Peltogaster sp.

Most people are familiar with how barnacles look like - sedentary creatures which filter the surrounding water for food while being stuck attached to rocks or other hard surfaces. Parasitic barnacles on the other hand looks nothing like those creatures. In fact, they don't look anything like what most people would expect an animal to look like. The most well-know example of a parasitic barnacle is Sacculina carcini, but that infamous species is only one of an entire order of such body-snatching parasites that infect crustaceans like crabs and crayfish.

Left: Peltogaster externa attached to their hermit crab host.  
Right: The externa (orange) and interna (green) of Peltogaster in its hermit crab host
Photos from Fig. 1 and Fig. 2 of the paper

These parasitic barnacles belong to a group call Rhizocephala and the body of the adult parasite can broadly be divided into two parts: The "externa" which is the bulbous reproductive organ that sticks out of the host's abdomen, and the "interna" which is found inside their host's body. The interna is a network of root-like tendrils which wrap themselves around the host's organs (hence the name "Rhizocephala" which roughly translates into "root-head").

Most depiction of rhizocephalans have those parasitic roots running throughout the entire body of the host - this is based on an illustration of S. carncini drawn by the famous artist and biologist Ernst Haeckel. Haeckel's original drawing has been copied by many others since it was first published in the book Kunstformen der Natur, and has been treated as the definitive depiction of the rhizocephalan interna. But the thing is, Haeckel has never actual seen a Sacculina in person - he simply based his illustration upon descriptions of the parasite in a monography published in 1884. So while Haeckel's original drawing is iconic and has been replicated countless time in many books, that depiction of these parasitic barnacle is not entirely accurate. Much like tropes in other areas of scientific illustration (such as depictions of extinct animals), Haeckel's depiction of Sacculina has been faithfully and unquestioningly used and copied ever since.

It is understandable that not much is known about the true three-dimensional structure of the rhizocephalan interna - because of its complex and delicate nature, it would be really difficult to tease apart all those roots which are tightly intertwined with host tissue to get an accurate picture of the parasite's extensive root network. But now there is technology available which can resolve this question. In the study featured in this post, a group of researchers used X-ray microtomography to obtain a 3D image of these parasites' root network inside their hosts. They performed this procedure on five species of rhizocephalan barnacles collected from the coast of Norway and the United States; four of the species were hermit crab parasites belonging to the genus Peltogaster, and one - Briarosaccus tennellus - was from the hairy crab.

From the microCT scans, they found that the barnacle's "roots" are not spread evenly throughout the body, but were wrapped around certain organs, with most concentrated near the hepatopancreas  - an organ found in crustaceans which is also known as the digestive gland, which would be prime place to suck up nutrients. And in contrast to Haeckel oft-cited and copied drawing, none of the roots actually penetrate into the muscles. While the roots of the four Peltogaster species were mostly wrapped around the hepatopancreas, the roots of Briarosaccus also extended to the host's brain and central nervous system, which may explain how some of these parasites can manipulate the behaviour of their crustacean host.

Parasite can often manipulate their host's behaviour and physiology to an amazing degree. While many of those interactions are very complex, with the use of techniques such as micro CT, we can begin to unravel the intricacies of how these body-snatchers interact with and manipulate their hosts.

Reference:
Noever, C., Keiler, J., & Glenner, H. (2016). First 3D reconstruction of the rhizocephalan root system using MicroCT. Journal of Sea Research 113:51-57