• Unicellular (some are colonial)
• Numerous cilia on the cell surface- for movement or to generate water currents
• Two different kinds of nuclei (compartments containing the genetic material)

None really. Most people aren’t even aware they are there.

Ciliates are a group of protists in which the flagella have become very numerous are usually referred to as cilia. Most ciliates use their cilia to swim and to draw particles of food into their ‘mouths’. Some use their cilia like legs and crawl across substrates, while a few are sedentary and use their cilia to create strong currents to collect food. Almost all ciliates are tiny, colourless and can only be seen with a microscope. The largest are just visible with the naked eye and may be coloured. The vary emormously in shape- from fast swimming footballs to highly flexible and contractile ribbons to cells with protective shells to colonies of spheres on stalks.

Ciliates are extremely numerous in, and important components of almost all aquatic environments on earth, from freshwater to almost saturated brine, from the tropics to the polar circle. In the ocean, there many planktonic forms that are mostly free-swimming organisms. There can be dozens of individuals in every millilitre of water. All but the finest sediments contain many, mostly ribbon-shaped forms. Surfaces have diverse communities of flattened scuttling species. Some surfaces, including plants and animals, may have attached species. One such group, called folliculinids, live inside vase-shaped shells and have been found in dense concentrations on patches of dead coral. No-one knows as yet whether the folliculinids are responsible for the coral death, or are merely rapidly colonising the vacant habitat.

Almost all ciliates eat other microrgansims and other small organic particles using a special ‘mouth’ region which usually has a complicated pattern of cilia around it. The structure and function of this mouth varies dramatically with the foraging strategy and the size of the food consumed. The smaller species mostly eat bacteria, larger species usually eat other protists. Most species don’t seem to be particularly fussy with their food, as long as it is of a manageable size. A few species are specialists on odd types of food, such as long filaments of blue-green algae, or on other ciliates larger than themselves. Like all protists, ciliate have no gut:- Food is packaged into a temporary compartment called a vacuole, in which digestion occurs. A few species have algal symbionts and a few derive energy from associations with bacteria.

Ciliates usually fall prey to small animals such as copepods and larger protists such as dinoflagellates and other ciliates. At times, ciliates can form more than half of the diet of the smallest planktonic animals. As ciliates may be so numerous, and are often capable of efficiently consuming prey items much smaller than animals can usually manage, it is thought that they represent at major link between microbial organisms and animals:- “repackaging” energy into particles large enough for animals to use, particularly when phytoplankton growth is low. The Great Barrier Reef seems to consume a large proportion of the ciliates and other small protists from the water that passes over it each day. This may well be an important and often overlooked source of energy for the reef ecosystem.

Cilates usually reproduce by binary fission- where one mother cell splits into two daughter cells. Generation times depend on the size of the cells (small species are usually faster) and on the availability of nutrients, but under ideal conditions, it may be less than a day. Ciliates can generally undergo a sexual process where a pair of cells come together and swop half of their genetic information, without necessarily undergoing reproduction at the same time. There are usually ‘mating types’ within a species: Individuals must be of different mating types to pair successfully. Mating types are thus similar to the sexes of animals, however some ciliates have over 50 different ‘sexes’.

Most ciliates are solitary and free living. A few attached species form colonies. There are also a wide diversity of ciliates that live inside animals. It has been long known that there are high densities of highly specialised ciliates living in the stomachs of some herbivorous mammals, ranging from sheep to wallabies. It turns out that some herbivorous reef fish have ciliates in their stomachs too. While these stomach ciliates are probably beneficial to the fish (or at least, not too much trouble), there are other species that are harmful parasites. Whitspot disease, which affects many tropical/ subtropical fish is caused by a ciliate. Some externally attached species may cause problems for animals they are attached if too dense.

Despite their small size individually, ciliates (and other small protists), due to thier abundance and rapid ‘metabolisms’, play a major role in the oceans ecosystems. In particular they are the major consumers of bacteria and the smallest ‘algae’ and therefore have a major effect on the way that the oceanic ecosystems ‘work’ in terms of nutrients and energy.