Creature Feature: Loxo and Mud Crabs

A zombie apocalypse is a terrifying concept that illogically plagues the minds of many of us. What would it be like to run from infected versions of your friends and family, who no longer think or act the same as they used to (and would also probably try to kill you)? How quickly would the infection spread, and how would you find safety? Luckily for us, this threat is currently limited to fictional characters within our popular culture. However, in the animal kingdom zombies really do exist! The cause is most often some nasty (but fascinating) parasites, and you’d best believe that these parasites are truly terrifying to their respective hosts. The parasite Loxothylacus panopaei, referred to simply as Loxo (an abbreviation of its Latin name), is one of them. Can you imagine something so terrifying that its only namesake is in an ancient language?

A black-fingered mud crab infected with Loxo carries an inflated Loxo egg sac on its underside. Loxo takes over the reproductive system of its crab host and causes changes in the crab’s behavior. Photo by Will Parson, Chesapeake Bay Program [Source].

Loxo is a parasitic barnacle, although it bears no resemblance to the average barnacle in its adult form [1]. Loxo uses mud crabs as its host and most often infects the white-fingered mud crab (Rhithropanopaeus harissi)and the flat-back mud crab (Eurypanopaeus depressus). The parasite’s life cycle is direct, which means it spends the majority of its life in one host and undergoes its entire reproductive cycle in that host [2]. This is in contrast to parasites that have indirect life cycles and use multiple host species. So only mud crabs have to live in fear of Loxo.

What does Loxo do that makes it such a terrifying “zombie” parasite? Well, it does lox-o things (get it? lots of . . . never mind). The female Loxo inserts herself into a host crab, where she creates a system of rhizoids, or roots, that spread throughout the crab’s body [2]. These root systems are very destructive and tear through the gonads (reproductive tissue) of the host, sterilizing them. Even if the host crab has not yet been a parent when it becomes infected, it will still have the opportunity to give birth . . . to the young of its resident parasite, that is [3]. Loxo hijacks its host’s reproductive system and forms its own egg sac, called an externa, on the crab’s underside where a female crab would normally hold her eggs [3]. The extra scary part is that infected male crabs are manipulated to do this as well! The now “feminized” male will behave like a female, keeping the Loxo externa clean of debris and protecting it as if the crab was guarding its own brood [4]. Even weirder, the apron (abdomen) of male mud crabs, which is narrower than that of females, is found to widen in infected individuals to better hold and protect the parasite’s young [4]. Crazy just how much this parasite can change its host!

A flat-back mud crab infected with Loxo. Notice the widened apron (abdomen) of the crab, providing more space for the parasite’s eggs. Photo by Robert Aguilar, Smithsonian Environmental Research Center [Source].

While Loxo alters male mud crab behavior in a very extreme way, there’s still more that it does to its hosts of both sexes. The movements of infected crabs are noticeably more exaggerated, and some studies have found that infected crabs move faster than uninfected crabs [5]. Loxo also alters the hiding behavior of its hosts. When exposed to predators, infected mud crabs have shown a strong preference for hiding in more complex habitats, such as oyster clumps, over simple habitats like mud or gravel [6].

Loxo is native to the Gulf of Mexico and infects less than 10% of its intended hosts there. However, its modern range includes much of the eastern Atlantic coast, where its infection prevalence can be upwards of 90% [7]! The cause? An Invasion. In the 1960s, oysters from the Gulf of Mexico were transported to the Chesapeake Bay as part of restoration efforts [8]. Unbeknownst to the restorers, those oysters were home to mud crabs—mostly flat-back mud crabs—some of which were infected with Loxo [8]. Unlike their Gulf of Mexico counterparts, the mud crab populations in the Chesapeake Bay at the time were evolutionarily naïve to Loxo, so they had no evolved defenses against this new threat. As a result, populations were decimated by this new infection, which rapidly spread up and down the east coast [3].

If mud crabs were capable of having nightmares, Loxo would almost certainly appear in many of them.

Loxo is not the only parasite of its kind. A related species, Loxothylacus texanus, has a similarly devastating effect on the Blue Crab, Callinectes sapidus, which is an important economic food source for much of the east coast [2]. Fortunately, L. texanus remains in the Gulf of Mexico, so Maryland’s favorite crab isn’t in any danger. Through scientific research on Loxo and other invasive species, we learn more and more about biological invasions, what allows them to happen, and how we can prevent future invasions from occurring.

If mud crabs were capable of having nightmares, Loxo would almost certainly appear in many of them. How lucky we are that no human zombie outbreaks have occurred. Yet. But with spooky season upon us, it’s not unlikely that hordes of human zombies and creepy creatures will come out of the woodworks as Halloween approaches.

Christofer Brothers is a first year graduate student in the Animal Behavior Graduate Group. He is interested in predator-prey interactions, sexual selection, and environmental factors that influence those dynamics. He also has an unhealthy love of puns.


  1. Mouritsen, K. N., Geyti, S. N., Lützen, J., Høeg, J. T., & Glenner, H. (2018). Population dynamics and development of the rhizocephalan Sacculina carcini, parasitic on the shore crab Carcinus maenas. Diseases of Aquatic Organisms131(3), 199–211.
  2. Alvarez, F. and Calderon, J. (1996) Distribution of Loxothylacus texanus (Cirripedia: Rhizocephala) Parasitizing Crabs of the Genus Callinectes in the Southwestern Gulf of Mexico. Gulf Research Reports 9(3), 205–210.
  3. Reisser, C.E. & Forward, R.B. (1991) Effect of salinity on osmoregulation and survival of a rhizocephalan parasite, Loxothylacus panopaei, and its crab host, Rhithropanopeus harrisii. Estuaries 14, 102.
  4. Kristensen, T., Nielsen, A. I., Jørgensen, A. I., Mouritsen, K. N., Glenner, H., Christensen, J. T., … & Høeg, J. T. (2012). The selective advantage of host feminization: a case study of the green crab Carcinus maenas and the parasitic barnacle Sacculina carcini. Marine biology159(9), 2015–2023.
  5. Gehman, A. L. M., & Byers, J. E. (2017). Non-native parasite enhances susceptibility of host to native predators. Oecologia183(4), 919–926.
  6. Brothers and Blakeslee. Alien vs. Predator: Hide and Seek. The Journal for Experimental Marine Biology and Ecology (In review).
  7. Hines AH, Alvarez F, Reed SA (1997) Introduced and native populations of a marine parasitic castrator: variation in prevalence of the rhizocephalan Loxothylacus panopaei in Xanthid crabs. Bulletin of Marine Science, 61, 197–214.
  8. Van Engel, W.A. and Dillon, William A. and Zwerner, David and Eldridge, Dana (1966) Loxothylacus Panopaei (Cirripedia, Sacculinidae) an Introduced Parasite on a Xanthid Crab in Chesapeake Bay. Crustaceana, 10, 110–112.

Main featured image of Loxo-infected black-fingered mud crab by Will Parson, Chesapeake Bay Program [Source]

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