On gill function in the musselMytilus EdulisL.

Abstract

The function of the gill of the mussel Mytilus edulis (L.) has been studied in intact animals and in animals with cut posterior adductor muscle, as well as on gill fragments and isolated gill filaments. Intact 3–4 cm long specimens, kept singly in glass jars containing 600 ml aerated sea-water at 1–2° or 14°C, were able to clear suspensions of various particles at high rates. The gills retained almost completely particles down to about 3–5 µm in diameter (yeast cells or latex spheres, and about half of particles 1–2 µm in diameter (Bacterium sp. or latex spheres). The immediate effect of cutting the adductor muscle was to decrease the retentiveness of the morphologically intact gills; later also the rate of water transport decreased. In intact, fully open mussels, 10⁻⁷-10⁻⁵ M dopamine (DA) or 10⁻⁷-10⁻⁶ M serotonin (5-HT) did not affect the rates at which the mussels cleared suspensions of yeast, whereas 10⁻⁵ M 5-HT strongly reduced clearances of suspensions of yeast and 18 µm Tetraselmis cells, the effect being greater on the clearances of yeast cells. In mussels with freshly cut adductor muscle, 5-HT had no effects on clearances of Artemia nauplii, but strongly reduced clearances of yeast cells. 5–6 days after operation, in mussels which had ceased to transport water, 10⁻⁵ M 5-HT produced clearances of nauplii that amounted to ¼-⅓ of the small-particle clearances measured before operation, whereas no clearances of yeast cells could be measured. Observation under the dissecting microscope on morphologically intact demibranchs of injured mussels showed that the rates at which suspended particles moved toward the gill surface varied from high to zero. The demibranchs also varied widely in efficiency with which they retained particles. The frontal cilia and cilia of the margin of the demibranchs were usually permanently active, maintaining steady currents of water and transport of particles along filaments and the free margins of the demibranchs. Also the latero-frontal cirri mostly remained active, whereas the lateral cilia eventually stopped beating. Addition of 5-HT in concentrations of 10⁻⁶-10⁻⁵ M restored fast movements of particles (water) toward the gill surface, that is, restored metachronal beating of the lateral cilia, whereas 10⁻⁷ M had little effect. 5-HT simultaneously reduced the efficiency of the gill in retaining particles. The reduction in retentiveness of the demibranchs could be correlated with a reduced angle of beat of the latero-frontal cirri. 10⁻⁶-10⁻⁴ M acetylcholine and 10⁻⁶ or 10⁻⁵ M adrenaline had no visible effects on the activity of lateral cilia or latero-frontal cirri. Small particles travelled on the ciliary tract inside the marginal food groove of the demibranchs at mean rates that varied from 0.21 mm sec⁻¹ at 5°C to 0.60 mm sec⁻¹ at 20°C. 10⁻¹ M 5-HT increased the rate of particle transport by ca 60% at 5°C and ca 20% at 20°C. The various systems of cilia on excised gill fragments or single filaments acted very similarly to what could be observed on the intact gills of injured mussels. In addition it could be observed that large particles or particles embedded in copious mucus were carried directly on the ciliary tracts of the frontal surface of filaments, whereas small particles were carried in the water current beyond the tips of the frontal cilia, at the level of the tips of the latero-frontal cirri at the end of their effective stroke. At 14°C, typical speeds of particles moving at this level were 0.5–0.8 mm sec⁻¹. It remains unsettled whether the currents of water that carry small particles are solely produced by the activity of the frontal cilia, or whether the beating of the latero-frontal cirri also contributes. The sensitivity of the lateral cilia and latero-frontal cirri toward 5-HT increased with age of the preparation. 10⁻⁵-10⁻³ M DA had no effects on the beating of the latero-frontal cirri. 10⁻³ M DA restored beating of the lateral cilia, but the metachronal activity was less stable and regular than the 5-HT-induced activation. 10⁻⁵-10⁻³ M DA did not influence the effects exerted by 5-HT on the lateral cilia and latero-frontal cirri. Resting latera-frontal cirri or cirri that beat slowly or irregularly are feather-shaped because of the branching off of the distal ends of the composite cilia fram the main stem. It remains to be settled whether this structure is always present also in normally beating cirri. The distances between the branching cilia were about 1.3–1.4 µm along the distal half of the cirri, and about 0.8 µm proximally. The literature on the function of the bivalve gill is discussed in the light of the present findings. It is argued that efficient particle retention in Mytilus edulis transporting water at high rates depends upon the integrated activity of the ciliary systems of the gill.