MAR 105 Plankton Lab
Name___________________________________________________________
Exercise #1
Plankton Races
Most plankton are very small, ranging in size from 2µm - 2000µm (remember, a micron (µm) is
1,000,000th of a meter, or 1000x smaller than a millimeter!). Life at such a small size is different than
what we would expect for larger organisms (such as ourselves) in the open ocean. When large
organisms (ex. fish, dolphins, whales) swim through a sea of water molecules, the water molecules are
proportionately MUCH smaller than the organisms and exert significantly less resistance than that
exerted on a smaller organism swimming through the same fluid. For very small organisms, the
surrounding water molecules are proportionately very large and make movement through the water
more difficult. For small plankton, swimming through water is the equivalent of a human swimming in
a sea of syrup. In this way, seawater is viscous for small organisms like plankton.
Imagine that you are swimming in the ocean. You propel your body through the water using body
strokes, but when you stop moving your arms and legs, you continue to move slightly through the water
because of inertia (e.g., momentum). Inertia – for large animals – dominates over the water’s viscosity
(which to us is negligible), but for smaller organisms, the reverse is true and viscosity dominates over
inertial forces. The ratio of inertial to viscous forces is known as Reynold’s number.
Reynold’s number = Inertial forces/Viscous forces
A Reynold’s number >1 indicates that inertial forces dominate over viscous forces, while a number <1
indicates that viscous forces dominate over inertial forces. Below are some examples of Reynold’s
numbers for common marine organisms.
Organism:
Reynold’s number
Whale
300,000,000
Tuna
30,000,000
Penguin
300,000
Small fish
30,000
Large copepod
300
Small copepod
0.3
Bacteria
0.00001
Question #1
Examine the above examples of Reynold’s numbers. For which organisms do viscous
forces dominate over inertial forces?
Question #2
Provide some examples of other marine organisms that would have a Reynold’s number
of ~30,000,000. (Hint: Tuna typically range in size from 3-12 feet in length)
Question #3
Given the values of Reynold’s numbers presented above, estimate what YOUR Reynold’s
number might be _____________
For organisms with low Reynold’s numbers, movement through the water is limited, but not impossible.
Phytoplankton, for example, frequently sink out of the photic zone under the influence of gravity
despite their low Reynold’s numbers. For a photosynthetic organism, sinking out of this sunlit zone can
be extremely detrimental since they require sunlight for photosynthesis, so many phytoplankton species
develop distinct shapes that increase their surface area and increase frictional resistance (drag), keeping
them in the photic zone much longer. This is an extremely important adaptation for phytoplankton
species and so many distinct shapes have evolved.
Question #4
Examine the phytoplankton guides on the tables. Describe (or draw) some of the shapes
phytoplankton have evolved to reduce sinking.
Question #5
Many phytoplankton species are capable of locomotion, via use of a flagella or cilia. Do
you think that sinking is still a problem for these species in the open ocean? Why or why not?
You will now test the effectiveness of different body shapes on reducing sinking speed in plankton.
Procedure:
Obtain a 250 mL graduated cylinder and fill to 250mL with Karo corn syrup.
Use the modeling clay provided to create 5 different shapes (5 different ‘types’ of phytoplankton). First,
you’ll need to measure out 15 clay spheres that are exactly 1 cm in diameter. Then you will begin
molding 4 different phytoplankton shapes, making 3 of each shape for replication purposes. The
remaining 3 balls of clay will remain as spheres. ONE OF YOUR SHAPES MUST BE A SPHERE, and at least
1 should be based upon known phytoplankton morphologies (use your guides for inspiration, or come
up with an anti-sinking shape of your own design). Remember, all of your clay pieces must begin as 1
inch spheres and you will create 3 of each shape for a total of 15 clay ‘phytoplankton’ pieces (including 3
spheres for comparison).
Obtain a timer and/or stopwatch. Have one member of your lab group drop the clay pieces – ONE AT A
TIME - into the syrup, while another prepares to record time. Be prepared to poke at the clay piece with
a ruler or other long object to initiate its descent (they frequently get stuck in the surface layer of syrup).
When the clay piece has reached the 210mL mark, begin recording the time of descent for the clay to
reach a designated volume NEAR (NOT AT) the bottom of the graduated cylinder. DO NOT BEGIN TIMING
THE DESCENT UNTIL IT HAS REACHED THE 210mL mark. For example, begin timing the descent at 210mL
and stop recording the descent when the clay piece has reached 80mL. This way, all the clay pieces will
be allowed to settle at the bottom without interrupting the descent time as clay pieces begin to gather
at the bottom.
Record your observations in the charts provided below:
Question #6
Which shape sunk the fastest? Which sank the slowest?
Question #7
What is the disadvantage of sinking out of the photic zone for a phytoplankton species?
Can you think of any benefits sinking might have for a phytoplankton species?
Exercise #2
Plankton samples (live and preserved)
Plankton are classified according to several parameters. These include:

Metabolism (autotrophic vs. heterotrophic)

Size

Life history (holoplanktonic vs. meroplanktonic)

Taxonomy (ex. crustaceous zooplankton vs. gelatinous zooplankton)
Autotrophic plankton use photosynthesis (solar energy) to produce organic matter and are commonly
referred to as phytoplankton. Because of the pigments needed for this process, phytoplankton tend to
be colored in shades of green, brown, or red. Heterotrophic plankton must obtain nourishment by
eating and are commonly referred to as zooplankton.
Holoplanktonic organisms are ALWAYS planktonic (they spend their entire lives in the water column).
Copepods are a classic example of a holoplanktonic organism. Meroplanktonic organisms are “parttime” plankton that exist in the water column for only part of their lives. Good examples of
meroplankton include the larvae of many common marine organisms, such as clam, crab, squid, and
even fish larvae.
Question #8
Jellyfish begin their lives as polyps suspended on the seafloor. These polyps later
develop into the common medusa form of jellyfish we see in our local waters during summer. Are
jellyfish holoplanktonic or meroplanktonic?
Question #9
As described above, phytoplankton are different colors based upon the pigments used
to capture solar energy necessary for photosynthesis. Zooplankton do not perform photosynthesis and
so lack such pigments. What ‘color’ do you think most zooplankton are in the surface waters of the
ocean? What color do you think zooplankton are in the deep sea?
Examine the plankton samples (live and preserved) using the microscopes set up for you in lab. Be
sure to examine both the live samples and preserved specimens. Use the available space below to
draw some of the plankton you observe.
Question #10 Based on your observations of the plankton samples located in the lab, what are some
defenses that plankton employ to avoid predation in the open ocean?
Exercise #3
Meroplankton match-up
Using the figures on the following page, match up the meroplankton to their adult counterparts. Label
the adult organism with the appropriate letter of their larval form. Use your guides for help (ex. to
determine what type of organism begins life as a “zoea”).
LARVAE
A = yolk sac larvae; B = crab zoea; C = polychaete larvae; D = nauplius; E = bipinnaria larvae; F =
megalops; G = ephyra; H = veliger; I = ephyra
ADULT