Distribution of a population of annelid polychaetes in the “trottoir” of the midlittoral zone on the coast of North-East Spain,Western Mediterranean

The discharge and mode of action of the microbasic mastigophores (b-Rhabdoids) of Anemonia sulcata Pennant (Anthozoa) tentacles were examined using light and electron microscopical techniques. The statistical analysis of different morphological parameters of 100 nematocysts before and after their complete discharge revealed four different phases in the process of discharge. The first phase consists of the opening of the three opercular flaps. The second phase is characterized by the extrusion of the shaft. Osmotic pressure generated by the nematocyst propagates the extremely fast shaft discharge. The third phase, the evagination of the long tubule, is propagated by intrinsic mechanical forces stored in the tubule's twists and coils. During the process of evagination the tubule length increases by 95%, its circumference by 17%. Its forward motion is produced by the dilatation of its circumference and undoing of its twists. This motion is supported by the pressure which is generated by the hydration of the capsular matrix. The fourth phase consists in the extrusion of soluble capsular material at the tip of the evaginated tubule. During discharge the net volume of the nematocyst increases by 182%. This expansion is only possible if a high osmotic pressure is generated within the cyst. The extrusion of the shaft is the crucial phase in the discharge of the microbasic mastigophores, because without the puncture produced by the shaft the tubule is not able to overcome even surfaces of low viscosity media. The function of these tentacular nematocysts is the mechanical capture of prey organisms and, most probably, the introduction of unknown toxins into the latter.