“Mr. Vaughn, what we are dealing with here is a perfect engine, an eating machine. It’s really a miracle of evolution. All this machine does is swim and eat and make little sharks, and that’s all”
This is how oceanographer Matt Hooper (Richard Dreyfuss) described the white shark in Steven Spielberg’s 1975 film Jaws. Certainly, when we think of an efficient and terrible marine predator, the shark always comes to our mind, and more specifically the white shark, which is the worst famous. However, if Mr. Spielberg had known of the terrible ways in which marine protists kill and devour their prey, perhaps the film would have been called “The Protist”. Jokes aside, if there are any creepy and dangerous beasts for their congeners in the ocean these are the protists.
Protists are eukaryotic (i.e., with nucleus) unicellular organisms ubiquitous in seas and oceans. We can classify them in various fashions, for example, by their way of obtaining energy: those that do photosynthesis (autotrophs or algae), those that eat other organisms (heterotrophs, also called protozoa) or those that combine both strategies (the mixotrophs). Here we will focus only on those that eat live prey, i.e. protozoa and mixotrophs.
Of the approximately 50 gigatons (50 billion tons) of carbon that algae produce annually in the seas and oceans, protozoa (perhaps with the help of mixotrophs) consume about 30- 60%. We think that the next relevant consumers, the copepods, only eat about 6 gigatons. Protozoa and mixotrophs do not eat only algae, they also feed on bacteria, other protozoa and even some animals much larger than them. But how do these tiny, mouthless unicellular beings eat them? The truth is that they have different prey capture and feeding strategies depending on the group, and they are all very curious.
The main feeding strategies in protists
Filtration: Many microorganisms use whale-like feeding systems, either by attracting prey into the oral orifice or by swimming and collecting prey. This feeding strategy is usually used for very small prey, such as bacteria or flagellates.
Engulfment: When prey begins to gain considerable size, many protists can catch and ingest them whole in a process resembling that of a boa eating a goat. In fact, some protists, such as the dinoflagellate Gyrodinium dominans have a very flexible body (cell) and can ingest chains of diatoms much larger than theirs (Fig. 1). Some foraminifera, distant relatives of amoebas and provided with an outer cover formed by calcium carbonate, can swallow even large copepods. The process is slow but effective (Fig. 2). Many protists use venom-laden stingrays or release toxins into the water to immobilize prey. Some toxins are so efficient that they can kill fish and other organisms, and even once accumulated by filters, such as mussels, they can lead to serious poisoning in humans.
Figure 1. Process of swallowing a chain of diatoms by the dinoflagellate Gyrodinium dominans. The red arrow indicates a G. dominans with a chain of diatoms inside. The blue arrow shows the size of the same species without prey inside. Photo by Albert Calbet
Figure 2. The image shows a foraminifer that has just captured two copepods. Photo by Albert Calbet
Tube or peduncle: Certain dinoflagellates have a retractable tubular structure that they insert into the prey to suck its contents, as if it were a straw on a Margarita Cocktel (Fig. 3). They use this mechanism to eat prey similar in size to theirs, but also to kill and devour, like tiny leeches, animals much larger than themselves, such as copepods, worms, and so on.
Figure 3. Peduncle feeding of a Dinophysis sp. on a myxotrophic ciliate (Mesodinium rubrum). Drawing Albert Calbet
Pallium or veil: This is perhaps the most curious and complex mechanism. Like sea urchins and starfish, they evaginate their stomachs (well, not an actual stomach indeed, but a membrane with digestive characteristics) in order to slowly digest their prey, such as large diatom chains (Fig. 4). Gradually, the trapped cells are consumed and the predator incorporates the dissolved nutrients into the membrane. Once finished, only a siliceous skeleton will remain.
Figure 4. Protoperidinium sp. dragging a chain of diatoms into its pallium. Photo by Albert Calbet
Piston: Not long ago, a very odd dinoflagellate was discovered and named also with a very odd name, Erythropsidinium. It has a small piston that can be expanded and hided very fast. It seems Erythropsidinium uses it to detect and, by a suction mechanism, catch prey that will be eventually swallowed (Fig. 5). The most interesting thing about this unicellular creature is that it also has a kind of primitive eye (ocelloid), with its lens and all. The function of this ocelloid is still subject of debate among the scientific community, but it could be used, in a very rudimentary way, to locate prey. Remember, we’re talking about a single-celled organism!
Figure 5. Representation of an Erythropsidinium showing its ocular lens and piston. Drawing Albert Calbet
Luckily for us, all these creatures, which could be taken from the most terrifying of Stephen King’s books, are no more than a few tens of thousandths of a millimeter. Imagine what would happen if they were our size!