What is a dinoflagellate?

A single-celled swimmer with two flagella, a weird nucleus, and a bad reputation in reef tanks.

The short version

Dinoflagellates ("dinos" for short) are single-celled organisms that live in nearly every watery place on Earth. Biologists estimate there are around 11,000 species. Most are far too small to see without a microscope. They show up as plankton in the open ocean, as slime on sand and rock, and, inside many corals, as the tiny golden partners that feed the coral.

Different dinos make a living in very different ways. Some photosynthesize like plants. Some hunt other cells like tiny predators. Some are parasites, some live inside other animals, and plenty switch between these lifestyles depending on what is on the menu. A few species are the culprits behind harmful algal blooms such as red tides, which is where most of their bad reputation comes from.

Why they are called "dinos"

The name comes from the Greek dinos, "to whirl". Every dinoflagellate has two different flagella (the whip-like tails that single cells use to swim):

• One is a ribbon-like flagellum that wraps around the cell in a groove and beats to the cell's left.
• The other is a rudder-like flagellum that trails out behind the cell and beats backward.

Together they make the whirling, corkscrewing swim that gives the group its name. That motion is also a handy clue for identification, as you will see on the ID guide.

Armor, or not

Many dinos are wrapped in a suit of armor: stiff plates made of cellulose (the same stuff plants use) tucked just under the cell membrane. Biologists call these cells "thecate" or "armored", and the plates are the "theca". The arrangement of those plates is the traditional way to tell species apart, and it is still the gold standard for most genera.

Other dinos are "naked" and have no plates at all. That makes them hard to identify from their looks alone, because the main thing taxonomists would normally use is missing. Amphidinium is one of the naked ones, which is why it is so difficult to name to species.

Prorocentrum is a bit of a special case. Its plates have been reduced to two big halves called valves, plus a few tiny plates around a pore at the top where the two flagella come out. See anatomy of a dinoflagellate for pictures.

The strangest nucleus in biology

Dinoflagellates have one of the weirdest nuclei known. In almost every other organism, chromosomes are tidy bundles that only condense into their familiar X-shapes when a cell is about to divide. In dinoflagellates they are permanently condensed, all the time, and they are not wrapped around the usual packaging proteins either. Under a microscope this gives the nucleus its characteristic look: a big, cloudy region that is easy to spot in a stained cell. This arrangement has a name, the "dinokaryon".

The genomes themselves are enormous. The average dinoflagellate genome is more than ten times the size of the human one, and some are around a hundred times bigger. Prorocentrum micans, a relative of the Prorocentrum that turn up in tanks, has a genome more than 70 times the human genome. When I was in graduate school, sequencing it with the technology of the day would have cost well over $100 million. The one exception to the "huge" rule is Symbiodinium, the coral symbionts, whose genomes are comparatively tiny (about half the size of ours).

This is a big part of why dinos are so hard to identify with DNA, and why I cannot always give you a species name. The genes are repeated many times over, in long tandem arrays, and many of them are slightly different from one another. My PhD was about untangling some of that.

Once thought to be a missing link

Because of all that strangeness, dinoflagellates were once regarded as a "missing link" between bacteria and the complex cells of animals and plants. They were even given their own name, the Mesokaryota ("middle nuclei"). When DNA evidence became available in the late 1980s and early 1990s, it turned out that dinos are fully complex cells after all, and are close relatives of two other surprising groups: the ciliates (the animal-like cells that sometimes eat dinos in your tank) and the apicomplexans (a group of parasites that includes the malaria parasite). Together the three groups are called the Alveolata, named for the little sacs, or alveoli, just under their cell membranes. So when a ciliate chases a dino across your microscope slide, it is a chase between cousins.

Where the pests fit

I worked out a family tree for the dinos using 668 genes from 58 species. A few things stand out for aquarists:

• Gambierdiscus, Ostreopsis, Coolia, and Alexandrium all sit in the same big group, the Gonyaulacales. That fits what I see in practice: Gambierdiscus and Ostreopsis are close relatives, and both are toxic.
• Prorocentrum belongs to a neighboring group (the Prorocentrales). My analysis supported the Prorocentrales, together with another group of shellfish-poisoning dinos (the Dinophysiales), being the sister group to the Gonyaulacales.
• Amphidinium sits on a different branch from all of those. It is one of the "naked" dinos.
• Symbiodinium, the coral symbiont, is on yet another branch (with the Peridinales as its closest relatives)..

Open water vs. your sand bed

When people talk about red tides they usually mean plankton drifting in the water. The pests in this guide are mostly benthic, meaning they live on surfaces such as sand, rock, and glass, where they form slimy mats. They are related to the bloom-formers, but they behave differently. Some benthic species make potent toxins, and some release them when a lot of cells die at once. That is why I keep nagging about identifying your dino before you start killing it. See the safety notes.

Want to learn more?

Next, try Is it a dino? to work out what you have, or anatomy of a dinoflagellate for a guided tour of the cell.