Vertical distribution, life cycle and production of the chaetognath Sagitta crassa in Tokyo Bay, Japan

Abstract. The daytime vertical distribution of Sagata crassa in Tokyo Bay was examined from February 13, 1988 to February 20, 1989 High densities of larger-size chaetognaths were found near the sea bottom, whereas the smaller animals tended to inhabit the upper layers. This feature of distribution is discussed in relation to the distribution of their main food organisms, e g. Pseudodiaptomus marinus, Acartia omoru, Centropages abdommalis and Oithona davisae. The two periods of replacement of two morphs were confirmed by the vanation only in mean body length of this chaetognath, unlike the previous authors who made additional morphological observations. It was hypothesized that S.crassa has at least five generations Two generations, including mostly the larger forms, had higher growth rates than the generation consisting mainly of the small form. Yearly respiration of S.crassa was 8.2 g C m~2 Yearly production of this animal was estimated to be 3.8 g C m~2. A feeding estimate revealed that chaetognaths require a prey production of 13.1 g C m~2 year"1. The impact of this chaetognath on the prey population in Tokyo Bay and the propriety of an estimated value of annual production of 5 crassa is discussed