The Syndiniales are an order of dinoflagellate

Dinoflagellate

The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...

s, found exclusively as endosymbionts of the tintinnid

Tintinnid

Tintinnids are ciliates of the choreotrich taxon Tintinnida, distinguished by vase-shaped shells called loricae, which are mostly protein but may incorporate minute pieces of minerals. Although appearing as early as the Ordovician period, tintinnids became abundant in the fossil record during the...

 ciliate

Ciliate

The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

s, crustacea and fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, protozoa

Protozoa

Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

, algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, other dinoflagellates. The trophic form is often multinucleate

Cell nucleus

In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

, and ultimately divides to form motile spores, which have two flagella in typical dinoflagellate arrangement. They lack a theca and chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s, and unlike all other orders, the nucleus is never a dinokaryon. A well-studied example is Amoebophrya, which is a parasite of other dinoflagellates and may play a part in ending red tide

Algal bloom

An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration...

s.