"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

February 12, 2024

Ascarophis globuligera

To land-dwelling humans, deep sea hydrothermal vents would seem like a vision of hell, amidst the crushing darkness you have plumes of superheated water, mixed with noxious sulfides, erupting from fissures on the seafloor. But for many deep sea animals, this "hell" is in fact a vibrant oasis in the middle of the abyss. This lively habitat is made possible thanks to bacteria that are able to extract energy from the sulphurous waters billowing from those vents. In the absence of sunlight, these chemoautotrophs form the foundation of the food chain. Some tube worms have been able to co-opt the power of these bacteria by housing the microbes in their gills, enabling them to grow to enormous sizes. Their tubes form dense, forest-like habitats for many other animals including other polychaete worms, fishes, crustaceans, and molluscs. This sets the stage for all kinds of complex ecological interactions, and that includes parasitism.

Left: Anterior of Ascarophis globuligera from Fig. 6 of the paper, Right: Photo of Thermarces cerberus (pink vent fish) by Dr Lauren Dykman, used with permission 

This post is about a paper reporting on three newly described species of Ascarophis nematodes that have been found in the guts of some deep sea hydrothermal vent fishes. Some of those worms were collected as a part of a larger study which focused on looking for parasites from hydrothermal vent animals, and along with freshly caught specimens, the researchers also examined preserved fishes collected by past expeditions. While they only managed to recover a few specimens of Ascarophis nematodes, some of which were fragmentary, those were enough to provide a scientific description for three different species - A. justinei, A. globuligera, and A. monofilamentosa.

The three species differed slightly in which fish species they infect - A. justinei is found in both the pink vent fish and a species of viviparous brotula, whereas A. globuligera has only been found in the pink vent fish, and A. monofilamentosa lives in a species of zoarchid fish named Pyrolycus manusanus. While it is not possible to conduct experimental infections to work out exactly how these nematodes transmit between hosts, their life cycles can be inferred based on what is known about other Ascarophis species which are found in shallower waters. This usually involves a crustacean, often amphipods, serving as the intermediate host for the parasite's larvae. Amphipods are plentiful around hydrothermal vents, and these crustaceans are eaten by a range of animals including deep sea fishes such as the pink vent fish, making them the ideal vehicle for Ascarophis to complete its life cycle.

The need for Ascarophis to reach an amphipod host may explain why each of those Ascarophis species has differently shaped eggs. For example, A. justinei has eggs which are regular, ovoid shape rather similar to other known species of Ascarophis, but the eggs of A. globuligera have a bulge on their side (which gave the species its name), and A. monofilamentosa eggs have a long filament dangling from them which is about fifteen times the length of the egg itself. These differently shaped eggs could mean slightly different transmission strategies. The extra ornament on the eggs of A. globuligera might serve to entice a hungry amphipod by resembling something edible (as with some tapeworm eggs that infect crustaceans by mimicking diatoms), or in the case of A. monofilamentosa, its long filament may prevent the eggs from drifting away into the empty abyss by wrapping them around a structure, or entangle them around something which might get eaten by an amphipod.

Some Ascarophis species are actually known to take a shortcut with their life-cycles -  instead of waiting for a fish host to come along, they become sexually mature and start laying eggs inside the amphipod, bypassing the need to enter a fish host to complete their life cycles. It is unknown whether any of the newly described deep sea species are capable of doing this, but in an ephemeral habitat like hydrothermal vents, it would be useful to have such an option as insurance.

There are many biomes on this planet which are completely inhospitable to humans. But that does not stop them from being as rich and vibrant as those that we are more familiar with, and wherever there is a thriving ecosystem, you will find parasites taking part in its web of interactions.

Moravec, F., Dykman, L. N., & Davis, D. B. (2024). Three new species of Ascarophis van Beneden, 1871 (Nematoda: Cystidicolidae) from deep-sea hydrothermal vent fishes of the Pacific Ocean. Systematic Parasitology 101: 2.