Atlas of Crustacean Larvae

21. Rhizocephala

21 general: Rhizocephalans are very successful and entirely parasitic members of the Cirripedia. With more than 250 species , they comprise about one quarter of all cirripede species, and their parasitic mode of life is therefore a major component in the success of the subclass (Høeg et al. 2005). Rhizocephalans parasitize other Crustacea, with most species infesting brachyuran and anomuran decapods, but they also occur on carideans, peracarids, stomatopods, and even balanomorphan cirripedes (Høeg and Lützen 1995, 1996). The majority of species belongs to the suborder Kentrogonida and are found exclusively on decapods, whereas species of the highly specialized suborder Akentrogonida also infest other crustaceans (Høeg and Rybakov 1992). Rhizocephalans occur in all marine environments, including the deep sea. Remarkably, they also infest semiterrestrial true crabs and hermit crabs, and they even occur on freshwater crabs. All rhizocephalans have separate sexes, and this is reflected in both the size and detailed morphology of their larvae (Høeg 1984; Walker 1985; Høeg and Lützen 1995, 1996). Larval development deviates little (if at all) from that found in other Cirripedia (Rybakov et al. 2002), but the metamorphosis following attachment involves highly specialized stages, unlike any found elsewhere in the subclass (Høeg 1985a, 1987a). The parasite is female and has an extremely simplified morphology. The parasite consists of an internal nutrient-absorbing system of rootlets (interna) and an external reproductive sac (externa) that contains an ovary, a mantle cavity, and organs specialized to host dwarf males, but it lacks appendages, segmentation, a gut, and sensory organs (Høeg and Lützen 1995, 1996). The life cycle consists of 4–6 naupliar instars, a cyprid, a kentrogon and vermigon (females), a trichogon and sperm-producing adult (males), and (in females) an endoparasitic phase and an adult consisting of interna and externa hosting cryptic dwarf males. larval types Nauplius: Most species hatch as a nauplius. Few species have been cultured through all stages, so the number of naupliar instars is uncertain, but it seemingly can vary from four to six. Cyprid: The nauplius phase is followed by a typical cyprid. Some deep-sea species, all freshwater species, and all members of the Akentrogonida have an abbreviated development and hatch as cyprids. Following cypris settlement, the ensuing metamorphosis involves highly specialized stages (kentrogon, vermigon, and trichogon) unlike any seen elsewhere in the Cirripedia. Male and female larvae differ in size (fig. 21.2F). This is most apparent at the cypris stage, where the sexes also differ in clear-cut details of their morphology (fig. 21.2C–E). morphology Nauplius: The naupliar larvae are always lecithotrophic and have three pairs of limbs: uniramous antennules, biramous antennae, and mandibles (fig. 21.1). Unlike non-feeding nauplii in the Thoracica, those in the Rhizocephala are very small, rarely exceeding 360 μm in length, and contain only limited amounts of nutrients that are stored in oil cells. The nauplii have only the merest rudiment of a labrum, and they lack both plumose setae for suspension feeding and antennal and mandibular endites (fig. 21.1B). Limb setation changes little (if at all) during development and therefore offers no clue to distinguish between instars. The instars differ principally by a gradual increase in their length, due to the development of the post-cephalic trunk. As in other cirripedes, the morphology of the last nauplius instar heralds the ensuing cypris by possessing an enlarged head shield and a distinct distal distension of the antennule, indicating the development of the cypris attachment organ. The nauplii always have a pair of frontolateral horns, a pair of frontal filaments, and (normally) a pigmented naupliar eye (fig. 21.1A, B). Cyprid: The non-feeding cyprids, in almost all details, are quite similar to those of thoracican and acrothoracican cirripedes , but they differ in the sensory armature on the antennules (fig. 21.2A). In the Kentrogonida, both male and female cyprids have an olfactory aesthetasc seta on the small fourth Jens T. Høeg Benny K. K. Chan Alexey V. Rybakov Jens T. Høeg, Benny K. K. Chan, and Alexey V. Rybakov Rhizocephala 112 Jens T. Høeg, Benny K. K. Chan, and Alexey V. Rybakov antennular segment, but it is relatively much longer in males (Walker 1985). Males carry an additional, even longer aesthetasc on the third segment (the attachment organ). These and other differences, seen easily with a light microscope, offer a reliable means of determining the sex of individual larvae (fig. 21.2C–E). Cypris length varies with...