Jean Claude Duchêne

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 larvae to control their position in the water column. The respective roles 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 direction 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, ranging between 2.02 and 4.03 mm · s−1, for 18 and 41 setiger-larval bodies, respectively. However, when no mucus secretion was produced, the passive sinking rate was also correlated with larval weight or number 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 suspends larvae in the water column, thereby facilitating long planktonic life with low energy expenditure. It is also used to capture nutrients and to return from the bottom to the water in the presence of unfavorable substrata. Mucus is especially important in this species because of its long planktonic period that is required to allow the morphological 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.

Experimental study of filtration activity in Ditrupa arietina (Annelida Polychaeta) using an automated image analysis system

Abstract An automated image analysis system designed to assess the activity of benthic organisms is described. This system is used to study filtration in the serpulid polychaete Ditrupa arietina. The video sensor of the system is composed of a black and white charged coupled device, a microprocessor, a memory and an interface board. It is driven by real-time routines, which are downloaded in permanent memory prior to each experiment. These routines control picture acquisition frequency and compute the differences in grey levels between the image recorded at a given time and a reference image (corresponding to no filtration in the case of D. arietina). These differences are used to detect numerical objects, which are stored in the memory board. At the end of each experiment, these objects are uploaded to a microcomputer where they are analysed using a second set of programs. This procedure involves several parameters, namely: the minimal object surface which is indicative of a real difference between two images (minimal surface), the research area (search radius), the research sites, and the definition of a set of conditions relating differences between images with a true filtering activity by D. arietina. These parameters were determined by comparing the results obtained using: 1) the automated system, and 2) classical frame by frame videotape analysis. They were then validated on several batches of worms by comparing total filtering durations per worm measured using the as previously calibrated automated system with those obtained using classical video observations. This step showed that the automated system is suitable for studying filtering activity in D. arietina. Our first results show that inter-individual variability is high, which has important consequences on experimental plans designed to assess the effects of environmental factors on filtration.

The effect of photoperiod on the timing of larval release in the Mediterranean brood-care polychaete Eupolymnia nebulosa (Terebellida)

The effect of light-dark cycle on the timing of larval release from jelly masses of the Mediterranean polychaete Eupolymnia nebulosa (Terebellida) was investigated. The larval emergence from the jelly mass occurs mainly during daylight. Results suggest that the larvae of E. nebulosa do not track the light - dark cycle endogenously.