Abstract
Sagitta crassa is the only autochthonous chaetognath in Tokyo Bay, central Japan. We analyzed the gut contents of S. crassa at a fixed station in May and September 1997. The species fed on prey organisms whose body widths were in a range between 1.4-6.0% of the length of S. crassa. Prey with body widths less than 2% of the length of S. crassa were minor components of the gut contents. Although the cyclopoid copepod Oithona davisae was abundant in the water column, the prey selectivity of S. crassa on O. davisae was low. Our results suggest that S. crassa consumes catchable prey organisms that are larger and easier to detect. As planktonic species diversity in the Tokyo Bay ecosystem is low, it has been assumed that a simple trophic pathway from the most dominant primary consumer O. davisae to the secondary consumer S. crassa exists. However, such a simple assumption is inadequate to understand the prey-predator relationships and food-web dynamics in Tokyo Bay.
