Ecological aspects of deformed chaetognaths and visual observations of their periphytes

Abstract

Preserved samples of chaetognaths (Sagitta crassa) in Tokyo Bay contained normal specimens and knotty and flabby deformed ones. On the body surfaces of both normal and deformed specimens, three different types of periphyte were observed by a scanning electron microscope (SEM): branch-like growths, large numbers of filaments and protuberances. Some branch-like growths frequently covered almost the entire body of even normal chaetognaths. Bacteria were only observed on the abnormal specimens. Bacterial colonization occurred inside the body rather than on the body surface. Due to the growth of bacteria, muscles degenerated and chaetognaths became abnormally knotty and flabby. Abnormal chaetognaths occurred in Tokyo Bay more frequently in July 1979 (12.4%) than in June 1982 (3.9%). Food containing ratio (FCR) values, an indication of feeding activity, of the abnormal chaetognaths were less than 10% and no feeding rhythm was found, whereas normal chaetognaths had high FCR values with a maximum peak (more than 60%) between 20.000 and 23.000 hrs. Abnormal specimens sometimes had spermatophores on their body surfaces. It is obvious that abnormal chaetognaths had still been alive in the sea and that some of them not only fed on copepods but also copulated. However, feeding activity and reproductive behaviour were far lower in abnormal chaetognaths than in normal ones. Abnormal specimens were not larger than 9.0 mm and their mean body length was smaller than that of normal chaetognaths. This suggests that bacterial infection directly affects the mortality of the chaetognath population.