Observations of valve-like structures and evidence for rectification of flow within the gill lamellae of the crab Carcinus maenas (Crustacea, Decapoda)

Abstract

The isolated gills of Carcinus maenas, perfused at pressure drops of 1–10 cm of water, exhibited flow rectification, the resistance to perfusion via the afferent vessel being many times lower than that for efferent perfusion. The asymmetry was greater at the lower end of this pressure range. The overall afferent branchial resistance for Carcinus of weight 65 g, and with no ventilatory component in the transmural pressure difference, was estimated to be 0.05 cm of water. μl−1 · sec. The corresponding overall reverse (efferent) branchial resistance was 0.36 cm of water · μl−1 · sec. LM, TEM and SEM examination of the gills indicated that haemolymph leaves each gill lamella via several discrete parallel efferent channels which drain different regions of the lamella, and that each efferent channel is nearly closed, at its junction with the efferent branchial vessel, by a diaphragm of loosely interwoven and very elongated cells. It is concluded that these cells may constitute efferent valves and that narrow apertures between them may contribute a major component to the branchial resistance and be primarily responsible for the rectification of flow. Relatively wide apertures lead directly from the afferent vessel into the lamellae and are not asociated with valves of any kind. The valves may be important in enabling changes in transmural pressure associated with ventilatory reversals to pump haemolymph unidirectionally through the lamellae. Similarly valves may allow the oscillating venous pressures associated with locomotor activity to improve gill perfusion during exercise. The elongated tails of the cells of the efferent valve contain numerous microtubules. The wider cell bodies contain the nucleus and many mitochondria. Unusual organelles composed of many short (about 0.25 μm long) microtubules and often lying close to the nuclear membrane may be microtubule organising centres. It is speculated that, in addition to their simple mechanical function, the valve cells may play a more dynamic role in regulating flow of haemolymph through different lamellar routes, or that they may monitor composition, pressure or flow of the efferent lamellar circulation.