To be a parasite or to have a parasite?

When you think of parasites, you probably imagine worms invading the human digestive system – pinworms, tapeworms, roundworms… Just the thought of that can send shivers down your spine. If it makes you feel any better, humans aren’t the only victims of such terrifying parasites – the ocean is full of them too.  
 
A few days ago, in one of the samples collected by our beloved EBS, we found many individuals from the Cryptocopoidae family – about 200, to be precise! While sorting through them and selecting the most beautiful ones for photography, I noticed something strange. A long, twisting, wriggling intruder. A nematode.  
 
Nematodes are invertebrates that live in almost every environment, from soils and freshwater bodies to the vast oceans. Interestingly, around 50% of all known nematode species are parasitic. Most nematodes are tiny, averaging about 1 mm in length, but nature loves exceptions. Among them is a true giant – Placentonema gigantissima – the largest known nematode, reaching up to 8.4 meters in length and 2.5 cm in width. This unusual species lives inside the placenta of the sperm whale, using the massive body of its host for shelter and food.  
 
The nematode we found wasn’t nearly that big, but its presence was still remarkable. It was a reminder that even as a 2-millimeter crustacean living in Antarctic waters, you are never truly safe – parasites are everywhere, ready to take advantage of any available host. Unfortunately, the Cryptocopoides individual we found was already dead. So, we decided to take a closer look, extract the nematode, and photograph it to share with you!  
 
In the same sample, we discovered another Tanaidacea species – Exspina typica – an unusual crustacean that is a parasite itself! This tiny tanaid burrows into the outer tissues of deep-sea sea cucumbers, hiding inside their bodies. It was spotted in 2008 when scientists working here – in the Ross Sea – noticed it through the transparent skin of its hosts! 
 
The deep sea is full of fascinating relationships, where even the tiniest creatures can fall victim to even smaller ones. Parasitism in the ocean takes surprising forms, and this small discovery is a reminder that nothing happens by accident in marine ecosystems. Every niche, no matter how hidden, finds its inhabitant. 

​Kamila Głuchowska
University of Łódź