Life history traits of the deep-water medusa Periphylla periphylla as revealed through failure and recovery of recruitment

Abstract

Extended recruitment failure and subsequent success of the coronate medusa Periphylla periphylla in a Norwegian fjord provided a unique opportunity to estimate age-related biological traits by combining individual size distribution in the population over time with previously published data. The average growth rate decreased exponentially with age, from 0.55% day−1 during the first year to <0.17% day−1 after four years. This species becomes mature at the third year, when females carry approximately 10 mature oocytes. At an age of 9 years, the medusa is on average 12 cm in diameter and carries over 1000 mature oocytes. Recruitment of juveniles, 0.5–1 cm diameter, was estimated to be 15 female−1 year−1 in Halsafjord; mortality from egg to juvenile 1 cm in diameter, using oocyte turnover times (T) of 1–12 months, was 93% (T = 12 months) to 99.95% (T = 1 month). Eggs and juveniles younger than 10 months are not capable of vertical migration and remain deeper than the sill depth where regular fjord circulation occurs. Therefore they are only vulnerable to deep-water renewal, and a fjord population of P. periphylla is estimated to gradually decrease recruitment with increased frequency of deep-water renewal and reach zero recruitment with renewal intervals of 10 months or less. It is hypothesized that differences in regular and intermittent deep-water renewal is the main factor causing variability in size (age) distribution between fjord populations.