Article: Fluid transport properties of Nautilus siphuncular tube: within-camera distribution of flow rate

There are two viewpoints concerning the nature of flow across the siphuncular tube of Nautilus and other ectocochliates: (1) flow is uniform along the length of the tube; (2) flow is mainly localized in the septal necks. We tested these alternatives in specimens of N. pompilius by measuring flow rates through septal necks sealed with epoxy. We found that the septal neck, and specifically the so-called pillar zone, is not a conduit for fluid transport. Fluid entering or exiting the camerae flows uniformly through the permeable conchiolinous membranes of the connecting ring. This situation is the result of two factors: (1) the connecting ring is provided with osmotically active epithelial cells whereas the septal necks are not; and (2) the pillar zone is not porous, but consists of a solid array of crystal matrix intergrowths. The actual function of the pillar zone appears to lie in the direction of optimizing the mechanical strength of the septum-connecting ring junction and of minimizing the metabolic cost of constructing the connecting ring. The microstructure of the septal neck in many fossil cephalopods is consistent with this interpretation. Evaluation of siphuncle function and adaptation in fossil forms, and of evolution in siphuncular structure, must take account of the fluid transport properties of the connecting ring.