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13 Páginas ; 5 Figuras ; 1 Tabla, The phylogenetic relationships within the Order Aplousobranchiata (Ascidiacea) are largely unexplored. In this work, we study the phylogenetic status of the genera Clavelina and Pycnoclavella. Traditionally, both genera had been included in the family Clavelinidae, until the new family Pycnoclavellidae was defined, removing the genus Pycnoclavella from Clavelinidae. Not all authors accept the validity of Clavelina and Pycnoclavella as distinct genera, let alone their belonging to different families. In addition, the assignment of species to these genera, as well as to the genus Archidistoma, has been controversial. We analyzed sequences of the mitochondrial gene cytochrome c oxidase subunit I belonging to ten species of Pycnoclavella (including several formerly assigned to Archidistoma and Clavelina), 11 species of Clavelinidae, and ten species of other aplousobranch genera belonging to seven families, plus two outgroups. Two different tree construction methods (maximum likelihood and Bayesian inference) showed similar results. Pycnoclavella and Clavelina appeared in distinct clades but formed a monophyletic group relative to representatives of the main families of the order Aplousobranchiata. Our phylogenetic results indicate that both genera are valid but should be included within a single family, with the name Clavelinidae having precedence. The monotypic clavelinid genus Nephtheis branches in our trees within the clade of the genus Clavelina. Our results also confirm that some forms assigned to Archidistoma and Clavelina have been misplaced and belong to the genus Pycnoclavella. Pycnoclavella martae n.sp. is described., The research was funded by projects REN2001-2312-C03-02 and CTM2004-05265-C02-01 of the Spanish Government.

A new ascidian species, Eudistoma purpuropunctatum, is described from the San Juan Archipelago in Washington State. Recorded frequently for many years from fast-current subtidal areas, it was formerly considered a variant of Cystodytes lobatus (Ritter). Notes on color, texture, and habitat are included.

Archidistoma is a genus created for a species discovered and described briefly by Garstang in 1891 as A. aggregatum. It has been found at different times only in the Plymouth region, the original locality, attached to rock surfaces in a few feet of water. Recently, however, it has been found near Cape Hatteras on the eastern American coast (Van Name, 1945), suggesting either harbour to harbour transportation by ship bottom, or that its inconspicuous appearance has obscured a wide distribution.This form is of considerable interest as it represents a type less specialized in zooid structure, nature of tadpole, and method of budding than any merosomatous ascidian except the diazonids. Diazonids are oviparous and have the simpler development and tadpole associated with small eggs, although there is a more elaborate branchial sac correlated with the relatively large size of the mature zooid.ColonyThe colony consists of a brownish tough matrix forming a thin encrustation on stones and shells, with individual zooids (a few millimetres long), partly embedded in the test but for the most part extending freely in clumps usually of three or four. Sand grains and round particles adhere to or become buried within the test, making this form the most inconspicuous of ascidians.Structure of ZooidThe zooid is divisible into a thorax and a long slender abdomen (Fig. 1). There is no postabdomen, nor is there hypertrophy of the ventral test vessels, thereby segregating Archidistoma as a type from that of the synoicids (polyclinids) and the clavelinids respectively.