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Species of the marine planktonic diatom Chaetoceros were collected over a three year period from Otsuchi Bay, Japan. The vertical distribution of Chaetoceros spp. over a 40 m depth was investigated. One species, C. coarctatum, was found to have chloroplasts of two different shapes. Chaetoceros coarctatum was also the only species found in association with the ciliate epibiont Vorticella oceanica. The numerical abundance of V. oceanica was closely correlated to that of its host at certain depths. Potential food sources for V. oceanica in Otsuchi Bay were identified. Vorticella oceanica is redescribed following examination by light and scanning electron microscopy.

Chaetognaths are an abundant component of the plankton and are usually the most important carnivores. Head or tail damage is often found in chaetognath specimens from the laboratory and the sea. Examination by light and scanning electron microscopy (SEM) of specimens with such damage has revealed that bacterial invasion occurs in chaetognaths under laboratory and natural conditions. Living chaetognaths which cease feeding are found to have large numbers of attached bacteria and some parts of their body become opaque. Once they are attacked in this manner the body appears to be a mass of bacteria. Examination of field samples has shown that similar bacterial attacks occur in the sea. Bacterial growth is common inside the muscle tissue of field specimens. Up to 13.3% of two dominant species of chaetognaths in Suruga Bay are attacked by bacteria.

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.

Association between copepods and bacteria was observed in many scanning electron micrographs. Particular sites on the copepods were selectively colonized by bacteria; the joints of segments and legs, swimming legs and depressed parts of the body surface were found to be densely covered with bacteria. In comparison, bacterial attachment to copepod skeletons in fecal pellets excreted by chaetognaths was not selective; bacteria were sparsely found all over the copepod. Between 9 to 30% of copepods in Tokyo Bay waters had attached bacteria in January and April 1983.

The chaetognath Sagitta crassa Tokioka, which is abundant in Tokyo Bay, may seize and ingest several copepods within the space of a few minutes under laboratory and natural conditions. This chaetognath consumed about a third of its daily ration within 2 h after receiving a supply of food. After partial ingestion of a copepod, S. crassa occasionally relinquished its prey, which was immobile and appeared to be paralyzed. Large numbers of bacteria were found on the exoskeleton of a released copepod, instead of the expected holes from piercing by chaetognath teeth. S. crassa had a mean digestion time of 2.2 and 2.8 h for Acartia clausi Giesbrecht and Tigriopus japonicus Mori, respectively. The digestive efficiency of Sagitta crassa was determined gravimetrically and it ranged from 56 to 93%, depending on the food items, with a mean of 83%.

Pelagic chaetognaths in Sagami Bay and Suruga Bay, Central Japan, were studied. Their community structure was very similar in both of these bays. Four genera and 26 species were identified. In Sagami Bay the habitat segregation by depths was clearly obtained. The population and species number of epipelagic (0–200 m) chaetognaths were larger in summer than in winter. The populations of meso- (200–500 m) and bathypelagic (500–1,000 m) species were smaller than that of epipelagic ones and fairly stable seasonally.

The feeding ofSagitta nagae, the most predominant chaetognath species in Suruga Bay, was studied. Specimens were collected with ORI-100 net (1.0 mm in mesh aperture) and MTD net (0.35 mm in mesh aperture) and were fixed with neutralized formalin. The main food organisms ofSagitta nagae were the copepods,Candacia bipinnata, Calanus pacificus andPareuchaeta russelli. The food-containing ratio (P/T, %) was higher at night and lower in the daytime. Here, P is the number ofSagitta containing food organisms in the gut and T is the total number ofSagitta examined. P/T was almost constant vertically in the upper 100 m layer. P/T was generally influenced neither by food density nor by the copepod numbers perSagitta in this study.Sagitta nagae took food throughout life. Food ingested bySagitta per day was calculated as 37.6% ofSagitta in dry weight.