3 SIBLING SPECIES OF DIDEMNID ASCIDIANS FROM NORTHERN NORWAY - DIDEMNUM-ALBIDUM (VERRILL, 1871), DIDEMNUM-POLARE (HARTMEYER, 1903), AND DIDEMNUM-ROMSSAE SP-NOV
Ja. Marks, 3 SIBLING SPECIES OF DIDEMNID ASCIDIANS FROM NORTHERN NORWAY - DIDEMNUM-ALBIDUM (VERRILL, 1871), DIDEMNUM-POLARE (HARTMEYER, 1903), AND DIDEMNUM-ROMSSAE SP-NOV, Canadian journal of zoology, 74(2), 1996, pp. 357-379
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.