Tuesday, January 31, 2012

Sacculina carcini: Seafood Anyone?



Sacculina carcini is a parasite that carries on a symbiotic relationship with crabs [1]. This particular genus practices a form of parasitic castration whereby hindering the reproductive ability of its host [2]. According to our text, “Certain parasites appear to make an amazingly boneheaded move: they castrate their hosts (as do many trematodes in mollusks and crustaceans in other crustaceans) or they end up killing the host (most parasitoid insects [3].” Unlike the parasites referred to by Combes, the parasite S. carcini only partially castrates their hosts enough to control their sexual reproduction. It should also be noted, since crabs are found in a variety of marine habitats, that the parasite is vastly allocated in terms of global distribution. In regard to the ecological impact, the parasite prevents the crabs from reproducing eggs other than that of the parasite. This may in turn lead to the steady destruction of crab populations in certain parts of marine habitats [4]. 

Symbiont Description-

Until S. carcini finds a crab to infect, it looks like a small slug floating in the water [1]. The female parasite finds a break in the crab’s protective outer covering, usually in the crab’s joint, and injects itself into the crab [1]. The parasite will grow root-like structures inside the crab that hinder the crab from reproducing anymore, and will continue its life gathering nutrients for the eggs of the parasite [4].  

Host Description-

The definitive host or the host where the parasite reaches sexual maturity is the crab. The parasite, S. carcini, has no intermediate hosts and thus has no acting vector hosts [5]. The parasite only shares a relationship with one host, the crab, so it is considered a holoxenous parasite. The host specificity of S. carcini helps it keep selective pressures on simply one host, and there is then no need to adapt to different kinds of hosts [3].


Life Cycle-

The overall life cycle of S. carcini is rather simple. First, a free-swimming barnacle larva of the parasite finds a crab to inject its inner worm-like body [2]. Next, the parasite makes root-like tentacles to obtain nutrients from the host crab. Finally, after achieving the desired amount of nutrients, the parasite is able to make its eggs, and the host crab will care for the parasitic eggs as it were its very own fertilized eggs [2].


Initially, the consequences of this interaction in nature do not seem terribly severe. However, the prevalence of S. carcini is widespread since their hosts, crabs, are found in most all marine habitats. Most of the crab population is found in the marine habitats of China, Japan, and the United States. Another fair amount of crabs can also be found in the Atlantic Ocean around the United Kingdom [5]. Nearly half of certain crab populations are infected with this parasite [2]. This continued infection could result in the steady decrease of the crab population as a whole. 

An example of regular parasite distribution-

S. carcini is an example of regular parasite distribution because every individual host is infected the same amount as everyone else. Even male and female crabs have the same chance of being parasitized [2]. Likewise, a healthy crab and a sick crab have the same probability of being infected by the parasite because the parasite is only looking for any crab’s joint, and no other one particular attribute of a crab is more desirable to S. carcini than any other one specific attribute. 


[2] Piper, Ross. Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Westport, CT: Greenwood, 2007.

[3] Combes, Claude. Parasitism: The Ecology and Evolution of Intimate Interactions; Translated by Isaure De Buron and Vincent A. Connors; with a New Foreword by Daniel Simberloff. Chicago: University of Chicago, 2001.

[5] Zimmer, Carl. Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures. London: Arrow, 2003.



  1. This comment has been removed by the author.

  2. I wonder what would happen if a human ate an infected crab. Would our immune system just kill the parasite?

    1. It would be very hard for a human to unknowingly eat an infected crab because the parasite is easily visible on the host's abdomen. However, if the parasite was not clearly identified, I would assume that the parasite would not survive the cooking process/boiling of the host crab.

    2. what would happen if you didn't boil it and ate it unknowingly?