Reproduction and growth of the deep-sea brittlestar ophiura-ljungmani (lyman)

Reproductive biology and population dynamics of Ophiura Ijungmani were studied by measuring disk diameters and by examining oocyte size-frequencies and fecundity from a sequential seasonal series of large samples from a 2900 m deep Permanent Station in the Rockall Trough (Northeast Atlantic). Egg size (max. 90 !lm) and fecundity (up to 57000 eggs per individual) suggests indirect planktotrophic development, while a clear annual cycle in gametogenesis, with probably synchronous spawning in JanuaryFebruary, confirms a marked seasonality that is also seen as an annual, summertime set of postlarvae on to the bottom. Analysis of size structure of the population indicates annually discrete year classes with a maximum longevitv of around 10 years. lndividuals show fastest growth and lowest survivorship as postlarvae with a higher survivorship and lower growth rate of about 1 mm. yr- 1 among sizes > 1.5 mm. Reproductive maturity is reached in the third year at 3.5-4.0 mm with individuals thereafter probably reproducing annually. Possible factors controlling the annual cycle in the supposedly seasonless deep sea are discussed in relation to the possible significance of a likely seasonal pattern in downward flux of organic particles derived from surface production.