"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

April 21, 2022

Aggregata sinensis

Apicomplexa is a diverse phylum of single-celled parasites. They are found in a wide range of different animals, and includes some well-known species which can infect humans such as the malaria-causing Plasmodium, the infamous and widespread Toxoplasma gondii, and the gut-busting Cryptosporidium. But it is not as if this group has any particular affinity for humanity - humans are just one species among many across the animal kingdom that are hosts for apicomplexan parasites. Most of the more well-studied apicomplexans are those that infect terrestrial animals, especially domesticated species, but far less is known about apicomplexan parasites that are found in the marine realm.

Top left: Aggregata sinensis oocysts in the membrane between the arms of an octopus. Top right: Oocysts in the branchial heart.
Bottom left: Sporocysts found within an oocyst. Bottom right: Sporozoite released from a sporocyst.
Photos from Fig. 1 and Fig. 2 of the paper

Aggregata is a genus of apicomplexan which specifically targets cephalopods - mainly octopuses. Octopus can become infected from eating crustaceans such as shrimps which harbours the asexual stage of the parasite. Once they get into the octopus gut, the parasite takes over the digestive tract, and undergo sexual reproduction in the cells of the gut lining. There are twenty different known species of Aggregata, and it seems that for octopuses, there is no escape from this genus of parasite - even deep sea species living around hydrothermal vents are targeted by their own specialised species of Aggregata parasite.

So there are no doubt many other species of Aggregata out there which are still undiscovered. The paper featured in this blog post describes a species of Aggregata called Aggregata sinensis which has been found in octopus from the eastern-central coastal waters of China and the northern tip of Taiwan. The parasite was found infecting two species of octopus - the webfoot octopus and the long arm octopus - both of which are commercially important species that are caught by the local fishermen. 

The parasite was rather common, and depending on the location, between 20-100% of the octopuses that the researchers examined were afflicted with A. sinensis. Because the way an octopus becomes infected is from eating parasitised prey, Aggregata infection initially starts in the digestive tract, but it doesn't stay there for long. In heavy infections, the parasite spills over into other parts of the body in a very visible way. As Aggregata proliferates in the octopus, it leaves tell-tale signs of their presence in the form of white cysts that speckle the octopus' body. Those white cysts are called oocysts, which are the results of the parasite's sexual reproduction. Aggregata can wreak a destructive toll on the octopus's health. As the parasite proliferates, they smother the gut lining and destroy the submucosa cells, which compromise the octopus' ability to absorb nutrients. 

As if that's not enough, those white oocysts are filled with microscopic spheres called sporocysts which need to depart from the octopus' body to continue the life cycle, and they do so in a destructive manner. The release of those Aggregata oocysts necessitates the rupture and shedding of the surrounding hosts cells, resulting in ulcers and atrophy of the gut lining and connective tissues. Once free in the surrounding waters, should the sporocysts find themselves in an unlucky crustacean, they unravel to reveal their payload of worms-shaped sporozoites. These squirm out and settle in the crustacean's gut where they undergo asexual reproduction, and start the life cycle anew.

A recent study on the phylogeny of Apicomplexa suggests that Aggregata belongs to a group called the Marosporida - which occupies a key evolutionary position within Apicomplexa, separate from the rest of the phylum. Which means that understanding parasites like Aggregata may also help us understand the evolution of the Apicomplexa phylum as a whole, and how they became one of the most successful and ubiquitous group of parasites on the planet.

Ren, J., & Zheng, X. (2022). Aggregata sinensis n. sp.(Apicomplexa: Aggregatidae), a new coccidian parasite from Amphioctopus fangsiao and Octopus minor (Mollusca: Octopodidae) in the Western Pacific Ocean. Parasitology Research 121: 373-381.