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Does buoyancy differ between copepod groups? A passive sinking experiment
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Hoi Iao 2015
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Abstract | |
High diversity in copepods has
allowed them to colonise many habitats and they are called the “insects of the
sea”. They travel vast distances daily in the water column – termed diel
vertical migration, and buoyancy control is very important. I observed that the
morphology of the antennulae differ between copepod groups, for example length
of antennulae which affects the drag of the copepod in water. Comparison among copepod
orders is extremely rare. I hypothesised that 1) sinking rate differs among
copepod groups, 2) sinking rate increases when antennulae length decreases and
3) sinking rate of the copepod increases when density increases. Passive
sinking experiments in freshwater and seawater were performed with preserved
samples of five groups of copepods: family Calanidae (order Calanoida), Oithona sp. (order Cyclopoida), Oncaea sp. (order
Poecilostomatoida), Farranula sp. (order Poecilostomatoida)
and Macrosetella sp. (order Harpacticoida). Pictures of the
antennulae length and size was measured in ImageJ software and density was
computed from wet mass and size. Sinking rate was significantly
different among groups in both freshwater (p=1.643x10-05) and seawater
(p=4.007x10-07) which supports the first
hypothesis. Interestingly, Calanidae had the longest antennulae,
lowest density and fastest sinking rate. There was a significant positive correlation between antennulae
length and sinking rate and a significant negative correlation between density and sinking rate, which is opposite
to hypothesis 2 and 3. The rigid position of the antennulae may have affected
the results. Further research can be done on the combined effects
of antennae length, angle and lipid content. Understanding the ways to control
buoyancy will help explain how copepods travel vast distances in the water
column.
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Introduction | |
Copepods have colonised many
habitats with different conditions (Huys and Boxshall, 1991: p9-12). In
the ocean, they can be found in the water column, in sediments and in deep-sea
vents. They also live in freshwater bodies, saline lakes, hot springs and even damp terrestrial environments (Huys and Boxshall, 1991: p9-12). There are
many parasitic forms and they have parasitized species in most animal phyla (Huys
and Boxshall, 1991: p11-12). It is interesting to look at the adaptations in
copepods that have allowed them to occupy this large variety of habitats. Copepods
are called “the insects of the sea” for their size, diversity and abundance (Huys
and Boxshall, 1991: p9). There is a large diversity in planktonic copepods
alone (some examples shown in figure 1).
Buoyancy plays a big role in marine
environments. Many copepods exhibit diel vertical migration traversing 50-150m (Thorisson,
2006). After feeding in the mixed surface
layer, satiated copepods stretch out their antennulae (see copepod body plan in figure 2) and slowly sink back into
the aphotic zone where they are less conspicuous to predators (Thorisson,
2006). When they are hungry, they swim up
to the surface to feed again (Thorisson,
2006). When Eucalanus crassus (Order Calanoida) feeds with its self-generated feeding
current, the first antennae (antennulae) act as parachutes to slow down sinking
and a slight negative buoyancy maintains the orientation for the flow field (Ruppert
et al., 2004, p 672). Morphology of the
antennules between copepod groups may differ in terms of length, number of
segments and organisation of setae (Huys and
Boxshall, 1991, Boxshall and Halsey, 2004). I have observed that generally calanoids
have long antennulae, while Corycaeus and Oncaea (Order Poecilostomatoida) have very short antennulae. As
longer antennules increases the drag of copepods in water, it is reasonable to
predict copepods with longer antennules sink slower.
Buoyancy can also be affected by the
relative body density of copepods to surrounding seawater. If the density of a
copepod is higher than density of seawater, it will sink. It has been proposed
that lipid content in copepods may also be a means to control buoyancy,
especially during diapause (Zarubin et al.,
2014, Heath et al., 2004, Pond and Tarling, 2011). Lipids are stored in an oil sac in
the prosome for energy-storage and reproduction (Zarubin et al.,
2014) and they have lower density, higher
compressibility and higher thermal expansion of esters compared to water (Pond and
Tarling, 2011). Copepods found in deeper water was
found to have higher lipid content and it has been suggested that they change
their lipid content at different depths to adjust for buoyancy (Zarubin et al.,
2014, Pond and Tarling, 2011).
This study determines to test if buoyancy
differ among five copepod groups: order Calanoida, order Cyclopoida, Oncaea and Corycaeus (order Poecilostomatoida) and order
Harpacticoida. I hypothesise
that sinking rate differs among copepod groups. If this hypothesis is
supported, I further hypothesise that sinking rate increases when antennule
length decreases, and sinking rate of the copepod increases when density
increases. Copepods have been well-studied at the single-species level, but
there are hardly any studies of multiple orders. Comparison among copepod
groups will help understand how different copepods use their respective habitat
differently.
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Figure 1 |
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Figure 2 |
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Materials and Methods | |
Five copepod samples were obtained
in the IMOS-SCIRO plankton Team in Brisbane, Australia: family Calanidae (order Calanoida), Oithona sp. (order Cyclopoida), Oncaea sp.
(order
Poecilostomatoida),
Farrancula sp. (order Poecilostomatoida) and Macrostella sp. (order Harpacticoida) (figure 1). They were collected with a 100 µm mesh net not deeper than 100m in Australian waters and
preserved in formalin.
Two vials (diameter 1cm, height
4.5cm) were filled up with 2cm3 of freshwater in one and unfiltered seawater
in the other. Each copepods was rinsed with freshwater and individually placed below
the meniscus in the freshwater vial. The time for the copepod to sink was timed
until it lied flat on bottom. Three repetitions were made with each individual
and the process was repeated in seawater.
Photographs
were taken of the copepods under microscope so that their antennules were
visible. Then they were placed on an absorbent glass-fibre filter to dry and were
weighed to obtain wet mass.
In the
software ImageJ, the mean antennae length of each copepod was measured. The
copepod shape was simplified into two ellipsoids (volume of ellipsoid given by
4πabc /3) and the length of the equatorial (a) and polar
(b) axes were assumed to be equal. The
radii (a and b) and length (c) of the prosome and urosome were measured (figure 3)
and volume of each copepod was calculated. Density was calculated by dividing
mass by volume.
A one-way
ANOVA was conducted to test the difference passive sinking rate among copepod group
in freshwater and seawater. Multiple regression was conducted to test the effect
of density and antennae length on passive sinking rate.
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Figure 3 |
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Results | |
Sinking rate was significantly
different among groups in both freshwater (p=1.643x10-5, f=29.46, df=4) and seawater (p=4.007x10-7,
f=65.153, df=4) (Table 1). Calanidae had the fastest sinking
rate in both freshwater (0.376 ± 0.007) cm/s and seawater (0.334 ± 0.023) cm/s. Farranula
sp. had the slowest
sinking rate in both freshwater (0.056 ±
0.001) cm/s and
seawater (0.057 ±0.004) cm/s.
Generally sinking rate increased as
antennae length increased (figure 4). Calanidae had the longest antennae
length and the highest sinking rates. Oncaea
sp. had the shortest antennae length but had the second highest sinking rates. The positive correlation between
antennae length and sinking rate was significant in both freshwater (p=0.00464, F2, 12=16.55) and seawater (p=1.78x10-6, F2,12=56.93).
Sinking rate decreased as copepod
density increased (figure 5). Calanidae had the lowest density and the
highest sinking rates. Macrosetella sp.
had the lowest density and the second lowest sinking rates after Farranula sp. The
negative correlation between density and sinking rate was significant in both freshwater
(p=0.00303, F2,
12=16.55) and
seawater (p=0.00115, F2,12=56.93).
Table 1. Freshwater and seawater sinking rates for each
copepod group. Data shows mean ± standard error.
Group
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Freshwater sinking rate (cm/s)
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Seawater sinking rate (cm/s)
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Calanidae
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0.376 ± 0.007
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0.334 ± 0.023
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Oithona
sp.
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0.089 ± 0.007
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0.088 ± 0.002
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Farranula sp.
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0.056 ± 0.001
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0.057 ±0.004
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Oncaea
sp.
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0.230 ± 0.055
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0.111 ±0.021
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Macrosetella sp.
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0.077 ± 0.006
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0.064 ±0.004
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Figure 4 |
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Figure 5 |
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Discussion | |
Sinking
rate was significantly different among copepod groups and the first hypothesis
is supported. However, the positive correlation between antennae length and
sinking rate and the negative correlation between density and sinking rate was
opposite to what was predicted. Interestingly,
calanidae had the longest antennules, lowest density and fastest sinking rate. Data of density was affected by the high percentage error
in the mass of copepods as the mass was extremely small (for example Oncaea sp. weighed 0.0002 g).
Since the samples have been
preserved, the antennules were rigid and the angle to which they were held
against the body was fixed, which may affect the effect of antennules on
buoyancy (Borazjani et
al., 2010). The angle could not be measured as
the calanoid, Macrosetella and Farranula could not balance to allow for photos
of dorsal or ventral view. Further research can be done on the combined effects
of antennae length, angle and lipid content (Zarubin et al.,
2014).
Buoyancy control is a dynamic
process that happens in copepod constantly. Understanding the ways to control
buoyancy will help explain how copepods travel vast distances in the water column
every day.
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Acknowledgements | |
I
would like to thank Julian-Uribe Palomino for his assistance in identifying
samples, taking photos and for sharing his knowledge on copepods. I also want to thank the IMOS-SCIRO plankton
Team in Brisbane Australia for their support and supply of samples and
equipment.
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References | |
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