POPULATION-DYNAMICS OF AURELIA-AURITA (SCYPHOZOA) FROM AN ISOLATED BRACKISH LAKE, WITH PARTICULAR REFERENCE TO SEXUAL REPRODUCTION

The annual cycle of abundance. growth and sexual reproduction of the s cyphomedusa Aurelia aurita was examined over a 12 month period in an i solated, shallow, brackish, man-made body of water: Horsea Lake, Engla nd. Strobilation started in December and ephyrae were present through June in varying numbers, suggesting a 7 month period of near-continuou s recruitment. Horsea Lake has a dense A.aurita population, with the p eak abundance of 24.9 m(-3) occurring in mid-May. Medusae (>10 mm) wer e present in the water column from March through January. Maximum bell diameter of 105 mm was observed in late September. However, medusa gr owth was very poor (G ranged between -0.03 and 0.07 day(-1) over a 12 month period), and monthly mean bell diameters were typically <50 mm. For most of the year, the abundant A.aurita population was most likely severely food limited as a result of a numerically and species-poor m esozooplankton community dominated by the calanoid copepods Acartia ma rgalefi and Acartia tonsa. Nevertheless, the very small A.aurita medus ae were still able to attain sexual maturity and reproduce. Ripe femal e medusae carrying fertilized eggs and planula larvae were first obser ved in May and June, although the main period of planula larva product ion was September-December. In November, all females were ripe, the sm allest being 19-20 mm bell diameter. At both Horsea Lake and a nearby productive estuary, Southampton Water, planula larva production increa sed linearly with wet weight, although compared with the larger A.auri ta medusae from Southampton Water. reproductive output at Horsea Lake was low. Aurelia aurita in Horsea Lake showed plasticity in its popula tion characteristics in response to environmental conditions, with bot h fecundity and size to maturity varying on a seasonal basis. Medusae appeared to partition the available food resources into either somatic growth when food was abundant or reproductive effort when food was sc arce.