Dinoflagellate - Morphology

Morphology

Dinoflagellates are unicellular forms with one to three flagellae. Usually, they possess two flagellae: one which extends towards the posterior, called the longitudinal flagellum, and the other forming a lateral circle, called the transverse flagellum. In many forms, these are set into grooves, called the sulcus and cingulum. The transverse flagellum is ribbon-like and coiled, provides most of the force propelling the cell, and often imparts to it a distinctive whirling motion, which is what gives them their name. The longitudinal flagellum acts mainly as a rudder, but provides a small amount of propulsive force, as well.

Dinoflagellates have a complex cell covering called an amphiesma, composed of flattened vesicles called alveoli. In armoured dinoflagellates, these support overlapping cellulose plates to create a sort of armor called the theca, as opposed to naked dinoflagellates. These come in various shapes and arrangements, depending on the species and sometimes on the stage of the dinoflagellate. Conventionally, the term tabulation has been used to refer to this arrangement of thecal plates. The plate configuration can be denoted with the plate formula or tabulation formula. Fibrous extrusomes are also found in many forms. Together with various other structural and genetic details, this organization indicates a close relationship between the dinoflagellates, Apicomplexa, and ciliates, collectively referred to as the alveolates.

Dinoflagellate tabulations can be grouped into six "tabulation types": gymnodinoid, suessoid, gonyaulacoid-peridinioid, nannoceratopsioid, dinophysioid and prorocentroid.

The chloroplasts in most photosynthetic dinoflagellates are bound by three membranes, suggesting they were probably derived from some ingested algae. Most photosynthetic species contain chlorophylls a and c2, the carotenoid beta-carotene, and a group of xanthophylls that appears to be unique to dinoflagellates, typically peridinin, dinoxanthin, and diadinoxanthin. These pigments give many dinoflagellates their typical goldenbrown color. However, some dinoflagellates have acquired other pigments through endosymbiosis, including fucoxanthin. This suggests their chloroplasts were incorporated by several endosymbiotic events involving already colored or secondarily colorless forms. The discovery of plastids in Apicomplexa has led some to suggest they were inherited from an ancestor common to the two groups, but none of the more basal lines have them. All the same, the dinoflagellate cell consists of the more common organelles such as rough and smooth endoplasmic reticulum, Golgi apparatus, mitochondria, lipid and starch grains, and food vacuoles. Some have even been found with a light-sensitive organelle, the eyespot or stigma, or a larger nucleus containing a prominent nucleolus. The dinoflagellate Erythropsidium has the smallest known eye.

Some species have an internal skeleton consisting of two star-like siliceous elements that has an unknown function, and can be found as microfossils. Tappan gave a survey of dinoflagellates with internal skeletons. This included the first detailed description of the pentasters in Actiniscus pentasterias, based on scanning electron microscopy.