Crabs and corals and water movement presentation

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

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).

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!

Backreef: Slow flow; Sedimentation can be a challenge for benthic organisms.

Forereef: All organisms are exposed to fast, high energy waves.

Terrestrial run-off (Sediment)

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.

A Closer Look: The Forereef

A Closer Look: The Backreef

The environment is much calmer, so water flow is slower and moves in one direction.

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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.

Why is Water Flow Important?

Gases

Wastes

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Dissolved nutrients

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 nutrients if there is a slow flow.

Water Flow

Sediment

Gases

Particle movement

Food

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Dissolved nutrients Wastes

Water flow also brings particles to the organism. This may include desired food particles (for example, zooplankton), or undesired sediment (sand or silt).

Sediment

Gases

Particle movement

Food

Gametes

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Dissolved nutrients Wastes

Benthic organisms further rely on water flow to remove unwanted sediment and to release their gametes (reproductive cells).

Sediment

Gases

Particle movement

Food

Hydrodynamic force

Gametes

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Dissolved nutrients Wastes

However, moving water also exerts hydrodynamic forces that push against an organism, which can become challenging for the organism.

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?

The scientific word for change: AdaptationCoral 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) is more delicate with thinner branches, which is found in calm water.

Forereef: Pocillopora eydouxi (Antler Coral) is more compact with thicker branches in higher energy wave zones.

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.

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!

Did Anyone Mention SYMBIOSIS?

Coral with No Crab Coral with Crab

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!

Pocillopora verrucosa Trapezia serenei

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?

Does this mean the live corals and crabs participate in a symbiotic relationship?

Trapezia serenei

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.

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.

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: Organisms are in their natural surroundings, so they should maintain their same behavioral patterns (movement, feeding, what they interact with, etc.)

Lab experiment: The scientist can make adjustments to control what the organism is exposed to, but the organism’s behavior may be different.

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.

Results: Field experiment

Crab No Crab

Results: Field Experiment

Stewart et al. 2006 Coral Reefs

Crab CrabNo Crab No Crab

Assignment: Graph Interpretation

What do these graphs tell us?

Dr. Stewart graphed her data after experimenting with two different 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.

Hint: Pay attention to the labeled axes, because they will tell you which experiment that graph represents.

No Crab Crab Crab

Crab Crab

No Crab

No Crab No Crab

Results: Crabs positively affect the survival, growth and health of both Acropora and Pocillopora species of coral

Stewart et al. 2006 Coral Reefs

Crab CrabNo Crab No Crab

None of the corals with crabs died, while many of the corals with no crab died.

Corals with crabs grew faster than corals with no crabs.

Corals with crabs had less bleaching than corals with no crabs.

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!

1 cm

Lab Experiment

crab

Results: Lab ExperimentCrabs increase sediment removal from corals

Experimental error? Bleaching occurred, which

may have affected the experiment.

Stewart et al. in review

Corals can clear some sediment on their own (white), but crabs help clear more (gray), especially the larger sediment.

SIZE: The medium-small size of sediment (0.25-0.5 mm) seemed to be the easiest for corals and crabs to clear.

A higher AMOUNT of sediment added did not seem to change cleaning efficiency.

Flow

Further Investigation: Would crabs still have a job in a “high flow” environment?

Low flow

High flow

0.00

0.40

0.80

1.20

1.60

2.00

no crab

crab

FlowLow High

Sed

imen

t re

mai

ning

(g)

No crabCrab

(N=8)

Sediment added to corals in field

Preliminary experiment with 8 corals.

If so, do the crabs have an effect on the size and shape of the coral colony as it grows?

Coral settlement

Crab settlement

Further Investigation: Do corals and crabs begin this mutualistic relationship when they are only juveniles?

1 cm

ConclusionsWhat 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.

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!

Baby Coral Baby Crab

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!

End of Lesson

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.

Work CitedText• 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 [email protected].

Photos1. 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).