Most dinoflagellates are photosynthetic plankton that form the basis of many aquatic food webs or mutualist symbionts, sharing and sharing alike with their hosts. Apodinium floodi is an exception. Unlike its photosynthetic cousins, it is an ectoparasite, and its host is the pelagic sea squirt Oikopleura labradoriensis. Sea squirts, despite their sac-like appearance, actually belongs to a group of animals call the urochordates, which are more closely related to vertebrate animals than all other invertebrates. Apodinium floodi uses a structure call a peduncle to attach itself to the tail of the sea squirt, and penetrate the host's notochord to absorb nutrients from its blood.
It is an evolutionary marvel - a single-celled algae which has evolved to make a living by sucking blood from a pelagic animal!
Contributed by Tommy Leung.