# Amphidinium | Algae ID Source: https://algaeid.com/identification/amphidinium/ [ID guide](https://algaeid.com/identification/) › Dinoflagellates # *Amphidinium* sp. **Be careful when killing it.** The scientific literature describes *Amphidinium* as only mildly toxic, but aquarists and aquaculture workers I have spoken to have had real health problems while trying to exterminate it. Ventilate, run carbon, and remove as much biomass as you can by hand first. See the [safety notes](https://algaeid.com/treatment/#safety). ## How to recognize it - **Movement.** These cells like to move along surfaces rather than swim up in the water. If you give them time to settle on a slide or dish, nearly all of them will be seen **sliding smoothly across the glass**. - **Shape.** Like [Ostreopsis](https://algaeid.com/identification/ostreopsis/), they are sesame-seed shaped, but they move differently and are generally much smaller. The small-cell form is also smaller than [Prorocentrum](https://algaeid.com/identification/prorocentrum/), but the large-cell form (see below) can be close to it in size, so do not rely on size alone. - **The apex.** Look for **two narrow grooves or folds at the top of the cell**. A typical dinoflagellate has a groove running around the middle of the cell. In *Amphidinium* that groove has moved to the very top, so only a small piece of the cell sits in the "northern hemisphere", which gives the cell a cap-like look. It can look a bit like the Millennium Falcon. - **Inside.** A large central pyrenoid and a branching chloroplast that fills the cell. *Amphidinium*: a smooth, plate-less oval with its grooves at the top. ![Amphidinium cell](https://algaeid.com/img/amphidinium-photo-1.jpg) Amphidinium. Look for the small cap at the apex (upper left) where the grooves sit, the large central pyrenoid, and the branching chloroplast. Image courtesy of Amy Bertelli. ![Amphidinium cell](https://algaeid.com/img/amphidinium-photo-2.jpg) Amphidinium with a pin pointing at the apex, where the characteristic grooves are. Image courtesy of Amy Bertelli. ![Amphidinium cell](https://algaeid.com/img/amphidinium-photo-3.jpg) Another view with the pin at the apex. The chloroplast fills the cell in a branching pattern. Image courtesy of Amy Bertelli. ![Amphidinium cell](https://algaeid.com/img/amphidinium-photo-4.jpg) Several short grooves ("fingers") are visible at the apex of this Amphidinium cell. Image courtesy of Amy Bertelli. ![Amphidinium cell](https://algaeid.com/img/amphidinium-photo-5.jpg) Amphidinium in portrait view: apex notch at the top, round pyrenoid in the middle. Image courtesy of Amy Bertelli. ![Amphidinium cell](https://algaeid.com/img/amphidinium-photo-6.jpg) The same cell, closer. Image courtesy of Amy Bertelli. ![Amphidinium cell](https://algaeid.com/img/amphidinium-photo-7.jpg) Amphidinium among thin strands and debris, a typical view of a sample from a bloom. Image courtesy of Amy Bertelli. ![Amphidinium sp. cells](https://algaeid.com/img/amphidinium-own-1.jpg) Amphidinium sp., 2012. The orange date stamp is from the camera. ![Amphidinium sp. cells](https://algaeid.com/img/amphidinium-own-2.jpg) Amphidinium sp., 2012. ## Videos Motion matters, so please watch these before you decide. The last one is the best view of the apex. [Video: Amphidinium sp. at 40X](https://www.youtube.com/watch?v=uPJYgkgCMGE) Amphidinium sp. at 40X [Watch on YouTube](https://www.youtube.com/watch?v=uPJYgkgCMGE) [Video: Amphidinium sp. at 100X](https://www.youtube.com/watch?v=JasNtU92OVQ) Amphidinium sp. at 100X [Watch on YouTube](https://www.youtube.com/watch?v=JasNtU92OVQ) [Video: Amphidinium sp. at 400X](https://www.youtube.com/watch?v=yezAo2HjW3w) Amphidinium sp. at 400X [Watch on YouTube](https://www.youtube.com/watch?v=yezAo2HjW3w) [Video: Amphidinium sp. at 400X, in focus](https://www.youtube.com/watch?v=Y578C6sd-wY) Amphidinium sp. at 400X, in focus [Watch on YouTube](https://www.youtube.com/watch?v=Y578C6sd-wY) ## Large-cell and small-cell forms Aquarists who have compared notes separate two kinds of *Amphidinium*: - **Large-cell *Amphidinium*** (roughly 30 to 60 µm) is the one in most of the photos on this page. It glides along sand and rock and tends to stay near the bottom. - **Small-cell *Amphidinium*** (roughly 10 to 15 µm) is a much faster swimmer and spreads to every surface. It is less often reported. That is why I sometimes quote a much smaller size for *Amphidinium* in emails. Both sizes are approximate. The [Dinoflagellate Guide](https://docs.google.com/document/d/127cKH5miePn4tsvzWNGNMuQivMh3fmxniPCVwLdF3ec/mobilebasic) has photos of both. ## Videos from other aquarists Other hobbyists have shared videos of their *Amphidinium* with me. Compare how the cells glide with my videos above. A close-up of an *Amphidinium* cell. Video courtesy of Dr. Bobby Ortiz ([@dogtor_bob on Instagram](https://www.instagram.com/dogtor_bob/)). *Amphidinium* cells under the microscope. Video courtesy of Dr. Bobby Ortiz ([@dogtor_bob on Instagram](https://www.instagram.com/dogtor_bob/)). *Amphidinium* from a home aquarium. This looks like *A. massartii* and not *A. carterae*. Video courtesy of a hobbyist. Another video of the same kind of cell. Video courtesy of a hobbyist. A third video. Video courtesy of a hobbyist. [Video: Dinoflagellates??](https://www.youtube.com/watch?v=7gNk0SzyKWM) Dinoflagellates?? [Watch on YouTube](https://www.youtube.com/watch?v=7gNk0SzyKWM) ## Which species? Almost no *Amphidinium* in aquaria can be named to species by light microscopy. They have no thecal plates (they are "naked"), and plates are what most dinoflagellate taxonomy rests on. Most isolates get dropped into *Amphidinium carterae*, which in the literature is probably many different closely related species under one name. If you read about "*A. carterae*", keep that in mind. Some of the ones I have seen look more like *A. massartii*. The genus may not even be a single natural group. ## Toxins *Amphidinium* makes toxins called amphidinols. They break down very rapidly, which makes them hard to detect after a bloom has collapsed. ## Natural enemies Ciliates and a predatory protist, *Oxyrrhis marina*, eat *Amphidinium*. This video shows *Oxyrrhis* going after *A. carterae* (not my best video, but watch closely): [Video: Oxyrrhis marina feeding on Amphidinium carterae](https://www.youtube.com/watch?v=jSJErgoqsrk) Oxyrrhis marina feeding on Amphidinium carterae [Watch on YouTube](https://www.youtube.com/watch?v=jSJErgoqsrk) See [ciliates](https://algaeid.com/not-dinos/#ciliates) and the [treatment page](https://algaeid.com/treatment/). A blackout can reduce numbers but is unlikely to get rid of them permanently.