The mucus of Sabella spallanzanii (Annelida, Polychaeta): Its involvement in chemical defence and fertilization success

In several species mucus typically forms a slippery coating that performs many specialised roles including the defence against infection and protection for eggs. In the present study we focused on the mucus of the polychaete Sabella spallanzanii in order to ascertain its involvement in chemical defence and fertilization success. The observations were carried out over an annual cycle according to different phases of the reproductive cycle. Mucus of S. spallanzanii showed a natural lysozyme-like activity with a maximum at pH 6.0 and I = 0.175. This activity was present in all the sampling times except for January (T₄) which corresponds to the reproductive period of this species. The electrophoretic analysis revealed at least ten major protein bands, with molecular weights ranging from 16 to 90 kDa, and six minor components, with molecular weights ranging from 14 to 116 kDa. This pattern was reported all over the year except for the reproductive period when not only the lysozyme-like activity decreased but the number of protein bands was also reduced. During this period microscopical observations of the mucus indicated that only fertilised eggs enveloped in the mucus were released out of the tubes where the worms live. This supports previous hypothesis that fertilization does not occur in the water column but it takes place in the tube just after egg release (in situ fertilization). This fertilization strategy, not involving egg dispersal in the water column, ensures a high success of fertilization. The mucus of S. spallanzanii therefore represents the ideal mean where eggs are released and fertilized as well as a compartment involved in defending the worms from bacterial attack serving as medium into which the antibacterial substances are exuded. The defensive role of lysozyme is important taking into account that this polychaete lives in eutrophic environments such as harbours where bacteria, including pathogens to man and marine organism, are abundant.