Biology · Biological Classification · NEET
They are Protista, not plants. NCERT places all single-celled eukaryotes (Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, Protozoans) under Kingdom Protista. They photosynthesise like plants, but they are single-celled eukaryotes, so Whittaker groups them in Protista. For NEET, always mark dinoflagellates as Protista.
Red dinoflagellates such as Gonyaulax sometimes multiply extremely fast in the sea. So many red-pigmented cells appear together that the whole sea surface looks red. This rapid, huge growth is a red tide. The large numbers also release toxins that can kill fishes and other marine animals.
Their cell wall has stiff cellulose plates on the outer surface. This is different from diatoms, whose wall is made of silica. Remember: dinoflagellates = cellulose plates (like armour); diatoms = silica (like a glass soap-box).
Most dinoflagellates have two flagella. One flagellum lies longitudinally (lengthwise) and the other lies transversely (crosswise) in a furrow between the cell wall plates. This two-flagella spinning arrangement gives them their name (dino = whirling).
They can appear yellow, green, brown, blue or red. The colour depends on the main pigment present in the cell. Only the red-pigmented ones (like Gonyaulax) cause red tides when they bloom.
Mostly marine (they live in the sea). NCERT clearly says dinoflagellates are 'mostly marine and photosynthetic'. If a question asks about mainly ocean-living photosynthetic Protista with red tides, that points to dinoflagellates.
Chrysophytes, Euglenoids, Dinoflagellates and Slime moulds are included in the kingdom:
Which of the following organisms are known as chief producers in the oceans?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Gonyaulax is the classic NCERT example. It is a red dinoflagellate that multiplies very fast and makes the sea appear red.
Yes. During a red tide, the huge number of dinoflagellates release toxins that can even kill marine animals such as fishes.
An algal bloom is any sudden fast growth of algae or algae-like protists that colours the water. A red tide is a specific type of bloom caused by red dinoflagellates like Gonyaulax that turns the sea red.
The name comes from their two flagella and whirling motion. One flagellum lies lengthwise and the other lies crosswise in a groove, making the cell spin.
Most are photosynthetic (autotrophic), which is why they can appear green, brown, yellow, blue or red depending on their pigments.
