Comparative ultrastructure of the root system in Rhizocephalan barnacles (Crustacea : Cirripedia : Rhizocephala)

Rhizocephalan barnacles are parasites of Crustacea. They lack even the rudi ments of an alimentary canal, but infiltrate their hosts with a nutrient-ab sorbing system of rootlets. We review the ultrastructure of the rootlets us ing light microscopy, SEM, and TEM in nine species from five families, repr esenting both suborders of the Rhizocephala: from the Kentrogonida Peltogas ter paguri, P. curvatus, Peltogasterella sulcata, Cyphosaccus norvegicus (P eltogastridae); Lernaeodiscus porcellanae (Lernaeodiscidae); and Sacculina carcini (Sacculinidae); and from the Akentrogonida Clistosaccus paguri (Cli stosaccidae); Chthamalophilus delagei, and Boschmaella japonica (Chthamalop hilidae). With the exception of Chthamalophilus delagei, the root system of the investigated species shares numerous apomorphies at the ultrastructura l level and displays at all levels specializations that maximize the surfac e area. The rootlets consist of a cuticle, an epidermis and a subjacent lay er 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 mum thick cuticle, w hich is never molted. The cuticle consists of an inner homogeneous layer wi th a slightly fibrous structure and an outer, less than 15-nm thick electro n-dense layer, from which numerous microcuticular projections extend into t he hemolymphatic space of the host. The microcuticular projections consist of the outer electron-dense layer and sometimes a core of the more transluc ent homogeneous layer. They vary among the species from being simple in Sac culina carcini to exhibiting complex branching patterns in Peltogasterella sulcata and Cyphosaccus norvegicus. Beneath the cuticle the epidermal plasm a membrane is thrown into irregularly shaped projections. The epidermal cel ls are joined by long septate junctions and exhibit the characteristics of a transporting epithelium. Experiments with acid phosphatase revealed activ ity both in the epidermis and among the microcuticular projections. The pro jections may therefore form a domain that is important in absorption and ex tracellular digestion of nutrients from the host. The axial cells contain a bundant endoplasmic reticulum and seem to convert absorbed carbohydrates in to lipid, which is stored in large droplets. Subepidermal muscle cells caus e sinuous movements of the rootlets, but it remains unknown how nutrients a re transported along the rootlets towards the external reproductive body. I n C. delagei the single, bladder-shaped rootlet lacks both the apical proje ctions in the epidermis, the electron-dense cuticle layer, and the microcut icular projections. We review previous studies on the rhizocephalan root sy stem and discuss functional and phylogenetic aspects of the morphology. (C) 2001 Wiley-Liss, Inc.