Engulfment of Ceratium spp. (Dinophyceae) by the thecate photosynthetic dinoflagellate Fragilidium subglobosum

The marine thecate dinoflagellate Fragilidium subglobosum (von Stosch) Loeblich (= Helgolandinium subglobosum von Stosch), previously believed to be an obligate phototroph, is shown to be mixotrophic, apparently feeding exclusively on Ceratium spp. Food uptake involves neither a pallium nor a peduncle but depends on direct engulfment. The amphiesma of F. subglobosum possesses bodies which might be involved in the initial digestion of the prey's theca and perhaps also in the capture of the prey. Ceratium cells are ingested within 5-15 min, depending on their size. The prey theca is gradually dissolved during engulfment, but the theca of F. subglobosum remains intact until engulfment is completed, although the feeding cell is able to increase its volume approximately threefold during the process. This is possible because the individual thecal plates detach from one another making the theca more flexible.