Population dynamics of Aurelia aurita (Scyphozoa) from an isolated brackish lake, with particular reference to sexual reproduction

Abstract

The annual cycle of abundance, growth and sexual reproduction of the scyphomedusa Aure- lia aurita was examined over a 12 month period in an isolated, shallow, brackish, man-made body of water: Horsea Lake, England. Strobilation started in December and ephyrae were present through June in varying numbers, suggesting a 7 month period of near-continuous recruitment. Horsea Lake has a dense A.aunia population, with the peak abundance of 24.9 rrr3 occurring in mid-May. Medusae (>10 mm) were present in the water column from March through January. Maximum bell diameter of 105 mm was observed in late September. However, medusa growth was very poor (G ranged between -0.03 and 0.07 day' 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 mesozooplankton community dominated by the calanoid copepods Acartia margalefi and Acartia lonsa. Nevertheless, the very small A.aurita medusae were still able to attain sexual maturity and reproduce. Ripe female medusae carrying fertilized eggs and planula larvae were first observed in May and June, although the main period of planula larva production was Septem- ber-December. In November, all females were ripe, the smallest being 19-20 mm bell diameter. At both Horsea Lake and a nearby productive estuary, Southampton Water, planula larva production increased linearly with wet weight, although compared with the larger A.aurita medusae from South- ampton Water, reproductive output at Horsea Lake was low. Aurelia aurita in Horsea Lake showed plasticity in its population characteristics in response to environmental conditions, with both 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 scarce.