. Allen 5 and Kennedy T. Paynter 2,3 Fertilization Success in Altered Bivalve Populations: Implications for Restoration 1 Virginia Institute of Marine Science, College of William and Mary, Eastern Shore Laboratory 2 University of Maryland Center for Environmental Studies, 3 University of Maryland 4 Virginia Institute of Marine Science, College of William and Mary 5 Oyster Recovery Partnership
Transcript
Mark W. Luckenbach1, Elizabeth North2, M. Lisa Kellogg3, Roger Mann4, Steve M. Allen5 and Kennedy T. Paynter2,3
Fertilization Success in Altered Bivalve Populations: Implications for Restoration
1Virginia Institute of Marine Science, College of William and Mary, Eastern Shore Laboratory 2University of Maryland Center for Environmental Studies,3University of Maryland4Virginia Institute of Marine Science, College of William and Mary5Oyster Recovery Partnership
Problem Description
Fertilization success in free-spawning, sessile marine bivalves is dependent upon gametes making contact.
Many of the shellfish populations that we seek to protect or restore currently exist at low population density, e.g.,
Restoration activities that add bivalves as broodstock do so with little basis for determining the density required to achieve high fertilization success.
Moreover, the addition of broodstock is often done without regard for the sex ratio, anticipated gamete production or potential fertilization success.
Problem Description
High fecundity in these species can lead to the expectation that a small population size is sufficient to effect a recovery.
For instance: 106 -107 eggs per ♀ at a density of 100 – 102 ♀’s m-2 yields1010 – 1013 eggs hectare-1
Assumptions about how many of these eggs are fertilized has the potential to propagate errors through demographics models that are several orders of magnitude.
Bolingbroke Sand, Choptank River
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36-60 61-85 86-110
Size Class (mm)
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emal
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Age (yr)
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Bolingbroke Sand
Low Sperm:Egg
Potentially Sperm Limited
Some Preliminary Findings with Crassostrea virginica(Kellogg et al. 2007 ICSR)
y = 3.8201Ln(x) + 64.305
R2 = 0.9998
y = 0.8836x + 0.3165
R2 = 0.9998
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ert
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y = 5E+06x2 + 1E+08x
R2 = 0.1567
1.E+08
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Shell Height (mm)
# S
perm
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Objectives
1) Better understand the factors that affect fertilization success, including…
gamete concentrations fertilization efficiency (avg. # sperm to fertilize an egg)turbulent mixing
2) Ultimately, we want to use this, coupled with density, size and fecundity estimates from the field, to estimate not only egg production, but also fertilization success in natural and restored bivalve populations.
Approach
1) Construct and parameterize a computational model which predicts fertilization success based upon contact between gametes in a turbulent medium.
2) Conduct laboratory experiments with Crassostrea gametes using field-relevant turbulent mixing conditions to test initial model predictions.
3) Refine the model predictions using experimental results.
4) Repeat 2 – 3 as necessary.
tfUFF ttt 1
ttt FUU 1
Predicts concentration of fertilized eggs over time when mixed with sperm of a given concentration:
where U = concentration of unfertilized eggs (number cm-3)
andF = concentration of fertilized eggs (number cm-3)f = fertilization constant (=1 if every contact results in fertilization)dt = time interval (s)ε = contact rate of sperm with each egg (number s-1).
Fertilization Success Model
Fertilization Success Model
The contract rate of egg and sperm (E) [After Rothschild & Osborn (1988), Evans (1989), Visser & MacKenzie (1998)]:
21
2222 2wvuRctE where c = concentration of ‘prey’ particles (e.g., sperm concentration, number cm-3)
andR = reactive distance of ‘predator’ particles (e.g., effective egg radius, cm)u = swimming velocity of ‘prey’ particles (e.g., sperm swimming speed, cm s -1)v = randomly directed motion of ‘predator’ particles (e.g., v = 0 for eggs)w = root-mean-square turbulent velocity, (cm s-1):
31daw where a = constant [1.37 or 1.9 according to references in Visser and MacKenzie (1989)]
d = turbulence length scale [d = R according to Visser and MacKenzie (1989)]
ε = turbulent dissipation rate (cm2 s-3).
Unfertilized egg conc. = 100 cm-3
Sperm concentration = 1000 cm-3
Fertilization efficiency = 0.01
Unfertilized egg conc. = 100 cm-3
Sperm concentration = 1000 cm-3
Fertilization efficiency = 0.03
Unfertilized egg conc. = 100 cm-3
Sperm concentration = 1000 cm-3
Fertilization efficiency = 0.1
Unfertilized egg conc. = 100 cm-3
Sperm concentration = 1000 cm-3
Fertilization efficiency = 0.2
Unfertilized egg conc. = 100 cm-3
Sperm concentration = 1000 cm-3
Fertilization efficiency = 0.5
Unfertilized egg conc. = 100 cm-3
Sperm concentration = 1000 cm-3
Fertilization efficiency = 1.0
Preliminary model run examplesTurbulent energy dissipation rates ε = 0.007 cm2 s-3
ε = 0.034 cm2 s-3
ε = 0.180 cm2 s-3
Solutions for contact rates when the following parameters were varied: A) sperm concentration (note the log scale), B) sperm swimming speed and contact (egg) radius, and C) turbulent dissipation rate (e) and the constant a. Open symbols indicate ‘base-case’ solutions.
A. B. C.
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Sperm concentration (# cm3)
Con
tact
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Radius (cm) or Swim speed (cm s-1)
Sperm Swimming Speed
Contact Radius
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Turbulent Dissipation RateConstant (a)
Turbulent mixing
Examples of initial release of dye (left) and final distribution of dye (right) in grid chambers.
Turbulent energy dissipation rates 1) measured in Chesapeake Bay and 2) occurring in the grid-stirred chambers with different motor speeds (colored lines).
Field data courtesy of Larry Sanford, UMCES Horn Point Laboratory
Average turbulent energy dissipation rates (e, cm2 s-3), average turbulent shear (g, s-1), and Reynolds number at different motor speeds (RPM) predicted for the grid-stirred chambers.
Grid-stirred chambersMixing controlled by motor speed
Summary of sperm dilution experiments with species, gamete concentrations, gamete ratios and mean % of eggs fertilized. Values for % fertilized are means of three treatment replicates.
Species Sperm Dilution
Gamete Concentration
Sperm ml-1 Eggs ml-1 Sperm:EggMean % Fertilized
101 9.63 x 106 104 9.63 x102 66.67 102 9.63 x 105 104 9.63 x101 8.94
C. virginica 103 9.63 x 104 104 9.63 x100 2.06 104 9.63 x 103 104 9.63 x10-1 2.22 105 9.63 x 102 104 9.63 x10-2 1.89 106 9.63 x 101 104 9.63 x10-3 2.28 107 9.63 x 100 104 9.63 x10-4 1.94 101 1.71 x 107 104 1.71 x 103 93.67 102 1.71 x 106 104 1.71 x 102 69.83
C. ariakensis 103 1.71 x 105 104 1.71 x 101 9.00 104 1.71 x 104 104 1.71 x 100 0.33 105 1.71 x 103 104 1.71 x 10-1 0.00 106 1.71 x 102 104 1.71 x 10-2 0.00 107 1.71 x 101 104 1.71 x 10-3 0.00 100 1.46 x 106 103 1.46 x 103 92
C. ariakensis 1021.46 x 104 103 1.46 x 101
12.5
1041.46 x 102 103 1.46 x 10-1
0C. ariakensis 100 9.19 x 105 102 9.19 x 103 99.25
102 9.19 x 103 102 9.19 x 101 81.63
1049.19 x 101 102 9.19 x 10-1
1.25C. virginica 101 9.16 x 105 102 9.16 x 103 97.5
103 9.16 x 103 102 9.16 x 101 53.5
105 9.16 x 101 102 9.16 x 10-1 0.33C. virginica 101 3.43 x 105 103 3.43 x 102 55.67
103 3.43 x 103 103 3.43 x 100 13.50
105 3.43 x 101 103 3.43 x 10-2 0.50C. virginica 101 1.42 x 106 101 1.42 x105 90
103 1.42 x 104 101 1.42 x103 94
105 1.42 x 102 101 1.42 x101 66.17
Summary of sperm dilution experiments with species, gamete concentrations, gamete ratios and mean % of eggs fertilized. Values for % fertilized are means of three treatment replicate.
SpeciesEgg
Dilution
Gamete Concentration
Sperm:Egg% FertilizedSperm ml-1 Eggs ml-1
C. virginica103 5.54 x 104 101 5.54 x 103 98.50
102 5.54 x 104 102 5.54 x 102 88.17
101 5.54 x 104 103 5.54 x 101 35.67C. virginica
103 6.98 x 104 101 6.98 x 103 97.17
102 6.98 x 104 102 6.98 x 102 97.17
101 6.98 x 104 103 6.98 x 101 67.67C. virginica
103 6.98 x 104 101 6.98 x 103 94.17
102 6.98 x 104 102 6.98 x 102 65.83
101 6.98 x 104 103 6.98 x 101 33.67
Examples of mean % eggs fertilized at varying sperm density
Percent fertilization as a function of sperm : egg ratios
xxef ln02859.09481.313233.401.0 where x was the ratio of initial sperm and unfertilized egg conc.
Fertilization Efficiency =
Model predictions of fertilization success after 30 min at different initial egg concentrations: A) 10 eggs cm 3, B) 100 eggs cm3, and C) 1000 eggs cm3. (1 RPM, ε = 0.00067 cm2 s-3; 3 RPM, ε = 0.007 cm2 s-3; 6 RPM, ε = 0.034 cm2 s-3; 12 RPM, ε = 0.18 cm2 s-3).
Observed fertilization success is very low at sperm:egg ratios ≤ 102 and generally only high above 103 . . .. . . even at high gamete concentrations.
Model predictions suggest that at high mixing rates fertilization success below this sperm:egg threshold should increase. . .. . . however, this neglects the high dilution rate at high turbulence.
Implications
Under conditions of low population density, such as exists for many overexploited bivalve species,
or when a single age class dominates the population, as often exists in areas of sporadic recruitment
or at restoration sites where single cohorts are planted,
population growth may be seriously constrained by low fertilization success.
Future Directions
Investigate fertilization success in the laboratory at higher mixing rates.
Make the model open, i.e. allow for dilution of gametes.
Extend the experiments to other bivalve species.
Conduct fine scale spatial surveys of “restored” bivalve populations to more accurately map densities and distributions of the sexes.
Acknowledgments
Funding was provided by the NOAA Chesapeake Bay Office and the Keith Campbell Foundation for the Environment.
We thank Larry Sanford and Steve Suttles for field data and construction of the turbulence mixing chamber, respectively.
Sperm swimming speed
Sperm and egg suspensions were added to opposite ends of a flat capillary tube.
Observed under an inverted light microscope with attached low light video camera.
With a frame grab frequency of 0.01 s-1 we measured the movement of individual sperm for 0.6 – 1.0 s.
8 cm
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Egg suspensionSperm suspension
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Number
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Measured distribution of swimming speeds for C. virginica sperm. Each bar is from a single video record and observations are ranked in increasing order.
Mean υ = 0.0057 cm s-1
A. Sperm swimming speed = 0.001 cm s-1 B. Sperm swimming speed = 0.0057 cm s-1
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ertil
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Species, sample size and range of shell heights used in determination of male size-specific fecundity.
Species nRange in Shell Height
(mm)Crassostrea virginica 55 17.4 – 135.7C. ariakensis (West Coast strain) 18 65.3 – 140.1C. ariakensis (South China strain) 23 62.1 – 88.0
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Shell Height (mm )
Estimated sperm (in billions) vs. shell height for Crassostrea virginica ( ), west coast strain of C. ariakensis ( ) and south China strain of C. ariakensis ( ).
