Crustaceamorpha: Parasitism

Parasitic amphipod on the jelly,
Solmissus. Image used with permission from the Monterey Bay Aquarium Research Institute.
Parasites are organisms that live in association with, and at the expense of, other organisms. The organism that provides the nutrition is called the host, and the organism obtaining the nutrition is the parasite. Parasites found on the exterior of the host are called ectoparasites (for example, Branchiura and some Isopoda), while those found inside the host are called endoparasites (for example, some parasitic copepods and Rhizocephala). The effect a parasite has on a host ranges from minor harm, allowing the host to live and reproduce normally and complete its normal life cycle, to completely interfering with reproduction or causing the premature death of the host. Some crustacean parasites, such as tantulocarids and rhizocephalans, find a host during the larval stages, metamorphose once attached to that host, and stay with it the rest of their lives. Others, such as the Branchiura, move from one host to another throughout their lives, staying on a single host just long enough to fill their bellies.

Some parasites look for very specific hosts to attach to, sometimes attaching only to a single host species; these are called obligate parasites. Other parasites will feed on any host available and are called facultative parasites. Obligate parasites are generally considered more evolved because of the adaptations they possess that require them to feed only on certain organisms. Often through time a host species can adapt defenses to the parasitic species. The parasite must then adapt to the new defenses or find a new host species. This cycle of adaptions is called coevolution, which leads to the specific host/parasite relationship of obligate parasites. Some of the adaptations of parasites are astounding. Because of the adaptations associated with a parasitic lifestyle, many parasitic crustaceans are practically or completely unrecognizable as a member of the taxa they are most closely related to except during early larval stages.

Here's an example of two crustacean taxa—one parasite and one host—and the adaptations they possess for parasitic life. Rhizocephala belong to the group Cirripedia (barnacles) and are endoparasites on decapods (shrimps and crabs). Rhizocephala are unrecognizable as members of the Cirripedia group—in fact, it is difficult to recognize them as a separate animal from their decapod hosts. As adults they lack appendages, segmentation, and all internal organs except gonads and the remains of the nervous system. Other than the minute naupliar stages, the only distinguishable portion of a rhizocephalan body is the externa or reproductive portion. A female nauplii settles on a host and metamorphoses as it penetrates the internal portion of the animal. It then ramifies, or grows in a similar manner as a root system, through the host centering on the digestive system. Once mature, the female produces a sac-like externa on the abdomen of the host. The externa is immature until a male nauplii settles on it and fuses with it. The externa then produces two types of eggs, small ones that will become female and large ones that become males. Because the externa is located in the same location as the host's egg sac would be, the host is fooled into thinking that the parasitic externa is its own egg sac. It cares for the rhizocephalan externa as if it were its own egg sac and never molts again (crustaceans don't molt until they release their eggs or young from the brood pouch). The host is so well fooled by the externa that even male hosts, which would never have carried eggs or young in a brood pouch, care for the externa as if they were females.

This table shows the crustacean groups that contain parasitic species.
Group Example Endo/Ectoparasite Hosts Relative #
Branchiopoda Cladocera Ectoparasites Hydra (related to portugese man few
Maxillopoda Copepoda Both other invertebrates and fish many (more than 7 higher taxa)
Maxillopoda Branchiura Ectoparasites fish all
Maxillopoda, Cirripedia Rhizocephala Endoparasites crabs and shrimp all
Maxillopoda, Cirripedia Ascothoracians Ectoparasites echinoderms and anthozoans all
Maxillopoda Tantulocarida Ectoparasites deep-sea crustaceans all
Malacostraca Isopoda Both jellyfish, fish few
Malacostraca Amphipoda Both jellyfish, fish few