An amazing presentation by Kira Withy-Allen Well worth watching.

1. Lesson 3: That’s My Homie!
Symbiosis among crabs and corals to
decrease sedimentation
By Kira Withy-Allen
University Laboratory School, Hawaii
Researcher: Dr. Hannah Stewart
Marine Science Institute
University of California, Santa Barbara

2. Let’s begin by looking at the big picture:
How the Environment Within A Barrier Reef
System Can Affect Benthic Organisms
• Yipee! Already a new vocabulary word! Benthic
organisms are those that live at the bottom of the ocean.
• Unlike pelagic organisms, which swim freely in the open
ocean, benthic organisms do not travel very far. Many
benthic organisms are actually attached to the substrate.
WHICH ORGANISMS DO YOU THINK WOULD
BE CONSIDERED “BENTHIC”?
Some include coral, seaweed, anemones, sea
cucumbers, crabs, sea stars, lobsters, various fish
(gobies, scorpionfish, eels, flatfish).

3. Why are Benthic Organisms Important?
• The majority of coral reef ecosystems are
made up of benthic organisms.
• The coral itself is a benthic organism!
• Do you think coral polyps usually move
once they’ve settled on a colony?
• NO! Coral polyps don’t move once they’ve
settled. This means that corals are very
vulnerable to the surrounding environment.
So let’s check what’s out there!

4. Terrestrial
run-off
(Sediment)
Forereef: All
organisms are
exposed to fast, high
energy waves.
Backreef: Slow flow;
Sedimentation can be a
challenge for benthic
organisms.

5. A Closer Look: The Forereef
High energy environment
with lots of waves and water
flow that goes in various
directions. This is a difficult
environment for organisms
to stay attached.

6. A Closer Look:
The Backreef
The environment is much
calmer, so water flow is
slower and moves in one
direction.

7. Why is Water Flow Important?
For benthic organisms, like
coral, water motion is
important for almost all
aspects of its ecology.
This picture represents a
single coral polyp. A reef is
usually made up of
thousands of coral polyps
mixed with coralline
algae, rock, or some other
form of substrate.
www.iwaswondering.org

8. Water Flow Water flow is important
for transport of gases
(like oxygen) and
dissolved nutrients to
the center of the
organism where they
can be used, and for
removing waste when
the cycle is complete.
It is more difficult for a
benthic organism to
exchange gases and
Gases
nutrients if there is a
Dissolved nutrients Wastes slow flow.
www.iwaswondering.org

9. Particle movement
Food Sediment
Water flow also brings
particles to the
organism. This may
include desired food
particles (for
example, zooplankton),
or undesired sediment
(sand or silt).
Gases
Dissolved nutrients Wastes
www.iwaswondering.org

10. Particle movement
Gametes
Food Sediment
Benthic organisms
further rely on water
flow to remove
unwanted sediment and
to release their gametes
(reproductive cells).
Gases
Dissolved nutrients Wastes
www.iwaswondering.org

11. Particle movement
Gametes
Food Sediment
However, moving
water also exerts
hydrodynamic forces
that push against an
organism, which can
become challenging
for the organism.
Gases Hydrodynamic force
Dissolved nutrients Wastes
www.iwaswondering.org

12. Benthic organisms have special mechanisms to
cope with different hydrodynamic challenges.
Assignment: Work in a group to answer the following questions.
Be realistic, but also use creativity in your ideas.
1) How would you stay attached if you were a coral polyp or colony?
2) If you could change something about your structure as a coral polyp or
colony, what would you do?
3) What is the scientific word that is used to explain organisms that change
something about their structure to fit in better with their environment?

13. The scientific word for change: Adaptation
Coral colonies of the same species generally follow similar growth
patterns, but differences such as location (forereef vs. backreef) may alter
the structure of the colony.
Organisms are sometimes able to adapt to their environment so they are
better suited for survival. For example, the coral colonies growing near
high energy flow might adapt by becoming more compact so that the
branches of the colony don’t break off easily, as shown below.
Backreef: Pocillopora damicornis (Lace Coral) Forereef: Pocillopora eydouxi (Antler
is more delicate with thinner branches, which Coral) is more compact with thicker
is found in calm water. branches in higher energy wave zones.

14. However, the process of adaptation
takes time. How do corals survive other
day to day challenges such as
sedimentation?
This coral colony is in an environment
that has slow flow and high sediment
(the backreef). The slow flow brings
sediment to the colony and once it
settles, the flow can be too weak to
move the sediment again.
Assignment:
Brainstorm at least 3
reasons why the coral
might be negatively
affected by sediment.

15. Does the sediment harm the coral?
YES! The corals could DIE!
Sediment could harm corals by:
• Preventing the zooxanthellae in the corals from accessing sunlight
to perform photosynthesis.
• Obstructing the coral’s ability to feed if its tentacles can no longer
catch zooplankton.
• Corals may have a hard time taking in oxygen from the
surrounding water.
• The corals cannot reproduce; If it tried to spawn, the gametes
might get stuck.
• The sediment does not allow for expansion of the reef if other
polyps cannot settle next to the older ones.
So how do you think corals survive in an environment
with high sedimentation? Please SHARE your ideas!

16. Did Anyone Mention SYMBIOSIS?
Coral with No Crab Coral with Crab
Pocillopora verrucosa Trapezia serenei
During her initial observations, Dr. Stewart found that coral colonies in places where
sedimentation was high were either:
1) Dead and covered in sediment (picture on left).
2) Very healthy with limited sediment (picture on right).
Dr. Stewart questioned why coral colonies that were exposed to the same amount of
sediment in the water would be so drastically different in health.
Upon surveying these coral colonies, she found that 94% of the living coral colonies had a
small crab living inside its branches. None of the dead colonies had a crab!

17. Does this mean the live corals and crabs participate in a
symbiotic relationship?
YES! Coral colonies are home to crabs, which means they have a symbiotic relationship.
Assignment: To understand more about the symbiotic relationship between corals and
crabs, we should investigate which kind of symbiosis they exhibit.
Brainstorm answers to the following questions for corals and crabs living in a
lagoon environment. This means the reef is close to land and there is slow
water flow going in one direction, which often leads to high rates of sedimentation.
1) Can you think of any reasons corals might need the help of crabs?
2) Are there any reasons crabs would need the help of corals?
3) Based on your ideas, which kind of symbiosis do you feel crabs and corals exhibit?
Hint: Review your vocabulary. What are the 3 kinds of
symbiotic relationships we have mentioned in this unit?
Trapezia serenei

18. Forming a Research Question and Hypothesis
Dr. Stewart observed that the crabs were almost always found in the
center of the coral colony, where they were potentially receiving
protection from predators.
Dr. Stewart also observed that if she placed a pinch of sand on the
corals, the crabs would actively remove it.
Main Research Question:
What biological mechanisms does coral use to
persist in the physically challenging nearshore
environment with slow water flow and high
sedimentation?
Assignment: From what you have learned about Dr. Stewart’s
observations thus far, form a hypothesis that you can either prove
or disprove through experimentation.

19. Dr. Stewart’s Research
Dr. Stewart’s Hypothesis: Corals can persist in
low flow, high sediment nearshore areas
because they share a mutualistic relationship
with crabs that clear sediment from the polyps in
return for protection.
Assignment: How would you test this
hypothesis? Would you do a study in the field or
the laboratory? Write a detailed description of
the steps you would take as a scientist
investigating this question.

20. Dr. Stewart’s Methods
Experiments were performed in both the field and the lab. In this case, the
laboratory portion was in a “Wet Lab,” where the experiments are run in tanks.
What is the difference between a field and a lab experiment? Take a few
minutes to discuss with a group member.
Field experiment: Lab experiment:
Organisms are in their natural The scientist can make
surroundings, so they should adjustments to control what the
maintain their same behavioral organism is exposed to, but the
patterns organism’s behavior may be
(movement, feeding, what they different.
interact with, etc.)

21. Dr. Stewart’s Methods:
Field Experiment
• In order to test her hypothesis, Dr. Stewart had to decide how to quantify
her observations by collecting data that would test her hypothesis.
• Data was collected on how much sediment was being cleared on corals with
and without crabs.
• Then, she assessed the health of all experimental corals over time by
looking for coral polyps that were bleaching or dead.
• The study was conducted daily over 24 days.
• Two different species of coral and their crabs were tested to make sure that
the pattern found was not just specific to a certain species. They included:
– Acropora hyacinthus (coral) and Tetralia nigrilineata (crab)
– Pocillopora verrucosa (coral) and Trapezia serenei (crab)
• 20 pieces each of small Acropora coral Pocillopora coral were placed on
tiles using Z-SPAR.
• Half of the corals had crabs, the other half had crabs removed.
• These tiles were nailed down into the substrate at a backreef site.
• Corals were exposed to ambient sediment for 24 days.

22. Results: Field experiment
Crab No Crab

23. Results: Field Experiment
Assignment: Graph Interpretation
What do these graphs tell us?
Dr. Stewart graphed her data after
experimenting with two different
No Crab Crab No Crab Crab species of coral (Acropora results are
on the left, and Pocillopora results are
on the right). Take some time as a
group to determine what Dr. Stewart
can conclude from her experiments.
No Crab Crab No Crab Crab Hint: Pay attention to the labeled
axes, because they will tell you which
experiment that graph represents.
No Crab Crab No Crab Crab
Stewart et al. 2006 Coral Reefs

24. Results: Crabs positively affect the survival, growth and
health of both Acropora and Pocillopora species of coral
None of the corals
with crabs died, while
many of the corals
with no crab died.
No Crab Crab No Crab Crab
Corals with crabs
grew faster than
corals with no crabs.
Corals with crabs
had less bleaching
than corals with no
crabs.
Stewart et al. 2006 Coral Reefs

25. Methods: Lab Experiment
• This experiment tested the size and amount of sediment that crabs are able
to remove from corals.
• The experiment was run for 24 hours.
• 20 pieces of small Acropora coral and 40 pieces of Pocillopora coral were
placed in a large tank.
• Half of the corals had crabs, the other half had crabs removed.
• Either “low sediment” or “high sediment” (twice that of the low sediment)
was placed on top of the corals.
• Different sizes of sediment (shown below) were tested on different corals.
Note how small the crab is compared to the largest size of sediment!

26. Lab Experiment
crab
1 cm

27. Results: Lab Experiment
Crabs increase sediment removal from corals
Corals can
clear some SIZE: The
sediment on medium-small
their own size of
(white), but sediment
crabs help (0.25-0.5 mm)
clear more seemed to be
(gray), espec the easiest for
ially the corals and
larger crabs to clear.
sediment.
A higher
AMOUNT of Experimental
sediment error? Bleaching
added did occurred, which
not seem to may have
change affected the
cleaning experiment.
efficiency.
Stewart et al. in review

28. Further Investigation:
Would crabs still have a job in a “high flow” environment?
Sediment added to corals in field Low flow
2.00
Sediment remaining (g)
1.60
1.20
No crab
no crab
Flow
Crab
crab
0.80
High flow
0.40
0.00
Low High (N=8)
Flow
Preliminary experiment with 8 corals.

29. Further Investigation: Do corals and crabs begin this
mutualistic relationship when they are only juveniles?
If so, do the crabs have an effect on
the size and shape of the coral colony
as it grows?
1 cm
Crab settlement
Coral settlement

30. Conclusions
What did we find?
Crabs increase sediment removal from corals. This is just the
beginning to exploring the mechanisms that benthic organisms use
to survive nearshore flow challenges.
Why is coral research important?
When corals bleach or die, the reefs do not continue to grow. This
leaves less shelter for reef organisms, less substrate for benthic
organisms to attach to, and less food for corallivores.
Why is it important to study sedimentation?
The human population is increasing on the island of
Moorea, which includes higher rates of erosion from the clearing of
trees for buildings, golf courses, and agriculture. This results in
higher amounts of sediment entering the ocean, which can harm
the coral reefs.

31. Like I said, that’s my homie!
Not only is coral the home for a crab, they also share
the deepest friendship found on the reef… mutualism!
So, like, I know
I’m totally new
around
here, but if you
let me move in
we could SO
be BFF’s 4 life!
Baby Coral Baby Crab
End of Lesson

32. Vocabulary Check
• Benthic organisms: Living things associated with the bottom substrate.
• Pelagic organisms: Living things associated with the open ocean.
• Substrate: The base upon which an organism lives.
• Ecosystem: A biological community or series of communities, including the non-living
component of an environment.
• Ecology: The study of interactions between an organism and its environment.
• Coralline algae: Green and red algae that deposit calcium carbonate that assists in
reef building.
• Zooplankton: Animals that cannot swim against a current. Often microscopic and the
primary source of food for many other animals.
• Sediment: Loose particles of sand, clay, silt, and other substances that settle at the
bottom of a water body.
• Adaptation: The process of change over time for an organism to increase its chance
for survival (and reproduction) in response to environmental conditions.
• Sedimentation: The accumulation of sediment deposits.
• Photosynthesis: The process in which green plants combine carbon dioxide and
water in the presence of light energy and chlorophyll to produce carbohydrates and
oxygen (see lesson 1).
• Mutualism: A relationship between two organisms in which they both benefit from the
interaction.
• Quantify: To express observations as a number or measure.

33. Work Cited
Text
• Stewart et al. 2006 Coral Reefs 25: 609-615
• Stewart, H. Researcher. Personal contact. University of
California, Santa Barbara. Current contact: Friday Harbor
Labs, University of Washington hstewart@u.washington.edu.
Photos
1. Hoover, John P., krupp.wcc.hawaii.edu (Slide 13).
2. National Academy of Sciences, www.iwaswondering.org
(Slides 7-11).
3. Stewart, Hannah. Post-Doctorate Fellow. University of
California, Santa Barbara.
(Slides 1, 4, 5, 6, 14, 16, 17, 22, 25, 26, 29, 31).
4. Withy-Allen, Kira. University of California, Santa Barbara (Slide 3).