• To be unicellular, or sometimes colonial
• To have flagella in at least part of your life cycle.
• To have one flagellum that circles the cell while the other trails behind.

Dinos

Most dinoflagellates are microscopic unicellular, free swimming organisms with two flagella: one wraps around the cell inside a groove while the other is directed behind the cell. The flagella are used for swimming. Many dinos have a kind of armour covering in which the whole body which is made out of abutting plates. In larger species this armour often gives rise to various stiff sails and spikes that may serve to stop the cell sinking too fast, or to make them harder to eat. Most small species have a basically rounded or top-shaped body, although there are several flattened forms that live on surfaces or in sediments. Some species can form into chains. Roughly half of the dinoflagellates known are coloured, usually golden brown but some are reddish or green. A few golden brown dinoflagellates live most of their lives as symbionts inside reef-building corals and several other groups of animals, where they are known as zooxanthellae. The most famous free-living dino is Noctiluca which, at up to 2 mm across, is about the only species that can be seen without a microscope. Adult Noctiluca look like tiny balloons with a tentacle; quite unlike any other dinoflagellate. At night they can produce a brief, eerie, glowing light when disturbed by the propeller of a boat or the swimming motion of a large fish or mammal.

Free-living dinoflagellates live in most wet environments including freshwater. In the open ocean they are extremely numerous, especially in more tropical waters and when concentrations of nutrients are high. There are frequently large numbers in surface sands and muds and associated with macroalgae and seagrasses. Basically all reef-building corals have ‘zooxanthellae’ dinos in their bodies. Symbiotic dinoflagellates are also found in many other cnidarians (including some jellyfish), giant clams, several other types of animals and in some large protists such as foraminifera. There are also many parasitic forms, but in most cases they are very difficult to recognise as dinos.

Golden-brown dinoflagellates, including zooxanthellae, can photosynthesise- ie. act like ‘plants’. Colourless species will trap prey, acting like ‘animals’. These dinos are usually specialists at catching (comparatively!) large prey, which is usually engulfed whole. Some species use extendable tubes that can suck out the insides of other cells, or veils that are extended over prey which is then digested without the dinoflagellate having even to ‘swallow’ it. Such devices can allow the consumption of organisms several times larger than the dinoflagellate. Noctiluca frequently eats small animals. In addition, many of the photosynthetic dinos are quite capable catching and eating quite large prey too. Just to make things even more complicated, some originally colourless dinos have acquired other photosynthetic symbionts (often green coloured). Dinos refuse to be categorised as ‘plants’ or ‘animals’ even by purely functional criteria!

Dinoflagellates mostly fall prey to small animals, such as small crustaceans and fish larvae, and to other protists, including, in many cases, other dinoflagellates. Because they are so plentiful, cover a large size range and fulfil so many different ecological ‘roles’, they are thought to be a very important source of food for the larger organisms in the ocean.

Most of the time, dinoflagellates reproduce by binary fission: they simply split in half. In most armoured species, each daughter cell gets half of the plates, which means they can look quite peculiar until they get around to building the missing parts. Occasionally a sexual process will be undertaken, where after a few modifications, two cells will fuse to make a single larger cell which contains genetic material from both of the ‘parents’. The fusing cells can be of the same size, or of two different sizes, depending on the species. In Noctiluca, the sexual cells are made an ‘adult’ splitting itself up into lots of small motile cells.

Although most dinoflagellates are free-living, it is the symbiotic forms (‘zooxanthellae’) that are the most famous. Energy generated by zooxanthellae through photosynthesis is taken up by the host (eg a reef coral). The energy from the zooxanthellae can amount to more than the daily requirement of the host. It is thought that the evolution of the association between dinoflagellates and corals may have been instrumental in the development of the coral reef ecosystem and might be essential for its maintenance. Certainly great damage is caused to reefs by ‘coral bleaching’, which occurs when extensive areas of coral suddenly lose all of their zooxanthellae. Coral bleaching may be caused by abnormally high water temperatures and it is feared that global warming is causing coral-bleaching to become a more common and widespread phenomenon.

Some dinoflagellates produce toxins that become concentrated in the bodies of organisms higher in the food chain, such as fish and shellfish. During dinoflagellate blooms humans eating seafood can be poisoned. About 2000 cases are reported worldwide each year and several different illnesses have been identified. Near tropical reefs, potentially fatal Ciguatera poisoning can result from eating fish contaminated with a toxin from a dinoflagellate that lives on the surfaces of seaweeds. Dinoflagellate blooms can also cause massive kills of fish and shellfish, often causing significant economic damage. It is feared that blooms of dinoflagellates are becoming more common due to human activities that increase the nutrients levels in the ocean (eg run-off from fertilised pastures, sewage disposal). It is also thought that the indiscriminate dumping of ballast water may allow toxic strains to spread rapidly over long distances.