3 SIBLING SPECIES OF DIDEMNID ASCIDIANS FROM NORTHERN NORWAY - DIDEMNUM-ALBIDUM (VERRILL, 1871), DIDEMNUM-POLARE (HARTMEYER, 1903), AND DIDEMNUM-ROMSSAE SP-NOV

Recent advances in taxonomy have disclosed the prevalence of cryptic s ibling species in marine systems. In didemnid ascidians, morphological variation between species is often slight, and many species may have been overlooked. Species boundaries within this group are often equivo cal, and patterns among taxa are reminiscent of those formed by reticu late evolution in plants. Specimens of the sessile colonial ascidian D idemnum albidum (Verrill, 1871) were examined for life-history and mor phological characters and found to constitute at least three sympatric sibling species. One of these, Didemnum romssae, is described here as a new species. Colonies were collected from hard-bottom assemblages i n Troms and Finnmark counties in northern Norway. Zooid morphology in D. romssae is similar to that of D. albidum, but varies within each sp ecies, even for traits usually considered specific. Didemnum romssae i s proposed as a distinct species on the basis of consistent difference s in (i) the shape and size of calcium carbonate spicules within the c ommon test; (ii) larval size and the number of lateral ampullae; (iii) timing of reproduction; and (iv) the absence of a seasonal nonfeeding , overwintering stage, which occurs in D. albidum. Examination of spec imens from museum collections considered synonymous with D. albidum co nfirmed the presence of a third species, Didemnum polare (Hartmeyer, 1 903), which differed from the other two species in zooid and larval mo rphology as well as spicule shape. Complexes of sibling species such a s these provide a tractable system for studying the consequences of li fe-history variation among closely related taxa.