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We used scanning electron microscopy (SEM) to describe cypris morphology in species of the barnacles Ibla and Paralepas, both of which are pivotal in understanding cirripede evolution. In Ibla, we also studied late naupliar stages with video and SEM. Special emphasis was put on the lattice organs, the antennules and the thorax and telson. In Paralepas we had settled specimens only and could therefore only investigate the carapace with the lattice organs. Cyprids of Ibla quadrivalvis and Paralepas dannevigi have five sets of lattice organs, grouped as two anterior and three posterior pairs. The organs are of the pore-field type and the terminal pore is situated anteriorly in the first pair, just as in the Rhizocephala and the Thoracica. In Ibla the armament of antennular sensilla resembles that found in the Thoracica but differs from the Rhizocephala. The absence of setules on the A and B setae sited terminally on the fourth antennular segment is a similarity with the Acrothoracica. The attachment disc is angled rather than facing distally and is encircled by a low cuticular velum. The thoracopods have two-segmented endopods and exopods as in the Thoracica, but the number, shape, and position of thoracopodal setae differ somewhat from other species of that superorder. Both Ibla and Paralepas cyprids have a deeply cleaved telson, but no independent abdominal part. In cypris morphology, Ibla and Paralepas show several synapomorphies with the clade comprising Rhizocephala and Thoracica and there are no specific apomorphies with either the Acrothoracica, the Rhizocephala or any particular subgroup within the Thoracica. This is in agreement with recent molecular evidence that Ibla (Ibliformes) is the sister taxon to all other Thoracica and the ibliforms therefore become the outgroup of choice for studying character evolution within the superorder. Paralepas, and other pedunculated barnacles without shell plates, are apparently not primitive but are secondarily evolved and nested within the Thoracica. J. Morphol., 2009. © 2008 Wiley-Liss, Inc.

Rhizocephalan barnacles are parasites of Crustacea. They lack even the rudiments of an alimentary canal, but infiltrate their hosts with a nutrient-absorbing system of rootlets. We review the ultrastructure of the rootlets using light microscopy, SEM, and TEM in nine species from five families, representing both suborders of the Rhizocephala: from the Kentrogonida Peltogaster paguri, P. curvatus, Peltogasterella sulcata, Cyphosaccus norvegicus (Peltogastridae); Lernaeodiscus porcellanae (Lernaeodiscidae); and Sacculina carcini (Sacculinidae); and from the Akentrogonida Clistosaccus paguri (Clistosaccidae); Chthamalophilus delagei, and Boschmaella japonica (Chthamalophilidae). With the exception of Chthamalophilus delagei, the root system of the investigated species shares numerous apomorphies at the ultrastructural level and displays at all levels specializations that maximize the surface area. The rootlets consist of a cuticle, an epidermis and a subjacent layer of axial cells that often, but not always surround, a central lumen. The rootlets are at all times enclosed in a less than 0.5 μm thick cuticle, which is never molted. The cuticle consists of an inner homogeneous layer with a slightly fibrous structure and an outer, less than 15-nm thick electron-dense layer, from which numerous microcuticular projections extend into the hemolymphatic space of the host. The microcuticular projections consist of the outer electron-dense layer and sometimes a core of the more translucent homogeneous layer. They vary among the species from being simple in Sacculina carcini to exhibiting complex branching patterns in Peltogasterella sulcata and Cyphosaccus norvegicus. Beneath the cuticle the epidermal plasma membrane is thrown into irregularly shaped projections. The epidermal cells are joined by long septate junctions and exhibit the characteristics of a transporting epithelium. Experiments with acid phosphatase revealed activity both in the epidermis and among the microcuticular projections. The projections may therefore form a domain that is important in absorption and extracellular digestion of nutrients from the host. The axial cells contain abundant endoplasmic reticulum and seem to convert absorbed carbohydrates into lipid, which is stored in large droplets. Subepidermal muscle cells cause sinuous movements of the rootlets, but it remains unknown how nutrients are transported along the rootlets towards the external reproductive body. In C. delagei the single, bladder-shaped rootlet lacks both the apical projections in the epidermis, the electron-dense cuticle layer, and the microcuticular projections. We review previous studies on the rhizocephalan root system and discuss functional and phylogenetic aspects of the morphology. J. Morphol. 249:9–42, 2001. © 2001 Wiley-Liss, Inc.

Rhizocephala is a group of crustaceans that exclusively parasitizes other crustaceans. It is taxonomically placed within the class Cirripedia, the barnacles, with which it shares a unique larval type, the cyprid. The main objective of the cyprid is to find and irreversibly attach to a suitable substratum and initiate metamorphosis. In the presumed sister group to Rhizocephala, the true barnacles or Thoracica, metamorphosis leads to a juvenile filter-feeding version of the adult organism. In Rhizocephala the female cyprid settles on the integument of a crustacean and undergoes metamorphosis into a kentrogon that possesses a hollow cuticular-tube structure, the stylet, which penetrates the integument of the host and acts as a guide tube for the prospective internal parasite. The first, hitherto unknown endoparasitic stage of a rhizocephalan, the vermigon, was recently discovered (Glenner and Hoeg [1995] Nature 377:147–150) and its migration through the hemolymph of the host, as well as its internal development, was described in Glenner et al. ([2000] Mar Biol 136:249–257). The present article provides detailed information on kentrogon and vermigon formation, the injection process, and the succeeding developmental stages up to the stage of the earliest primordium reported from the literature. The anlage of the ovary is traced back to the free-swimming cypris stage and it is implied that the mesoderm and ectoderm of the endoparasite are already differentiated in the cyprid. J. Morphol. 249:43–75, 2001. © 2001 Wiley-Liss, Inc.

Knowledge about the Rhizocephala (parasitic barnacles) has increased exponentially over the past two decades. This introduction broadly reviews this progress, touching on rhizocephalan morphology, life-cycles, larval biology, and the effects on the crustacean hosts. Members representing both orders, the Kentrogonida and Akentrogonida, are discussed. The recent discovery regarding the vermigon stage further highlights the intricacy of the kentrogonid life-cycle. Because rhizocephalans are found in most world regions, occurring on their respective hosts from the deep ocean to freshwater, their importance is now being recognized.

Rhizocephalan parasites are dioecious organisms, in that one or several dwarf males are implanted into the external part of the female parasite soon after it emerges from the interior of the host animal. The structure of the female externa and its resident males is crucial for understanding both the reproductive biology and the taxonomy of these specialized parasites. We use scanning electron microscopy and histological methods to study the anatomy of juvenile and the mature externae of the rhizocephalan barnacle Loxothylacus texanus parasitizing the blue crab Callinectes rathbunae. We put emphasis on the implantation of males and the histology of the female reproductive organs. In the virgin externae, male cyprids attach around a cuticular hood covering the mantle aperture, which is partially blocked by a plug of cuticle so only trichogon larvae, not cyprids, can access the mantle cavity. This resembles the situation known from Sacculina carcini. The mature externa is characterized by a visceral mass that contains the ovary, paired colleteric glands, a single male receptacle, but paired receptacle ducts. The proximal attachment of the visceral mass is located at some distance from the basal stalk, as is characteristic for the genus Loxothylacus. The internal anatomy of the mature externa of L. texanus is in most features similar to that seen in other species of the Sacculinidae, which comprises the majority of rhizocephalan species. However, the single receptacle creates a situation where the two implanted males cannot be kept separate as in most other rhizocephalans, but pass through spermatogenesis in a common chamber. This may have unknown effects on the reproductive biology such as male–male competition. J. Morphol., 2010. © 2009 Wiley-Liss, Inc.