CONTROL OF POSITION IN THE WATER COLUMN BY THE LARVAE OF POECILOCHAETUS SERPENS, (POLYCHAETA) - THE IMPORTANCE OF MUCUS SECRETION

We investigated mechanisms used by Poecilochaetus serpens planktonic l arvae to control their position in the water column. The respective ro les of swimming and of mucus secretion were evaluated. Active swimming did not depend on larval weight or age (7.67 mm . s(-1) for upward di rection and 7.44 mm . s(-1) for downward direction). Mucus secretions act as a buoyancy system. The sinking rate, when mucus is secreted, is directly related to both larval weight and the number of setigers, ra nging between 2.02 and 4.03 mm . s(-1), for 18 and 41 setiger-larval b odies, respectively. However, when no mucus secretion was produced, th e passive sinking rate was also correlated with larval weight or numbe r of setigers, but the absolute differences were small: 3.08 and 7.46 mm . s(-1) for 18 and 41 setiger-larval bodies, respectively. Mucus su spends larvae in the water column, thereby facilitating long planktoni c life with low energy expenditure. It is also used to capture nutrien ts and to return from the bottom to the water in the presence of unfav orable substrata. Mucus is especially important in this species becaus e of its long planktonic period that is required to allow the morpholo gical development of palpae and nuchal organs which play an essential role in the feeding process of benthic adults. Last the length of the maintenance of planktonic larvae close to the bottom is probably long, allowing them to partially escape from the advection of water masses. Copyright (C) 1997 Elsevier Science B.V.