A passing glance at the parasite in today's post might lead you to think that it is a worm, perhaps a nematode. But a closer look would reveal that not only is it much, much smaller than most nematodes, it also has a visible nucleus - like what you'd find with say, a cell. Despite how it looks, this parasite is not a worm, but a gregarine - which is a group of single-celled parasites that infect all kinds of invertebrates, including insects, crustaceans, sea cucumbers, and even sea squirts.
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| Left: Selenidium elongatum from Myxicola sp. Quadra. Right: Selenidium elongatum from M. aesthetica scale bar = 20 μm. m = mucron; n = nucleus. Photomicrographs from Figure 1 of the paper |
Gregarine belongs to the phylum Apicomplexa - which includes Toxoplasma gondii, Plasmodium (the malaria parasite), and Cryptosporidium among its ranks. But the cells of gregarines grow much larger than those human parasites, with some species reaching half a millimetre in length (and one species, Porospora gigantea, exceeding 10 mm in length). They also come in all kinds of different shapes, including one species which is shaped like a microscopic rubber chicken, and they cling to the host's tissue using their mucron, an organelle which functions like the suckers of a fluke or a tapeworm.
The study featured in this post looked at gregarines and other symbionts living in two species of feather duster worms from Harriot Bay in British Columbia. As their names indicate, those worms are shaped like feather dusters, they live in tubes and use their long feathery appendages to filter food particles and plankton from the surrounding waters. The two species that the researchers examined were Myxicola sp. Quadra, which lives in tubes on muddy seafloors, and Myxicola aesthetica, a shallower water dweller that attaches to firmer substrates like rocks or shells.
The researchers examined about 50 of those worms and found that nearly all of them were infected with gregarine parasites, consisting of two species in the Selenidium genus - S. mesnili and S. elongatum. Those gregarines lived in the gut of the marine worms in a comparable way to how parasitic worms inhabit the gut of vertebrate animals. Though they belong to the same genus, the two Selenidium species are different to each other in many ways. The cells of S. mesnili are shaped kind of like skinny lemons, while S. elongatum, as its name indicates, has a long cell that makes it look like a single-celled version of a roundworm.
Aside from their size and shape, they also differ in other ways. Selenidium elongatum lives in the intestine of its worm host, and is found in both species of feather duster worms that the researchers sampled. Meanwhile, S. mesnili was only found in Myxicola sp. Quadra, and it lives exclusively in the host's pharynx and oesophagus. These differences might have arisen from the way these gregarines obtain nutrients from the host's digestive tract, or it might have something to do with the life cycles and transmission routes of these parasites. But Selenidium were not alone in the guts of those feather duster worms, living inside the gut of Myxicola aesthetica next to S. elongatum was a species of ciliate called Pennarella elegantia that swam freely in the worm's gut content.
Gregarines are poorly known but they seem ubiquitous in invertebrates, and their relationship with the host isn't always parasitic - there are evidence to indicate they can sometimes be beneficial to their host. And there are many more of them out there which are waiting to be discovered. What these gregarines show is that if you know where to look and what to look for, you will find a rich vibrant world even within the guts of a mud-dwelling worm.
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