EFFECTS OF ENRICHMENT ON REPRODUCTION IN THE OPPORTUNISTIC POLYCHAETESTREBLOSPIO BENEDICTI(WEBSTER): A MESOCOSM STUDY

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Abstract
The influence of organic enrichment on growth and planktotrophic development of the spionid polychaete Streblospio benedicti Webster was examined in two mesocosm experiments conducted at the MRL facility, University of Rhode Island [USA]. Specimens of S. benedicti were collected and their reproductive traits monitored near the conclusion of a two-year eutrophication experiment, and in the middle of a sludge addition experiment. Nutrient (N, P, and Si) enrichments at 8 .times. and 32 .times. the average aerial input into Narragansett Bay, Rhode Island, resulted in increases in body length, segment number, and length per segment, and a doubling of brood size in S. benedicti females. These increases were substantially higher during May (12.degree. C) than August (20.degree. C). Enrichment effects were stronger in the 8 .times. than 32 .times. nutrient treatment. In the sewage sludge experiment body size increased 20% over control values at the highest (8x) sludge treatment level (nitrogen loading equivalent to the 8.times. nutrient treatment) but no significant increase was noted at the 4.times. sludge level, which received half as much nitrogen as the 8.times. sludge treatment. Mean brood size increased by a factor of 4.6 over controls in the 8.times. sludge treamtent and by a factor of 2.3 in the 4.times. sludge treatment. Within the range of adult body sizes observed, brood size enhancement occurred independent of increased length or segment number in both nutrient and sludge enrichment treatments. The ability to translate elevated food supply directly into increased reproductive output may underly opportunistic dynamics in macrobenthos. Brood size enhancement of the magnitude observed probably contributes to the high S. benedicti densities found in polluted or organically enriched settings.