Bivalves in more nutrient-rich environments like the East Pacific invest less in each egg, producing smaller eggs and larvae shells (P1), while those in less nutrient-rich areas like the Caribbean invest more in larger eggs and P1s. To test this, the author compiled egg and P1 size data for bivalve genera in both regions. Preliminary results found Caribbean eggs and P1s were larger on average, supporting the hypothesis. More data is needed to strengthen the results, but nutrient availability seems to impact bivalve reproductive strategies.

1. Abstract
Since the closure of the Central American Seaway (3.5 million years ago), the Caribbean has been
more nutrient poor than the east Pacific. Bivalve genera occupying more nutrient-rich environments
are hypothesized to invest less energy in single eggs and produce more eggs of smaller sizes. Genera
occupying less nutrient-rich environments are thought to invest more energy per egg, reflected by
larger single egg sizes. To test these hypotheses, we created a database of egg and larval shell sizes
for marine bivalve genera. We compared these measurements for East Pacific and West Atlantic
bivalve genera to determine if egg and prodissoconch 1 (P1) sizes were affected by the environmental
conditions these bivalves experienced. Preliminary results are consistent with our hypothesis:
Caribbean egg sizes were larger than Panamanian egg sizes. More data for additional bivalves could
be collected from the literature and museum specimens to assess the robustness of these
preliminary results.
Examining the Effects of Nutrient Availability on Scallops in the East Pacific and West Atlantic
Background
• During the Pliocene Epoch, the Isthmus of
Panama formed (3.5 MYA), separating the
Western Atlantic (to the East) from the
Eastern Pacific (to the West). These regions
are known today as the Caribbean and
Panamanian Provinces, respectively (Figure
1).
• Since the emergence of the Isthmus, the
Panamanian Province has been more nutrient
rich, and the Caribbean province has been
more nutrient poor.
• Bivalve species have a larval shell known as
the prodissoconch. The prodissoconch is
often divided in to two portions: the
prodissoconch 1 (P1) and prodissoconch 2
(P2) (Figure 2).
• P1 size correlates with egg size, and trends in
P1 size can be interpreted as trends in egg
size.
Eastern Pacific-
Panamanian Province
Western
Atlantic-
Caribbean
Province
Figure 1: Images showing the Isthmus of Panama
and the time period of its emergence.
Figure 2: Photomicrograph of P1 and P2 boundary on clam
species Limaria licombi. Scale bar= 100 μm. (Malchus &
Sartori 2013).
Materials and Methods
• Using Google Scholar, searched the literature
for egg, P1, and P2 size data.
• Compiled a database in Excel of P1 and egg
size data.
• Used R Statistical Programming Language to
generate histograms of egg and P1 size
trends on the provincial scale, and also on
the familial scale.
• Used R to run Mann-Whitney U-tests for
Provincial data, and Binomial tests for
familial data.
Results
P1 Size (Micrometers)
Frequency
0 200 400 600 800 1200
05101520
P1 Size Comparison Between Caribbean and Panamanian Genera
Egg Size (Micrometers)
Frequency
0 50 100 150 200 250
051015
Egg Size Comparison Between Caribbean and Panamanian Genera
Figure 3: (A) shows egg size comparison histogram between
Panamanian and Caribbean Provinces. (B) shows P1 size comparison
histogram between Panamanian and Caribbean Provinces. Red bars
indicate Panamanian provincial data and blue bars represent
Caribbean provincial data. Red lines represent median for
Panamanian Province and blue lines represent median for Caribbean
Province. (C) shows the egg sizes and (D) shows the P1 sizes for
species belonging to the family Pectinidae. Similar charts (not shown)
were plotted for all families in Table 1 (A) and (B).
A. B.
Picture Family
Panamanian
Median Egg Size
Caribbean
Median Egg Size
1. Arcidae 70.55 85.15
2. Carditidae
3. Crassatellidae
4. Limidae 140 154.25
5. Mytilidae 71.5 75
6. Nuculidae
7. Ostreidae 42.5 104.4167
8. Pectinidae 46.1 67.75
9. Propeamussidae 117.5 131.25
10. Veneridae 62.25 63.5
Picture Family
Panamanian
Median P1 Size
Caribbean Median
P1 Size
1. Arcidae 120.6 131.16
2. Carditidae 166 173
3. Crassatellidae 105 141.5
4. Limidae 232.5 190.75
5. Mytilidae 108.5 141.75
6. Nuculidae 252.54 246.25
7. Ostreidae 66.875 109.75
8. Pectinidae 82.175 92.14583
9. Propeamussidae 157.25 180.875
10. Veneridae 94 92.5
A.
B.
C.
Hypothesis
• Bivalves in more nutrient-rich areas,
such as the Panamanian Province, will
invest less energy per single egg,
yielding smaller eggs and smaller P1
sizes (see figure 3).
• Bivalves occupying more nutrient-poor
areas, such as the Caribbean Province,
will invest more energy per egg, yielding
larger eggs and larger P1 sizes.
P1 Size (Micrometers)
Frequency
110 120 130 140 150 160 170 180
012345
P1 Size Comparison Between Caribbean and Panamanian Genera
Figure 3: Illustrates an expected P1 comparison chart between
Panamanian and Caribbean genera. Similar outcomes would be
expected for an egg size comparison chart. Blue indicates
Caribbean data, red indicates Panamanian data.
Tim Smith Advisor: Dr. Paul Harnik Hackman Summer Scholar Program & Independent Study
Discussion
• The Panamanian province shows a trend towards smaller
egg size characters, at the family and genus level.
• When examining variation across all Genera, the U-test
indicated low statistical significance of the results (Table
1c).
• When comparing egg size/ P1 size within families, the
binomial test also failed to indicate significance of the
results (Table 1a-c), but was considerably closer to the
p=0.05 target. Generally smaller data pools have a more
difficult time attaining statistical significance. The family
data contains far less data, and so the increase in
statistical significance at this level may be important.
• The genus-level results could be complicated by long-
term evolutionary differences among bivalves, whereas
the family-level results could indicate the specialization of
genera within families in different marine provinces. One
possible force driving this specialization could be the
different nutrient regimes in each province.
• More data needs to be collected in order to increase the
robustness of the data. If the same trend continues and
egg and P1 size data are collected for more families, we
may see that the p-value for the binomial test falls below
0.05.
Panamanian Caribbean U Test Binomial Test
P1 Median 105 130.6 p= 0.1754 p=0.09375
Egg Size
Median 75 102.3778 p= 0.4363 p=0.1171875
Table 1: (A) Panamanian and Caribbean median egg size chart. (B) Panamanian and
Caribbean median P1 size chart. For (A) and (B), the colored number is the larger
median of the two provinces. (C) Median Egg and P1 sizes on the genus level between
the provinces. U test results indicate the significance of the difference between the
results at this level. Binomial Test results indicate significance of the Family level data..
Histogram of Pectinidae$EggSize
Egg Size (Micrometers)
Frequency
60 65 70 75 80 85
0.00.51.01.52.0
Histogram of Pectinidae$P1
P1 Size (Micrometers)
Frequency
60 80 100 120 140 160
0246810
C. D.
Acknowledgements- I’d like to thank the Hackman summer scholars program for providing funding for this project. I’d like to thank Dr. Paul Harnik for his guidance through the project,
and continuing involvement as my independent study advisor. I’d like to thank the “Encyclopedia of Life” (eol.org) for the images of each family, and the makers of “R” statistical programming
language.
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