Cymothoa indica (et al.)

Tongue-biters are among the most (in)famous parasites found in fish, but they aren't the only type of isopods that parasitise fish, nor is the mouth the only spot ripe for parasitism - there are many other parts of a fish's body where an isopod can make itself at home. Why, right behind the fish's mouth are its gills, and this cosy, well-aerated and blood-rich location is where some isopods reside. There are also others that cling to the fish's skin where they gnaw and suck on host tissue, and even some that just burrow into the fish's body cavity for extra coziness.

Photo collage showing a range of cymothoid isopods on various fishes: (a) Cymothoa indica male (smaller one in the photo) and female attaching to the buccal chamber of Datnoides polota; (b) Cymothoa indica attaching to the mouth of Jonhius sp.; (c) Nerocila loveni attaching to the skin in the ventrolateral region of Deveximentum Interruptum; (d) Nerocila orbignyi attaching to the tail skin of Mugil cephalus; (e) Agarna malayi attaching to the gill cavity of Nematolosus nasus; (f) Joryma sawayah male (smaller one in the photo) and female attaching to the gill cavity of Nematolosus nasus.

From Figure 1 of the paper

So there are many different ways to parasitise a fish and cymothoid isopods are particularly adept at doing so. But some isopods are pickier than others when it comes to which fish they parasitise, and it seems to have something to do with where they live on a fish. The study featured in this post looked at factors that may have driven the preference of these parasites. To do this, the researchers studied fish collected from commercial trawlers at harbours and fish landing centres along the north-eastern coast of India, from Petuaghat down to Gopalpur.

The researchers examined a total of 5798 fish, of which 923 (from 59 fish species) were parasitised by 21 different species of cymothoid isopods. With this massive dataset, they were able to compare the host preference of tongue-biters, gill-biters, and the skin-biters, noting how many different species of fish each of them parasitise, and the characteristics of the fish they infect. From their analyses, it seems that generally speaking gill-biters tend to be most specific - they stick to a single fish species and are mostly found in pelagic schooling fish. In contrast, tongue-biters tend to infect fish that hang out near the seafloor, and are less selective about their host species. And the skin-biters are happy to just go after whatever fish they come across.

This trend might have something to do with the life histories of those isopods. The gill-biters have free-swimming larvae that reach their host by getting sucked into the respiratory current of fish swimming through the water column, and if those fish are in a school, there would be plenty of hosts available nearby for the next generation of gill-biters. On another hand, tongue biters have larvae that hang out on the seafloor, waiting to ambush any foraging fish that come near, so they are more likely to encounter a wider range of fish. But even though tongue-biters can infect more fish species than the gill-biters, each species of tongue-biter definitely has a "type".

For example, take Cymothoa indica, a tongue-biter which is found in a wide range of fish species across seven different families - while that seems like it has pretty broad taste, its hosts all tend to be shallow water fish that live and feed near the seafloor. Similarly, another tongue-biter - Catoessa boscii - infect seven different fish species, but all those fishes are similarly shaped, as they are mostly deep-bodied fishes such as jacks and scads. Meanwhile, the skin-biters have larvae that roam freely around the water, and can launch its attack from the seafloor or while rapidly looping in the water column. Essentially if it runs into a fish, it just latches on and starts gnawing.

Parasitic isopods are found in/on fish all over the world, and they have significant impact on fisheries and aquaculture. But despite their ubiquity, they are relatively under-studied, with most of the published research on their taxonomy, biogeography and patterns of host associations coming from only a handful of specialist researchers across the globe. Studies like the one featured in this post can provide us with some much needed insight into the secret lives of these widely found parasites.

Reference:

Mohapatra, S. K., Swain, A., Ray, D., Behera, R. K., Tripathy, B., Seth, J. K., & Mohapatra, A. (2024). Niche partitioning and host specialisation in fish‐parasitising isopods: Trait‐dependent patterns from three ecosystems on the east coast of India. Ecology and Evolution 14: e70298.