Crinoid Bio-Medium Aquaculture: Nutrient Flows in Classroom Aquaria

  • Crinoid Bio-Medium Aquaculture: Nutrient Flows in Classroom Aquaria
    Purchase access to the 3D stereolithography (.STL) files and accompanying lesson plan for the Crinoid Bio-Medium Aquaculture model!

    Model Price: $2.99

    Hits the Standard: HS-LS2-3: Construct and revise an explanation based on evidence for the cycling of matter and nutrient flow of energy in aerobic and anaerobic conditions.

    Lesson Brief: Producers such as plants and consumers such as fish are important components of many ecosystems. Energy and nutrients flow between them and other components. Providing a substrate, or physical habitat, for another component, decomposers, can increase the efficiency of nutrient recycling. Using a linked fish – plant system, students will add a decomposer element, in the form of nitrate-reducing bacteria, to the system and document how this alters nutrient flow through the system.

    Background: Box and arrow diagrams are used in ecological science to show the magnitude and location of different pools of a nutrient as it moves through a system. The boxes represent different locations, different forms of the nutrient, and different biotic groups in the community as they use the nutrient. Arrows show the flow of the nutrient from one box to another. Together, these form a graphical balance sheet for the nutrient in a given ecosystem.
    Nitrogen is vital to all life. It is an important especially in proteins. When animals break down nitrogenous compounds, they release waste products such as ammonia, NH4. Plants take up nitrogen in the form of nitrates, compounds containing NO3-. In a natural ecosystem, there are many different routes between these forms of nitrogen. In addition to producers and consumers, a functioning ecosystem has decomposers. These organisms break down complex organic matter into simpler forms, and alter the form of nutrients making it available to producers and different consumers. Many decomposers are bacteria that alter ammonia to nitrite, NO2-, and other bacterial species that alter nitrite to nitrate which is then useable to plants. Bio-media provide a physical substrate for bacteria communities that help change ammonia from fish waste into plant-useable nitrate.

    Activity: Print out and assemble several sets of the Crinoid bio-medium model (Figure 1). If two aquaculture set-ups or two aquaria are available, outfit one with a chamber of Crinoids downstream of the existing filter. Leave the other aquarium as it is. Allow the water to circulate as normal for at least several weeks. Using commercially-available test strips, test the nitrogen levels in the tanks. Create a box and arrow diagram for the nitrogen cycles in the two tanks. If only one aquarium or aquaculture set-up is available, test nitrogen levels before using Crinoid media and several weeks after introducing.

    Analysis: Did the ammonia, nitrite, or nitrogen levels differ between the treatments with and without the Crinoid media? Was the surface area provided to the nitrogen-altering bacteria sufficient? After examining the box and arrow diagrams, suggest ways that this system could become more self-sustaining, in other words, require less inputs and removals.


    Figure 1: Assembly of the Crinoid bio-substrate. Print 1 scolex and 16 arms. Glue arms halves together ensuring careful match of halves of pegs. Insert pegs into holes. Printing can be facilitated by printing enough for 1 Crinoid at once, and by ensuring flat side of arms are down. Holes for 4 arms are not visible in the figure.

  • Hi Jeff,

    From a perspective of 3D printing, might it be more robust to have a thicker stem which plugs into the base? A rectangular socket would also prevent rotation of the attachments (though due to their symmetry, it doesn't look like that is an issue).


  • Great question, @dowdt! You are correct in that the symmetry here does not matter. The arms are radially symmetric about the center. Good idea on the socket for other models though. I ran a test print and got all 8 arms in without any problems. I think the 2 mm socket would be too thin if it was sticking out further without support. In this model, it sinks into the hole until the wide circular base on the arm is flush with the flat area around the socket, preventing movement.
    Despite all these design plans though, I will monitor and ask people to report any problems with the size or breakage. I will rapidly make an changes necessary, and get new files out to anyone with an old version.
    Thanks for the feedback and questions!

  • Any opinion on the size of the pegs on the Crinoid arm? Should I make them thicker just to be on the safe side? Let me know if you have an opinion one way or the other---I am happy to modify.

  • @dowdt -- When your are right, you are right. We had a peg on a Crinoid model break as you suspected it might. We have doubled the diameter of the pegs and are running prototype tests now. Once we are satisfied that we have a robust model, we will be sending out new Crinoid files to everyone that has purchased this model.

  • The new Crinoid bio-medium model is now available. The download will appear the same but we have switched out the old model files for the new, more robust version.

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