Hydromechanical mechanisms of colony organization and cost of hefense in an encrusting bryozoan, Membranipora membranacea

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In bryozoans, hydrodynamic interactions among colony members can substantially affect filtering rates and access to food‐rich water for zooids within a colony and for the colony as a whole. Hydrodynamic theory suggests that highly integrated (area‐filling) colony architectures lead to strong interference between neighboring zooids, whereas less integrated architectures do not. The highly integrated bryozoan Membranipora membranacea displays two morphological specializations that modify hydrodynamic interactions: internal excurrent zones (chimneys) and inducible defensive spines. In this study, possible hydrodynamic mechanisms underlying the cost of inducible defense and chimney organization were tested experimentally by culturing spined and unspined colonies under controlled flow conditions.Spines reduced colony growth rate when growth was limited by clearance rates, but not when transport of particles from upstream limited growth. This supports the hypothesis that the cost of inducible defense is primarily a reduction in clearance rate (i.e. spines cause hydrodynamic interference with feeding currents) rather than metabolic investment in spine construction. Chimneys were more closely spaced in spined than unspined colonies, consistent with a mechanism for organizing chimney formation based on responses by zooids to local hydromechanical cues rather than a rigid, colony‐level control of zooid growth form (astogeny). Direct manipulation of hydrodynamic cues resulted in substantial changes in chimney morphology, again supporting the hydromechanical organization mechanism. More generally, these experimental results support the theoretical assessment that the highly integrated architecture is hydrodynamically unfavorable. The success of Membranipora in some habitats suggests that its architecture confers compensating advantages, such as improved ability to compete for space. This tradeoff in growth habit may explain shifts in relative prevalence of bryozoan architectures with depth and across other environmental gradients.