Life Cycle, Abundance, Gonadic Stages and Size Frequency Distribution of the Chaetognath Sagitta elegans Verrill in the North-eastern Irish Sea

Plankton was collected to the east of the Isle of Man from February 1986 to November 1987. Sagitta elegans overwintered mainly as gonadic stage II, with a small number of stages I and III. Stage III became dominant in the population in March; animals in stage I were the main component of the population in April with the onset of spawning. Small animals in stage I of c . 2 mm length were recorded from April until the end of the summer. Animals of this species died after spawning and no mature specimens stage III were caught by mid-autumn. From the present data a 1 year life cycle is proposed for Sagitta elegans in the study area. However, in June-July of both study years a second important spawning took place which was more pronounced in 1987. More work is needed to investigate whether this second spawning peak was due to early maturation of animals born 3 to 4 months earlier. S. elegans was more abundant in summer and attained a maximum length of 23•6 mm. Minimum size was found to be 1•2 mm and animals hatched in March-April reached a mean size of 11 to 13 mm by November in both study years