Waters et al.—Respiration in blastoid hydrospires
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Figure 5. CFD simulations of a hydrospire of Monoschizoblastus rofei.(1) Hydrospire modeled in passive flow mode with external current velocity of 10 cm/s. Arrows illustrate water entering the hydrospires through the aboral hydrospire pores and largely exiting through the adoral pores. Minimal water volume exits the spiracle. (2) Hydrospire modeled in active flow mode with external current velocity of 0.5 cm/s and spiracular exit velocity of 0.9 cm/s. Although the orientation of the hydrospire has rotated 90º, the fundamental water flow through the hydrospire is the same (compare to Fig. 3.4). (3) Hydrospire modeled in active flow mode with external current velocity of 2.0 cm/s and spiracular exit velocity of 0.9 cm/s. Although the orientation of the hydrospire has rotated 90º, the fundamental water flow through the hydrospire is the same (compare to Fig. 3.4). (4) Hydrospire modeled in active flow mode with external current velocity of 5 cm/s and spiracular exit velocity of 0.9 cm/s. The exit velocity of water at the spiracle is too low to maintain optimal water flow through the hydrospire, which suffers reduced flow out the spiracle and significant respiratory leakage. (5) Hydrospire modeled in active flow mode with external current velocity of 5 cm/s and spiracular exit velocity of 2.5 cm/s. Under these conditions, the hydrospire achieved optimal flow within the hydrospire in contrast to the same hydrospire under passive flow conditions shown in Figure 5.1. (6) Hydrospire modeled in active flow mode with external current velocity of 10 cm/s and spiracular exit velocity of 5 cm/s. Under these conditions, the hydrospire achieved optimal flow within the hydrospire in contrast to the same hydrospire under passive flow conditions shown in Figure 5.1.
In this simulation, the water flow through the hydrospire was optimized for efficient gas exchange. Finally, we ran a CFD simulation with an external current velocity of 10cm/s and spiracular exit velocity of 5cm/s (Fig. 5.6). In this simulation, water flow through the hydrospire was optimized for efficient gas exchange (compare to Fig. 5.1).
Discussion
CFD simulations of blastoids living in currents ranging from >0.5 cm/s to 10 cm/s indicate that a velocity of water leaving the spiracle ~50% of the external current velocity produces water flow through the hydrospire that is more or less optimal for gas exchange. Our CFD simulations indicate that passive water flow through the hydrospire only produces optimal flow through hydrospires at very low current velocities (<0.5 cm/s). Therefore, we hypothesize that active cilia-driven flow through blastoid hydrospires is necessary to maintain effective respira- tion at the range of current velocities (>0.5 cm/s to 10 cm/s) in which a majority of blastoids likely lived. Blastoids likely lived in environments with fluctuating, and likely dynamically fluc- tuating, current velocities (according to the range of sedimen- tary regimes in which blastoids are found), suggesting that a system of active maintenance of water flow through the hydro- spire was necessary for optimal gas exchange.
active cilia-driven flow through blastoid hydrospires, many questions remain that will drive our research into the future. The pattern of water flow through the hydrospires and relative change in water velocity in the system are sufficient to develop the hypothesis of active cilia-driven flow in blastoid hydro- spires, but the results should be quantified to provide more precise hypotheses of hydrospire function. Blastoid hydrospires come in many configurations. We have modeled the simplest configuration of hydrospires, but an obvious next step is to conduct similar analyses on more complicated hydrospire morphologies. Although time consuming and tedious, such studies are required to test the validity of our hypothesis. Given the morphological disparity seen in blastoid hydrospires, there is no a priori reason to believe they all were optimized for the same range of current velocities. Indeed, Dynowski et al. (2016) have demonstrated that fossil crinoids can adopt a range of feeding modalities to adapt to dynamic current condi- tions. Modern crinoids can reorient themselves dynamically (Waters, personal observations) in response to change in current velocity and direction and we assume blastoids had similar capabilities even if we do not have a biomechanical understanding of the mechanisms involved. Some blastoids were bottom dwellers and likely had different optimal condi- tions for respiratory function from those described here.
Although our CFD simulations support a hypothesis of
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