FOOD REGULATION OF GROWTH AND MATURATION IN A NATURAL-POPULATION OF AURELIA-AURITA (L.)

Growth and maturity development of the moon jellyfish, Aurelia aurita, were recorded in Vagsbopollen, a small and semi-enclosed bay on the N orwegian west coast, and compared to those of medusae transferred to e xcess food and starving conditions, respectively. Mesozooplankton were extremely scarce in Vagsbopollen. The abundance and biomass of the me dusae in the poll were higher than those typically found in open water s, reaching a maximum of 22 ind. m(-3) and 710 mg C m(-3) in June. The average diameter of medusae in the poll increased to 8 cm until the l ast part of June. with an instantaneous growth rate between 1.5 and 20 % day(-1), thereafter retarding somewhat, giving a negative growth rat e of up to 2.6% day(-1). Starving medusae showed a negative growth rat e of up to 13.4% day(-1), and all the medusae were dead after 49 days. Well-fed medusae showed a very stable growth over a 56 day period, di verging from the poll population from early June, and with a growth ra te between 3.8 and 9.8% day(-1). Medusae from the poll population bega n carrying planulae on their oral arms when at least 5 cm in diameter, whereas not even the largest medusa of 15.6 cm diameter among those i n the well fed group produced any planulae. For the first time, it is thus explicitly shown that the size and maturity of A. aurita are exte rnally controlled through food availability. Scarcity of food reduces the growth rate, but also changes the energy allocation towards reprod uction, which thus occurs at a smaller size than for well-fed medusae. Its plasticity makes it possible for this species to exploit environm ents with low advection of food and develop high abundance in such env ironments, without losing fecundity.