The Food of Plankton Organisms. II

Extract

Last year preliminary results on the feeding of some of the commoner plankton organisms were published (1922). Further progress is made this year, more especially on the living animals, chiefly cœlenterates, kept in the plunger jars. The results of these experiments, specially undertaken to find out whether medusæ naturally caught and ate small fishes, answer the question undoubtedly in the affirmative. A large number of the smaller medusæ were kept alive, and most of these were seen to catch and eat fishes. The usual method of catching the food is with the outstretched tentacles, which, drawn out to very fine threads, react when anything living touches them, stinging the prey with their stinging cells; the tentacles then contract and helped by others, and often also by the lips and sides of the umbrella, manage to convey the food to the mouth, from which it reaches the stomach and is digested. A certain amount of selection is apparent, for the tentacles often reject food. Moreover, medusæ of one species do not as a rule eat one another, although taking other species voraciously. If many small fishes are available the smaller crustacea are neglected by many of the medusæ, although these may be taken when other food is searce. Sagitta bipunctata and Tomopteris helgolandiea are popular with nearly all those that can eat fishes. Certain medusæ, e.g. Sarsia tubulosa and S. prolifera, have never been seen to eat anything but crustacea (copepods or decapod larvæ), and fishes have not been seen inside them.