BULLETIN OF MARINE SCIENCE, 48(2): 420-431, 1991 LARVAL RELEASE FROM THE EGG MASS AND SETTLEMENT OF EUPOLYMNIA NEBULOSA (POL YCHAETA, TEREBELLIDAE) Michel R. Bhaud ABSTRACT The study of distribution and abundance of soft-substrate organisms can be examined by the recruitment mechanisms of the species involved. A study was carried out on Eupolymnia nebu/osa (Polychaeta, Terebellidae), starting from the release of larvae from the egg mass through the initial tube formation. The liberation of larvae, e){tending over 10 days, was counter-balanced by a marked synchrony at the time of settlement which occurred over a 2-day period for nearly 70% of the larvae. Despite the morphological differences present at the time of liberation, settlement took place at the same morphological stage. The duration of the planktonic life varied in relation to the time ofrelease from the egg mass. The larvae were provided with sufficient energy reserves to reach the first benthic stage. At this point, the most essential requirement is the construction of the first tube. Later, two basic require- ments arise: food, and material for the prolongation of the tube. Both are satisfied by input from sedimentation. The synchronized settlement period caused a negative inter-individual effect, which tended to distance the individuals from each other, resulting in an even distri- bution pattern. The dynamics of benthic communities are subject to fluctuations, which are due for the most part to the different life history strategies of the species involved. Therefore, study of the most abundant species, whose presence in a community is likely to be the most influential, would appear to be essential. In the Bay of Banyuls, Eupolymnia nebulosa is one such species; the characteristics of its life cycle have been described previously (Bhaud, 1988a; 1988b; Bhaud et aI., 1987; Bhaud and Gremare, 1988). These can be summarized as follows: the growth of oocytes within the coelom occurs between September (10-30 ~m) and March (150-170 ~m). During the spawning period, which extends from the beginning of March until the end of May, the females deposit gelatinous masses containing fertilized eggs close to their tubes. This spawning period is synchronized. In observations over 5 years (1983-1987) the most marked peak occurred during the last quarter of the moon in April. Juvenile growth takes place during the seasonal increase in water temperature. An experimental study has shown that there is a positive correlation between an increase in temperature and growth rate, when sufficient food is available (Bhaud, 1988a). In spite of this information, release oflarvae from the egg mass into the plankton, and the transition from the water column to the sediment, remain largely unknown. Therefore, a study of these phases between spawning and recruitment has been undertaken to address the following questions. What conditions control the liberation of the larvae? What is the duration of planktonic life? What type of relationship is established between the first benthic stages and the sediment, and does the latter act simul- taneously or separately as a physical support and as a source of nourishment for the larvae? MATERIALS AND METHODS Collection of the Egg Masses. - In the study area, E. nebu/osa has a large vertical distribution ranging from the infralittoral upper levels (Laubier and Paris, 1962) to more than 300 m at the mouth of a submarine canyon (Reyss, 1964). Adults inhabit a tube and can be found at the interface of rocks and 420 BHAUD: RELEASE AND SETTLING OF E. NEBULOSA 421 coarse sediment. During the reproductive season, egg masses were always submerged and were located with a glass bottom viewing tube and removed by hand. Additional samples were taken by divers between 5-10 m depth. Samples were usually collected from a 70-m stretch of a small semi-enclosed bay close to the laboratory. Daily observations were made in order to determine the spawning dates. Range of Recruitment Times. - The following procedures were developed to determine the range of recruitment times of individuals emerging from a single egg mass at the beginning of the reproductive season. They were based on the progressive withdrawal or addition of sediment, contained in petri dishes, upon which the larvae settled. The beginning of the recruitment period was established using a tank filled with 50 liters of sea water and containing a single egg mass of known spawning date. A series of 12 petri dishes containing substrate were deposited on the bottom of the tank. The dishes were removed at a rate of one a day and isolated in another tank supplied with filtered water (0.2 !tm). The appearance of tubes in the withdrawn dishes signalled the beginning of settlement. The end of the recruitment period was established in a similar way: the petri dishes containing sediment were kept in a tank supplied with filtered running sea water, and were transferred at a rate of one a day to a tank containing a single egg mass of known spawning date. The end of the settlement period was evident by the absence of tubes in the introduced dishes after this point. The same procedures were carried out in May 1986, at the end of the reproductive season, with the following alteration: the transfer of the petri dishes from the tank with the egg mass to the incubation tank was made via an intermediary tank for a period of 2 days to allow complete settlement to occur, while avoiding possible contamination from the dishes already placed in the tank. Sediment Used. -Silt was obtained following sedimentation from laboratory seawater supplies and sieved on mesh of 60 !tm. A particle analyzer was used to give the cumulative size distribution. This process indicated that 76% of the particles were less than 10 !tm. A rough estimate of the quantity of organic material, 1.13 mg·g-' dry weight, was obtained by the loss of weight on ignition at 400·C of a dried sediment without carbonates. This sediment constitutes the standard sediment referred to in the subsequent text. Laboratory Rearing of the Juveniles. - The juvenile stages were kept in a tank 60 x 40 x 16 em, filled with 50 liters of sea water. Water was circulated through the tank at a rate of 0.5 liters·min-' and was drained from the surface by a siphon. Suspended particles in the sea water were allowed to settle out at two points of decantation. The egg masses were placed on a platform 7 cm from the bottom. Petri dishes with standard sediment were placed on the bottom of the tank. Food was added after juveniles became established. This food supplement was a suspension of 1.5 mg dry weight of tetra mine which had been pulverized in a Potter tube containing 12 ml of filtered sea water. The calorific value of the suspension was 4.10 cal, mg-' as determined with a calorimetric bomb of the type designed by Phil- lipson (1964). After addition of food, the water supply to the tank was stopped for 4 h to allow the particles to settle to the bottom to ensure that food was not a limiting factor. Distribution Pattern for the First Tubes. -A single egg mass was placed on a support close to the the surface of the tank, the bottom of which was covered with a fine layer of standard sediment. The quantity used was 3 mg (dry weight)'cm-2, giving a sediment thickness of about 0.1 mm. This enabled the larvae to reach the hard bottom of the tank which was a prerequisite for successful settlement (Bhaud, 1990). When the first tubes appeared, two photographs were taken. A grid dividing the photographs into units of known area allowed the numbers of tubes in each square to be counted and the parameters of the distribution calculated. In addition, the spatial distribution of first settling juveniles was characterized by measurement of the distance between the opening of an individual tube and those of its neighbors. Larval Requirements at the Time of Settlement.-Between 50 and 60 planktonic larvae, which had just emerged from the mucus mass, were introduced to a container holding 80 ml of sea water. The water was changed daily. A 2-ml suspension of silt which served as both food and material for tube- construction was added after the introduction of the larvae. The optimum quantity of food for this number of individuals was not known and so temperature was used to control the consumption of food by the larvae. The experiment was carried out at four different temperatures (6, 9, 12, and l5OC, each with two replicates), over a period of 30 days. Juvenile Requirements. - Larvae fed by means of cilia located around the mouth and the ventral side of the first tentacle. Ifadditional food is not added to the aquarium, the sediment in front of the tube is utilized for food and tube construction. An evaluation of juvenile requirements for construction material and food was carried out by supplementing the cleared zones close to the juvenile tubes with additional sediment using a pipette held 5 cm above the substrate. Four different characteristics are given to justify the use of this species for the experimental analysis of settlement: (1) after the release of all the larvae from the egg mass, an experiment of short duration (2 to 3 days) is sufficient to obtain settled stages; (2) settlement is easily detected by the appearance 422 BULLETIN OF MARINE SCIENCE, VOL. 48, NO.2, 199] Table I. Temporal data on the release of larvae (2 and 3), the disintegration of the mucus mass (4 and 6), and the appearance of the first tubes (5 and 7) as a function of the spawning date of the egg mass (1) 4 6 7 I 2 3 Mucus mass 5