Rhizocephala

Rhizocephala is an order of parasitic crustaceans within the class Maxillopoda, subclass Thecostraca. These organisms are primarily known for their unique and highly specialized parasitic relationship with other crustaceans, particularly decapods such as crabs and lobsters. Rhizocephalans are characterized by their complex life cycle and highly altered body form, which lacks the typical crustacean features like appendages or a segmented exoskeleton in the mature reproductive stage.

The life cycle begins with a free-swimming nauplius larva that develops into a primary infective stage called the cypris. The cypris attaches to a host crustacean and develops into an internal, sac-like structure known as the interna, which penetrates the host's body tissues. The externa, a reproductive and often conspicuous part of the parasite, protrudes from the host's body and resembles a small mantle or sac. Rhizocephalans manipulate and control the physiology and reproductive processes of their hosts, often sterilizing them and redirecting energy toward parasite reproduction.

Rhizocephalans are notable for their ability to induce significant morphological and behavioral changes in their hosts. They can alter host molting, mating, and care behaviors to favor the reproductive success of the parasite. Despite their parasitism, rhizocephalans are ecologically significant, influencing population dynamics and interactions within aquatic environments.

Throughout history, rhizocephalans have been of interest both for their biological complexity and for their potential impacts on fisheries and crustacean populations. Their highly specialized parasitic strategy exemplifies the diverse ways in which organisms adapt to exploit host resources within ecological systems.