Horrifying Underwater Parasites – It’s Halloween everyday UW!
Written by Roy Kittrell 29 September 2023.
For this month’s Halloween article, I will be focusing on the horrifying but fascinating world of underwater parasitism, as seen from a diver’s point of view!
While most people would consider parasites to be ‘gross’ or ‘disgusting’, from a naturalist’s point of view they are actually fascinating, highly evolved organisms with extremely specialized life cycles, sometimes requiring more than 4 different animals across multiple phyla to complete. What is also fascinating about parasites is that they are not a specific kind of plant or animal, anything can become one in the same way that anything can become predator or prey.
From my years of diving, I have come across and learned about multiple different types of parasites that live in the reef, and as a diver you will be very likely to come across them yourself.
1. The Cardiocetes copepod, Vampires of the ocean
This is a small arthropod in the Class Copepoda; which resemble something close to tiny shrimp. However, unlike shrimp that filter feed on marine detritus, they have instead evolved to be blood sucking parasites of fish, burrowing deep into their gills and body tissue to feed off their host until they can create spirals of egg sacs to start the next generation. This feeding weakens the host significantly and also decreases its lifespan.
The picture here was from the Flow dive trip to the Lembeh Strait, where we saw an unfortunate whip coral goby that had been parasitized by not one but two separate copepods, and was noticeably weak and smaller than normal because of it
2. The horror of the Tongue Replacing Isopod
The tongue replacing isopod, or Cymothoa exigua as it’s known from its latin name, is pretty much exactly what it sounds like: a small isopod that swims into the mouths of different fish, and then latches onto their tongues with its front appendages. This severs the blood vessels, and causes the tongue to die and fall off. The isopod then, horrifyingly, becomes a new functional tongue in the fish, eating small parts of whatever the fish itself eats and in some cases also feeding from the host’s blood and mucus. This is, apparently, one of the only cases in the entire animal kingdom where one animal replaces the organ of another.
I have yet to observe these on any of my own dives; however, I have seen many pictures of them inside the mouths of clownfish, so the next time you are watching a family on top of their anemone, have a look inside their mouth and see if you can see this amazing isopod!
3. Gymnodoris nigricolor, the zombie nudibranch
This spooky little nudibranch is quite tiny at only 2-3mm long, and unlike most nudibranchs which are extremely colourful this nudi is jet black. What makes it so strange though is its life cycle; it can be found most of the time hanging onto the thin tissue of the fins and tails of gobies. It will spend its whole life there, doing what we don’t actually know. Because that’s the creepy thing about this parasite; it doesn’t seem to be eating the fin tissue or damaging the fish, it just seems to hang on there for no reason. Which to me is much creepier than if it was actually harming it’s host.
I saw the subject below on a Flow dive trip to Tioman island; our spotter An pointed at it on the sea floor excitedly and I dutifully took pictures despite not thinking much of it at the time, but I’m glad I did now because it is such an enigma!
4. Sacculina, the Crab-Castrating Parasitic Barnacle, real life Bodysnatchers
As I mentioned, any kind of organism can evolve into a parasitic lifeform, and barnacles are no exception. Sacculina, a type of barnacle in the infraclass Rhizocephala, has developed the ability to parasitise crabs in a surprisingly horrifying way.
The barnacle latches onto an adult crab, and then molts into a form called a kentrogon that can inject itself into the soft tissue of the crab at one of its joints. Once inside, it grows to adult form, emerging as a sac, known as externa, on the underside of the crab’s rear thorax, where the crab’s eggs would normally be incubated. Parasitic Sacculina destroy a crab’s genitalia, rendering the crab permanently infertile. It also loses the ability to molt and regrow limbs, shortening the crab’s lifespan. Even more interestingly, if Sacculina finds a male crab instead of a female; it will release hormones that effectively turn the male crab into a female, even to the point of performing female mating dances.
Once a female Sacculina has been fertilised by a male, the host crab looks after it’s ‘brood’ as it would its own eggs, even releasing them in the same way on the top of a rock where the current is strongest.
Next time you see a crab on one of your dives, try to keep an eye on its abdomen. If it looks like there’s a small balloon growing on its underside, there is every possibility you are seeing a true marvel of the natural world!
5. Dendrogaster, Sea Star Parasites
Dendrogaster, a type of crustacean in the family Dendrogastridae, parasitise the inside of sea stars as seen in the picture. How any biologist was able to determine that this was a crustacean is beyond me, as it looks nothing like any arthropod I have ever seen.
They find their way inside the sea star’s body and inhabit the coleomic cavity, which is the main hollow area inside. It’s thought that like Sacculina these parasites also have the ability to disrupt the reproduction of the sea stars. Unfortunately, this means you will rarely if ever see one of these remarkable creatures, but just knowing that these exist is enough to be unsettling to even the most hardened of naturalists!
Roy Kittrell
-Roy Kittrell is an avid naturalist, diver and nature photographer. You can find more of his work on his Instagram @roythedivebro