Blue Crabs, Malathion and the Flume

I would like to say thank you to Kelly and her sweet dog Abby for allowing me to work with them this summer to gain knowledge from a hands on authentic science research experiment.    Your time, patience, experience and humor were greatly appreciated.  – Lisa

Kelly and Abby at the Flume

Kelly and Abby at the Flume


Who We Are

Lisa Coates, Kelly Correia, Amanda Guillory, Erin Erben, Suzi Uhling, Dustin Gabel, Amy Biven, Maribel, Nicole, and Azita

What We Did

In the lab, Kelly wanted to see how the pesticide Malathion that the city of Corpus Christi sprays every four days for mosquitoes affected the crabs when they were given two doses in 8 days. We separated the crabs into three groups: acute, control, and treatment. The control crabs did not receive any dose of Malathion, but were kept in the same type of glass bowls as the others for comparison. The acute crabs received one dose, and the treatment crabs received 2 doses. Every twelve hours, Kelly flipped the crabs over on their backs using two spatulas to time how long it took to flip them, and how long it took for them to flip themselves back over. Then, she noted their behavior during the flipping and measured to see if their eyes retracted or not when she waved a mini rubber spatula near them. After eight days, all of the crabs were taken to the hatchery where Kelly had a flume that had two pathways for the crab to choose from: the one with food, and the other without food. This was to see if the Malathion affected the crabs in finding and tracking their food source.

 Prepare lab for crabs

– Make sea water 


– Clean, fill tanks and set up filters

 Crabbing/ cleaning

AMGo crabbing at local estuaries (Laguna Madre, Packery Channel, and Oso Bay) 

 Set up crabs in lab

Label Each Bowl 1-35 for juveniles

Randomly sort crabs in holding bowls in order to be sexed and sized.  It needs to be random for the data to be unbiased.


Exposure night – 36 hours after feeding

8:20 almost crab time

  • Locate datasheet (X2), clip board, pencil, stopwatch, spatula, crab tongs, ruler, video camera
  • One person: will be handling the stop watch and recording the righting time
  • One person: will be handling the video camera and recording other measurements and comments that are made throughout the experiment

8:30PM Crab flipping/recording time

    • Select a crab and measure and sex it (if you have not already done so)
    • Place in control bowl and flip and record RT, eye, eye touch, comments, etc.
    • Place in treatment bowl (make sure the measurement, sex, and treatment information is recorded!)
    • Record the ACTUAL time that the crabs are placed in their respective bowls
    • Try to get the crabs recorded and placed in the treatment bowl within 5 minutes
    • A for Acute (A1, A2..)
    • C for Control (C1, C2…)
    • X for Exposed (X1, X2…)
    • R for Recovery (R1, R2…)

9:30PM Round 2(these are the acute exposure measurements 1 hour post exposure

    • SAME ORDER flip and record RT, eye, eye touch, comments, etc.
    • Record the ACTUAL time that the crabs are flipped


    • Crabs need to be flipped and recorded every 12 hours.  After 96 hours the crabs are re-exposed and the entire process starts over

    • 8:30AM: flip, eye, consistency, eyeT, ect. – videotape
    • Clean!
    • Watch videos
    • Record data
    • 8:30PM: flip, measure – videotape

Hatchery Time

  • Day 12 (84,96 hours expo2)
    • 830AM: flip
    • 9:30AM: drive to hatchery à acute exposure crabs only
      • Each crab in flume
        • 5 minutes acclimation time in flume
        • 20 minutes search time after food is added and barrier is removed
        • 25 minutes clean time for flumes/setup for next round
      • Day before:  Reset the salinity of the water before leaving!
      • Clean everything before leaving!
      • Bring crabs back to the lab for quarantine (Total: 8 days)
      • Flume job
        • Timer –
          • Time until the crab starts tracking
          • Time until food is located
        • Return to school and clean everything before leaving
      • 830PM: flip
  • Day 13 (HATCHERY ALLLLLLL DAY, if we wanted to test all of these crabs in the flume)à chronic exposure and control crabs
    • Hatchery at 8:30


  • Day 14 (clean up, prepare for next round)
    • CLEAN
    • Crabs are placed in “clean” bowls and allowed to detox for a week.
    • Clean all available bowls
    • Everything that goes in the acid bath needs to be clean today

Procedures for cleaning

  • Rinse all pitchers and stirring sticks X3 with soap/water
  • Wipe off counters/any spills from pouring
  • Put hotplate/stirplate away – wipe everything down with Clorox wipes
  • Consolidate remaining solution from trashcans into one container
  • Trashcans: rinse thoroughly 1-2 times with soap in the sink
    • AFTER RINSING IN SINK, bring them outside and wash them X3 with soap and water
  • Glassware:
    • Beakers: X3 soap/water – prep for acid bath
    • Graduated cylinders: hand wash (Follow EPA protocol)
    • Volumetric flask: cover lid with parafilm and place in far left corner by the sink (this need to sit for minimum of 7 days before the stock solution can be dumped and cleaned)


What We Learned

Overall, the crabs that were treated with Malathion behaved differently than the control crabs. The treatment crabs were less aggressive, and acted more carefree. Their eyes did not retract normally, and it was harder for them to track the shrimp, if they even tracked it at all.

Swimmerettes: The very back legs on a blue crab that look like paddles and allow it to propel itself while swimming.

Fun Fact: When a crab feels threatened, it can eject its legs so that it can get away if caught.

Crabs molt just like a snake sheds.





Questions We Have

Why were we able to catch more adult males than females?  The males tend to prefer the lower salinity waters.

Why did it take longer for the crabs who molted in the lab shells to harden?  (In their natural habitat crab shells harden within 3-4 after molting, yet the crabs in the lab took about 3 weeks)

Are the crabs in the estuaries safe to eat due to the pesticides in the waters?


Connections to Teaching

Teks 7.9  Expressions, equations, and relationships. The student applies mathematical process standards to solve geometric problems. The student is expected to:

(A)  solve problems involving the volume of rectangular prisms, triangular prisms, rectangular pyramids, and triangular pyramids;

(D)  solve problems involving the lateral and total surface area of a rectangular prism, rectangular pyramid, triangular prism, and triangular pyramid by determining the area of the shape‘s net.