Dinoflagellates, good or bad?

Dinoflagellates are protists organisms -those that do not fit within the three natural kingdoms: animalia, plantae or fungus- with ability to move with rotating movements. All dinoflagellates have the common feature of having two flagella located at right angles allowing them to perform such rotational movement which makes them easily recognizable (although some species such as the genus symbiodinium they are virtually immobile).

They are known about 2.400 species which have a highly variable size ranging from 20 and 300 µm in the most common ones and up to 2 mm in the case of species such as noctiluca. In the marine aquarium species are usual smaller as symbiodinium and Amphidinium, While some like ostreopsis are large and can be seen without a microscope (with good lighting and visual acuity).



So, where is the problem?

Dinoflagellates are organisms present in any aquarium, in fact they are necessary for smooth operation. They constitute an important part of the first step of the food chain so if we use a microscope we will find them in a greater or lesser extent. We even have famous dinoflagellates such as zooxanthella, symbiotically present in most corals.


The problem arises when conditions in the aquarium break the biological balance and some dinoflagellate species spread uncontrollably, smothering the rest of the aquarium inhabitants. If the dinoflagellate species in question has the ability to produce toxins (usual in ostreopsis, gambierdiscus and prorocentrum to name a few), then we can get ready for a disaster.

No species of dinoflagellate, including the dreaded ostreopsis, should be of concern whenever we follow some basic guidelines to ensure the biological balance of the aquarium. The problem often arises when we try to bring cleanliness it to the limit, in order to improve its appearance and color of corals.

The good, the Bad and the Ugly

As he commented above, not all dinoflagellates are bad, some are good and even necessary. A common feature of these organisms is their amazing reproductive capacity, which can turn even the most beneficial ones in a beautiful aquarium into a hideous swamp, in a few hours.

We can classify them into two functional groups: autotrophic and heterotrophic:


Autotrophic dinoflagellates

Are the ones we should be concerned about because they do not depend on external organic sources to thrive. They are photosynthetic organisms capable of producing their own food even when they barely have any nitrates and inorganic phosphate. Their main advantages are their enormous resistance to extreme situations, besides their reproductive capacity. They tend to occur suddenly when the aquarium water reaches an extraordinary cleanliness, in which most microorganisms perishes for lack of food. With no other organisms that can stop them, this type of dinoflagellate can multiply so fast that when we realize it's late and we will find an aquarium full of brown and ochre slime suffocating fish and invertebrates.

These dinoflagellates possess chloroplasts enabling them to synthesize their own food even under a minimal amount of light. Some species can form cysts called pellicles which allow them to remain in the aquarium for months although we have completely sterilized or kept in complete darkness. Once the light or the right conditions come back, they will reappear and thus problems.

Most of these dinoflagellates have a very curious behavior, typical of pathogens and parasites. With the presence of light they are benthic, they secrete mucus which adhere to any surface, including algae, coral and fish. They spend hours synthesising food and extending vertically in search of the light source (if we turn off the pumps in the aquarium we can see brown filaments grow towards the surface). When the light source disappears and can no longer synthesize food, they detach from the mucus that had been producing and undertake a pelagic stage, carried away by the currents to colonize new areas.


Heterotrophic dinoflagellates

They are those who need a source of organic nutrients for living. They are usually good organisms in an aquarium because their reproductive capacity is moderate and therefore easily controllable. Some of them like oxyrrhis marina  eat other dinoflagellates, so they help control. Their presence in aquariums, while it is usual, it is limited to very few species and very low population densities.

…and the devil

If one of these autotrophic species is capable of producing toxins.

This is the case of the well known ostreopsis (but there are quite a few such as as certain species of prorocentrum, gyrodinium and gambierdiscus to name some of the most common)


With own name: ostreopsis

An almost unknown dinoflagellate among aquarium hobbyists just four years ago, It is now a popular one. This ostreopsis, is the main cause of well known “red tides” .

Ostreopsis on the microscope
ostreopsis on coral
Red tide

and why is it so special?

Very easy: it combines all possible ingredients to ruin an aquarium:

  1. High reproductive capacity. Under favorable conditions we can see how it spreads before our eyes. Just turn off the circulation pumps and turn on the lights to see threads of ochre mucus ascending and reaching the surface in a matter of a few minutes.
  2. Size. It is one of the largest dinoflagellates that can be found in our aquarium, reaching up to 60 µm in its elongated part. This prevents most ciliates from preying on them and control their expansion. On the other hand it is not uncommon on the eyes and gills of fish causing them itching and stress.
  3. Toxicity. It is capable of producing a powerful toxin that will kill any organism that ingests it, especially gastropods and herbivorous fish that can be confused with other algae smell dimethylsulphide.
  4. Ease to form pellicles. When this dinoflagellate feels threatened or can not find the optimal conditions to thrive, it will protected itself by forming a cyst capable of resisting for weeks and even months out of water.


On the left we can see a picture of ostreopsis ovata on an electronic microscope where the binding of thecas and flagella can be seen, a picture of a coral suffocated by this species of dinoflagellate and finally an image of a red tide caused by ostreopsis

Ostreopsis cases in marine aquariums are increasingly more frequent. This dinoflagellate has become a real plague, threatening especially those with ultra low nutrient systems (ULNS) that are seeking a pastel coloration in corals.

The first symptoms of an incoming problem

The speed at which dinoflagellates can proliferate dinoflagellates is so high that you will normally detect them when the aquarium is already dirty and its inhabitants start showing a condition. The first symptoms of a dinoflagellates attack are usually the following:

  • Brown and ocher slime appearance on illuminated and high flow areas. These opportunistic scoundrels love clinging on the most troubled and illuminated areas, that provides them with energy and inorganic nutrients to synthesize their own organic nutrients .
  • Occurrence of snails and small herbivorous invertebrates dead.
  • Nervous behavior of some fish.
  • Significant pH decrease in the absence of light.

How to combat autotrophic dinoflagellates, basic rules

The first thing to do is identify the species or at least the family to which it belongs. Some dinoflagellates are banal organisms, non-toxic and generally very sensitive that can be eliminated with any of the commercial products designed for it (Dino X, Dino Remove or Dinoxal), others can be eliminated by using powerful oxidants such as hydrogen peroxide or by the installation of an ozone generator. However, most of the dinoflagellates that proliferate in the aquarium are often harder to remove and we should relay on completely opposite solutions,  aimed at promoting biological competition with other microorganisms. Therefore if do not identify the species of dinoflagellate and use any of the above products, it is possible that we worsen the issue by contributing to eliminate many of the organisms they compete with.


What we should do

Turn off the skimmer

This is usually the most effective measure to stop the spread of dinoflagellates. Contrary to what most people think, the skimmer is little more than an efficient apparatus for removing polar contaminants and bacteria, as well as helping to oxygenate the water. Dinoflagellates love environments rich in oxygen and poor in biodiversity so by turning off the skimmer we will soon notice significant relief in the fight against them.


Program the pumps at minimum power

Pumps not only oxygenate the water, which is something dinoflagellates like, it also it helps them to colonize the entire aquarium transporting them all over. It is very important to reduce the pump flow to a minimum or even shut down those that are not strictly necessary.


Provide the competing microorganisms, increase biodiversity

How insignificant as it may seem, add a couple of kilograms of new live rock, use live food (copepods, amphipods, mysis…) or some sand from a mature tank can greatly help restore the biological balance in the aquarium and stop the advance of dinoflagellates.


Feeding the aquarium

We must not forget the causes that have fostered the occurrence of dinoflagellates, specially have turned it into an almost sterile environment by limiting food intake or have few or no animals that slightly stain the water and thus provide a minimum nutrient contribution to conserve biodiversity.

It is frequently a sudden drop in inorganic nutrients because we have made repeated water changes, installed a too powerful skimmer or abused of phosphate resins which lowers significantly the biodiversity and concentration of the competing organisms. Therefore we should not feel confident if by doing a water test we find that nitrates and/or phosphates are high: also in this seemingly unfavorable scenario we can cause the occurrence of dinoflagellates, especially if we commit any of these excesses that lead to a sharp nutrient reduction.

I recommend the usage of phytoplankton on a regular basis, either live or in gel by a dosing pump. It also helps a lot the sporadic usage of wet foods like mysis, calanus or brineshrimp


Install an ultraviolet sterilizer

We talked about a common fact of autotrophic dinoflagellates: they are benthic at daytime and pelagic at night.

If we install a UV sterilizer, and keep it working at night it will kill many of them as they are floating in the water. We will not remove them completely but if the sterilizer is powerful enough, dinoflagellates will decline significantly.


Usage of activated carbon

When dinoflagellates begin to die, toxins are released into the water. It is proved that activated carbon (especially bituminous kind) helps remove these contaminants effectively. Unlike so popular ion exchange resins, activated carbon does not detain the proliferation of other microorganisms which outcompete dinoflagellates.


Siphon the slime at sunset using a filter sock

It is late in the day when the greater the masses of dinoflagellates are and when the easier it is to remove them. As will be discussed below is not sensible to make water changes, therefore we can proceed installing a filter sock in the sump (I recommend the usage of one not exceeding 150 µm) and suck the slime with a flexible hose.


Keep track of the evolution under a microscope

Any of the above actions often produce a satisfactory response in the fight against dinoflagellates. The best way to check this is by microscopic observation: we can see that we are on track when dinoflagellate mobility decreases (only in some species since others have lost motor function or have it very atrophied), if we find detached thecas or a drop their concentration. It is also desirable to see if other organisms such as nematodes are showing up, ciliates, heterotrophic dinoflagellates…

symbidinium zooxanthela

What we must NOT do

Water changes

Dinoflagellates love water changes. Water changes add the trace elements that dinoflagellates need to thrive, especially iron, iodine and silicon. Therefore we must avoid making the minimum addition of new water to the aquarium for at least one month after dinoflagellates have disappeared from our sight.


Usage of carbon sources

Have you ever used alcohol to revive a barbecue? that's what happens if you use carbon sources in an aquarium with dinoflagellates: they will spread inevitably within minutes. We should especially avoid any substance containing sugar, vinegar or ethanol as the well known NO3:PO4-X from RedSea or NP-Out from Xepta Reef. Denitrifying bacteria usage is not contraindicated but in most cases is irrelevant.


Using phosphate absorbing resins

In most cases I know (and there are few) usage of ion exchange resins triggers the occurrence of a dinoflagellate attack. When we use these products, We provoke a rapid drop of inorganic phosphate, essential for life of small microorganisms as ciliates, other heterotrophic dinoflagellates, nematodes or copepods. Many of these organisms die suddenly leaving a free way for dinoflagellates. It is for this reason that high phosphate aquariums where we find uncontrolled masses of dinoflagellates and cyanobacteria are more and more frequent; possibly the worst imaginable scenario, not ugly and unpleasant, but because it is often very difficult to return it to its biological balance.


Add trace elements

As I mentioned in the first point, dinoflagellates need certain trace elements to thrive. It is very important to avoid any by-product of iron, iodine, silicon and potassium. Especially potassium iodide commonly used in SPS aquariums.


Slime blowing

Dinoflagellates produce mucus to keep sticked to any surface. It is easy and tempting to use a pump and remove this slime from rocks and substrate as they apparently vanish in the aquarium. What we are actually doing is spread the problem and dinoflagellates will colonize new areas of the aquarium. Corals will also be affected and we will soon see dinoflagellates strings clinging from their tips, especially in areas with higher water flow.


Using oxidizing agents

Unless we are trying to eliminate certain species of amphidinium, symbiodinium or any other dinoflagellate considered banal, we must avoid the use of ozone generators, hydrogen peroxide or any commercial product not properly tested with the dinoflagellate species we want to deal. This is why I always recommend to try identify the species (or at least the family to which the dinoflagellate that we want to eliminate belongs). Most commercial products are completely ineffective against certain species of dinoflagellates such as ostreopsis and can cause greater damage by limiting the spread of other beneficial microorganisms while weakening corals and other invertebrates.


Swapping fish or invertebrates

It is common sense, if our aquarium suffers a dinoflagellate infection it is not very ethical to swap even a snail with other hobbyists as we will be transmitting them our problem. If it is a dinoflagellate species with capacity to form pellicles, as is ostreopsis, quarantine of at least three months is recommended before selling or swapping any animal from our tank, yet there is no guarantee that we will not transmit these dinoflagellates.


Sterilize the aquarium or any of its elements

Does not work in most cases specially on those dinoflagellates mentioned in the previous point. Some people think that dipping the rocks in freshwater will eliminate dinoflagellates, and are right, but they do not consider that with just one surviving dinoflagellate (attention to forming pellicles species) in a barren rock without any microorganisms that can outcompete, they will face a real problem. I speak on the experience of having dipped rocks ostreopsis in freshwater with high concentrations of hydrogen peroxide to find a major problem a few days later after reintroducing them in an also sterilized aquarium .

Other methods of combat

There are microorganisms with the capacity to kill certain autotrophic dinoflagellates, among which the following stand out:

  • Parvilucifera sinensis. It is a tiny parasitoid of most dinoflagellates. When parvilucifera detects the presence of a dinoflagellate approaches and penetrates it through the gap between thecas. Then it begins to multiply rapidly and within hours the dinoflagellate will be filled with tiny replicas of itself. The dinoflagellate explodes due to internal pressure, releasing dozens of new parasitoids which keep on attacking other individuals. The problem is that parvilucifera is very delicate and desirable to most microorganisms, especially ciliated ones, so it does not always work. If we use against dinoflagellates as ostreopsis we will get temporary results as some get pellicle protected and when parvilucifera has disappeared they will show up again.
  • Fungus. Some dinoflagellates can be very sensitive to attack by certain fungi.
  • Other dinoflagellates. It has been proved that dinoflagellates oxyrrhis marina prey on dinoflagellates as ostreopsis ovata.


It is important to insist that the best defense against dinoflagellates is to maintain a proper biological balance in the aquarium avoiding situations where extreme cleanliness lead dinoflagellates to find no outcompeting organisms. If our aquarium fulfills this condition there is no reason to fear the dinoflagellates, we can even introduce animals from an aquarium with visible dinoflagellates without risk to infection.