In this article we will discuss about:- 1. Habitat and Habits of Dinoflagellates 2. Structure of Dinoflagellates 3. Reproduction 4. Economic Importance.
Habitat and Habits of Dinoflagellates:
(i) The din-flagellates are important component of phytoplankton. Most of them are marine but some occur in fresh water. Some din flagellates such as Gymnodinium and Gonyaulax grow in large number in the sea and make the water look red and cause the so called “red tide”.
(ii) Some periplast covered golden din flagellates called zooxanthellae occur in a number of marine protozoan protists (ciliates, radiolarians, foraminifera’s) and invertebrates (sponges, corals, jelly fishes, gastropods, etc.).
(iii) Some marine din flagellates show bioluminescence. It means they emit light, e.g., Noctiluca, Gonyaulax, Pyrocystis, Pyrodinium.
(iv) Nutrition is photosynthetic.
Structure of Dinoflagellates:
(i) Din flagellates are basically unicellular motile and biflagellate, golden brown, photosynthetic protists. Predominant colour is golden brown but yellow, green, brown and even blue forms also occur due to change in proportion of various pigments. A few are no motile, no flagellate, amoeboid, and filamentous.
(ii) Cells are generally covered by a rigid coat the theca or logical of articulated and sculptured plates of cellulose. Periplast may occur instead of theca. Because of the presence of sculptured plates, these protists are often known as armoured din flagellates.
(iii) The theca contains two grooves, the longitudinal groove called the sulcus and the circular groove known as the cingulum or annulus or girdle.
(iv) The two flagella are different (heterokont), one transverse flagellum and other longitudinal flagellum. The longitudinal flagellum is narrow, smooth, directed posteriorly and lies in the sulcus. The transverse flagellum is ribbon-like and lies in the cingulum or annulus. The two types of flagella beat in different directions. This causes spinning of dinoflagellates while swimming in water.
(v) The nucleus is larger in size and has been named as mesokaryon by Dodge (1966). Chromosomes do not have histone or RNA.
(vi) Plastids or chromatophores have chlorophyll a and chlorophyll c.
(vii) Mucilage bodies or vesicles occur below the cell membrane.
(viii) A non-contractile vacuole called pusule is present near the flagellar base. Pusule may take part in floatation and osmoregulation. Contractile vacuoles are absent.
(ix) Varieties of eye spots occur in dinoflagellates. Some of them are like ocelli.
(x) Trichocysts are found in a number of dinoflagellates. Nematocysts have also been reported in a few dinoflagellates.
Reserve food is stored in the form of starch and oils.
Reproduction in Dinoflagellates:
(i) Asexual reproduction is commonly through cell division. Cysts occur in a number of dinoflagellates.
(ii) Sexual reproduction has been reported in some dinoflagellates (e.g., Ceratium). It is isogamous and anisogamous.
(iii) The life cycle involves zygotic meiosis in Ceratium, Gymnodinium and Woloszynskia. Gametic meiosis occurs in Noctiluca.
Glenodinium, Peridinium, Gymnodinium, Gonyaulax, Ceratium, Noctiluca.
Economic Importance of Dinoflagellates:
Some dinoflagellates (e.g., Gonyaulax catenella) are poisonous to vertebrates. When they are in large number, they produce the toxin called saxitoxin into the sea water which kills fishes and other aquatic animals. Marine shell fishes (sea mussels-molluscs) eat large number of dinoflagellates.
The poisonous substance of dinoflagellates reach the shell fishes. The poison is not harmful to the shell fishes but the consumption of these infected mussels by man causes severe illness called paralytic shell fish poisoning (PSP) and may be fatal.
It is a heavy armoured dinoflagellate which occurs in marine, brackish and fresh waters. Sexual reproduction is anisogamous. Male gametes are small. Cytoplasm possesses a large mesokaryotic nucleus and a number of scattered chromatophores.
Noctiluca (= The Night Light):
It is a colourless dinoflagellate which is an important constituent of coastal plankton of both temperate and tropical seas. Nutrition is holozoic. Gametes are similar (isogametes). This alga is famous for bioluminescence as it was the first dinoflagellate where bioluminescence was reported.
It is characterised by the presence of a fragile long tentacle that functions as a flagellum. The transverse flagellum is reduced into a tooth-like structure. The longitudinal flagellum is small. The sulcus is developed into an oral groove and a cytostome. Tentacle develops beneath the cytostome.