Cymbasoma sp.

Floating amidst the ocean's plankton is a tiny monster, it has no mouth and it must mate, after which it will give birth to a new generation of little monsters that will grow within the bodies of worms. Everything about this tiny crustacean sounds like a science fiction monster, starting with the group's name - Monstrilloida, meaning "tiny monster" - coined by a scientist who found their life cycle and appearance to be delightfully bizarre.

Left: Copepodid stage of a female Cymbasoma dissected from a Haplosyllis worm, Right: Adult stage of a female Cymbasoma 
Photos from Fig. 2 and Fig. 4 of the paper

Adult monstrilloid are free-swimming and they don't feed, but as juveniles, they live as parasites that can grow inside various marine invertebrates including snails, mussels, and polychaete worms. In polychaete worms, they can grow pretty large in relation to their host, and when they reach adulthood, they bust out of the host like it's a novelty birthday cake. In that way, their life cycles are comparable to the hairworms that parasitise crickets and mantids. 

Unlike other planktonic copepods that often swim by flicking their long antennae, the antennae of adult monstrilloids are fixed, so instead they have powerful swimming legs that allow them to kick their way through the water.  And while the adult stage of these weird little crustaceans are sometimes found in plankton trawl samples, their juvenile stage are much more elusive. Out of the 195 known species of monstrilloids the parasitic juvenile stage has only been identified for seven species, since they are hidden away in the bodies of their hosts. As a result, it has been over a century since anyone has investigated those parasitic juveniles in detail.

In this study, scientists in Japan were examining pieces of sponge that had been washed up on Tancha Beach at Okinawa Island. Those sponges turned out to be home for hundreds of Haplosyllis polychaete worms, but the worms themselves were also occupied by monstrilloids. This was also the case for sponge worms from Diamond Beach on another part of the island, which turned out to be an absolute haven for the little monsters, with over half of the worms hosting monstrilloids. This abundance of monstrilloids at Okinawa Island presented an amazing opportunity for scientists to get a better look at the parasitic stage of these copepods. 

In order to find out more about these enigmatic crustaceans, scientists first had to coax the host worms out of their spongey home, and they did that by taking chunks of the sponges and kept them in water without aeration. As oxygen level dropped, the worms were forced to abandon their sponge to seek more oxygenated water, at which point they could be collected and examined under the microscope. Monstrilloids are relatively large and highly visible as the bulk of the copepod stretches out the worm's body wall to transparency. 

Among these sponge-dwelling polychaete worms, the scientists found the larvae of two monstrilloid genera - Cymbasoma and Monstrilla, the former is coloured pale pink while the latter is teal green, but only the female copepods are so eye-catching due to their ovaries. The males are colourless and transparent. These larvae also live up to the monstrilloid name - they are banana-shaped, with a single eye, enclosed in a translucent sheath, and have a pair of long feeding tubes which it uses to slurp up nutrients from the host's body. When they reach maturity, the copepod uses those same tubes to make its exit by tearing a hole through the worm's body wall. Once free of the host's body, the monstrilloid shrugs off its juvenile exoskeleton to transform into an adult and takes its place among the zooplankton. 

In order to complete its life cycle, monstrilloids have to survive in three very different environments - the open ocean as adults, the sea floor (briefly) as nauplii, and inside the body of animals as juveniles. In the words of one of the scientists who study these little monsters, they are simply an awesome group of crustaceans.

Reference:

Emelianenko, V., Savchenko, A. S., Husnik, F., & Huys, R. (2025). Live observations of endoparasitic stages of Cymbasoma sp. and Monstrilla sp.(Copepoda: Monstrilloida) utilizing sympatric sponge-associated Haplosyllis spp.(Polychaeta: Syllidae) in Okinawa. Plankton and Benthos Research 20(Spec): s235-s241.

Selenidium elongatum

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.

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.

Reference:

Park, E., & Leander, B. (2024). Coinfection of slime feather duster worms (Annelida, Myxicola) by different gregarine apicomplexans (Selenidium) and astome ciliates reflects spatial niche partitioning and host specificity. Parasitology, 151: 400-411.

Veneriserva pygoclava

There are many ways to become a parasite, and there are parasites with vastly different ancestries that end up joining the same path on the road of parasitism. In some cases, sharing the same path can also mean adopting a certain shape. This post is about Veneriserva pygoclava, a worm that lives inside a worm, more specifically it is a polychaete worm that has evolved to parasitise another type of polychaete worm which are commonly called "sea mice".

Top left: Ventral view of an infected Aphrodita longipalpa with a Veneriserva pygoclava parasite inside. Bottom Left: MicroCT scan image of an Aphrodita longipalpa with Veneriserva pygoclava female highlighted in yellow and juvenile highlighted in blue. Right: A female Veneriserva pygoclava (top) and a male (bottom).
Photos from Fig. 1 and Fig. 3 of the paper

The genus name of this parasitic polychaete translates into "Venus' servant" though this worm is certainly a servant for nobody but itself. You'd think that living inside the body of another animal would restrict how big it can get, but the female Veneriserva grows to about seven centimetres long, which is twice as long as its host. Surprisingly enough, being longer than the host is not unusual among these kinds of parasitic polychaete worms. Despite its size and the amount of space it occupies within the host, it does not seem to cause any injuries or damage to the host's internal organs.

Living this endoparasitic lifestyle requires some specialised adaptations, and over the course of its evolution, Veneriserva has ended up with a body plan which is very similar to that of tapeworms. Despite both being called "worms", tapeworm and polychaete worms are from entirely separate animal phyla and their path to this "tapeworm body plan" (for the lack of a better term) were very different.

Tapeworms evolved from free-living flatworms, which are fairly simple animals, at least in terms of their body plan. A flatworm has no body cavity, its gut is more or less a blind-end sac (with some branches in larger flatworms), and it doesn't even have a circulatory system. If anything, in order to adapt to a parasitic lifestyle, tapeworms have evolved to become more complex than their free-living ancestors. Over the course of the tapeworm's evolution, they have gained a new attachment organ - the scolex - which is a heavily modified head, while the rest of the body has become an efficient conveyor belt of reproductive organs. These parasitic flatworms have even evolved a brand new type of "skin" called a tegument which allows it to absorb nutrients as well as protect itself against the host's enzymes, and some tapeworms even have the most complex central nervous system among all the flatworms, enabling them to navigate and maneuver in the dark, fleshy tunnels that are their host's intestinal tract.

In contrast, polychaetes are segmented worms, and are actually more similar to us in their body plan, equipped with a full body cavity, muscular gastrointestinal tract, and a closed circulatory system with blood vessels. But Veneriserva has abandoned much of that, because when you're living inside another animal, being built like a tapeworm seems to be the way to go.

Veneriserva does have a mouth of sorts, but it is not connected to any digestive tract to speak of. In fact, the digestive tract has been reduced down to a throat with a blind-end. Instead, the mouth of Veneriserva serves as a grabber to hold the parasite in place, functioning much like a tapeworm's scolex. Additionally, Veneriserva has also evolved its own version of the tapeworm's tegument, which is covered in fine microscopic finger-like projects (rather like the lining of your small intestine, just inside out) allowing it to absorb nutrients through its skin. There are also patches of cilia on the skin which may serve to stir the host's bodily fluids in order to bring more nutrients into contact with the parasite's skin.

However, when it comes to sex, there is one key difference between Veneriserva and tapeworms. Tapeworms are hermaphroditic - any tapeworm can mate with any other individual of the same species, or even with itself if it is desperate and alone. In contrast Veneriserva have separate female and male sexes which are clearly distinguishable - male worms are tiny compared to their much larger partners (see accompanying photo).

This "attachment organ + loads of gonads" type of body plan that tapeworms and Veneriserva have both independently evolved is also found in other internal parasites. For example, acanthocephalans - thorny-headed worms - are parasitic worms which live in the gastrointestinal tract of vertebrate animals, and are somewhat related to rotifers. Despite being in a different phylum, they share some key anatomical similarities to tapeworms, with their own version of the tegument, a body dominated by gonads, and a prickly anchor at its "head" to stay attached to the host's intestinal wall. Another example is Thyonicola, the parasitic snail which uses a thin stalk to attach itself to the intestines of its sea cucumber host, while the rest of the body is simply a long tube of reproductive organs and developing eggs. There are even some parasitic dinoflagellates that have evolved to resemble tapeworms. 

Judging from how common this "tapeworm-style" anatomy is across different parasite groups, it seems that when you are an internal parasite, you have to get into shape - and that shape happens to be that of a tapeworm.

Reference:

Tilic, E., & Rouse, G. W. (2024). Hardly Venus’s servant—morphological adaptations of Veneriserva to an endoparasitic lifestyle and its phylogenetic position within Dorvilleidae (Annelida). Organisms Diversity & Evolution 24: 67–83

Endovermis seisuiae

Polychaete worms are common in the marine environment, living in just about every habitat ranging from the seashores, to the open ocean, the deep sea, next to boiling hot hydrothermal vents, or even on mounds of methane ice. The type of polychaete worms which most people are familiar with are beachworms and sandworms that live inside sand or mud burrows on the seashore, and are often collected by anglers who use them as bait for fishing. But the polychaete worm that is featured in today's post does not live in sand burrows - instead, it has evolved to live inside another polychaete worm, wearing them almost like someone wearing a mascot costume.

Endovermis seisuiae inside its scaleworm (Lepidonotus sp.) host (from Fig. 1 of the paper)

Endovermis seisuiae is very appropriately named since "Endovermis" basically means "inside worm". There are only 19 other species of polychaete worms that are known to have evolved this macabre life-style, and most of them belong to either the Oenonidae family or the Dorvilleidae family. But Endovermis hails from the Phyllodocidae family, a group of polychaete worms which are mostly free-living predators, or dwell in tubes which have been vacated by tubeworms.

But Endovermis has taken this lifestyle to a truly galaxy brain direction  - why settle for living in a tube created by another polychaete worm, when you can live inside the polychaete worm itself? The hosts of this parasitic polychaete are scaleworms, which are polychaete worms known for having iridescent scales. In this study, the researchers found E. seisuiae living inside of two species - Aphrodita sp. and Lepidonotus sp. - both were located at over 200 metres below sea level off the coast of the Wakayama Prefecture in Japan.

Endovermis can grow alarmingly large in comparison with its host. The two parasitised scale worms which the researchers found were 14 mm and 27 mm long, while the Endovermis living in each of them grew to 13 mm and 21 mm long respectively (depending on the host species). In both scale worms, Endovermis grew to be about as long as the host itself, though the scaleworm hosts have wider bodies than the parasites. So it is a very cosy fit for the parasite, and it takes up substantial room in the host. In fact, those scaleworms caught the researchers' attention in the first place because they noticed something squirming around inside their body cavity. This size parity between Endovermis to its scaleworm host would be like if you find out that there is a whippet living inside the body of a greyhound. 

So how does a worm like that get inside a host which isn't that much bigger than itself? There were no obvious scars on the body of the scaleworm as you would expect if a full-size Endovermis had simply tunnelled its way into the host's body. Since Endovermis produces tiny eggs which are only about 0.1 mm wide, the researchers suggested that it might enter the host as a microscopic larva, drifting into their body via the nephridial canals - which are the equivalent of kidneys in some invertebrate animals. Once inside, it would sit in the body cavity, feeding on the host's body fluids or even internal organs, and eventually getting to be almost as big as the host itself.

In nature, sometimes you get surprise bonus content for a worm - which is also another worm. Simply more worm for your worm.

Reference

Jimi, N., Kimura, T., Ogawa, A., & Kajihara, H. (2021). Alien worm in worm: a new genus of endoparasitic polychaete (Phyllodocidae, Annelida) from scale worms (Aphroditidae and Polynoidae, Annelida). Systematics and Biodiversity 19: 13-21.