Taxonomic publications: past and future - Senckenberg Museum

Burckhardt & Mühlethaler (eds): 8 th GfBS Annual Conference Abstracts 34
3D-Reconstructions of the chaetal arrangement contradict ideas on a common
inheritance of the chaetal inversion in Sabellariidae and Sabellidae (Annelida)
D. Kieselbach & H. Hausen
Sabellida and Sabellariidae are considered to be sister groups in a number of recent
investigations. This assumption is based mainly on a special distribution of certain
types of chaetae along the body called chaetal inversion. In taxa, which are assumed
to be close relatives of Sabellariidae and Sabellida, capillary chaetae form the
notopodial bundles, whereas uncini or homologous hooked chaetae occur only in
neuropodia throughout the body. This meets the situation in the thoracic setigers in
Sabellida, but abdominal notopodia bear uncini and abdominal notopodia bear
capillary chaetae. In Sabellariids uncini occur only in notopodia of the abdomen and
lack completely in the thorax and parathorax.
The only correspondance between Sabellida and Sabellariidae is the notopodial
position of uncini within the abdomen. To figure out, whether this unusual distribution
evolved once in a common lineage to Sabellariidae and Sabellida the sabellids
Branchiomma bombyx, Sabella pavonina and Fabricia stellaris and the sabellariid
Sabellaria alveolata were closely examined by SEM and LM. Exact computer-aided
3D models of the chaetal arrangement were performed by reconstruction of complete
series of plastic embedded semi-thin sections. Like in several other polychaete taxa,
all investigated sabellids show transverse rows in all noto- and neuropodia
irrespective of the type of chaetae they bear. The thoracic and abdominal transverse
rows have a single formative site at the ventral resp. dorsal edge of each row. In
contrast to abdominal neuropodia thoracic notopodia exhibit an additional, short
longitudinal row with an own caudal formative site. This character is regarded as
derived having evolved after the invention of the chaetal inversion. A minor difference
to Fabricia stellaris is found in Branchiomma bombyx and Sabella pavonina. Here the
transverse rows of the abdominal neuropodia are transformed into semicircles or
spirals shortly after segment formation. In all Sabellidae the abrupt transition of
capillary chaetae and uncini between thorax and abdomen is accompanied by a
sudden change of the parapodial morphology. In contrast Sabellaria alveolata shows
a gradual transition that can clearly be seen in the first one to two abdominal
neuropodial fascicles, since they show a mixed composition of chaetae typical for the
parathoracic and abdominal region as well as a mixed growing pattern. Moreover,
there is no change in parapodial morphology between parathorax and abdomen in
Sabellariidae at all.
Our results do not support ideas on a common evolutionary event that underlies the
distribution of chaetae in Sabellida and Sabellariidae. Thus a position of Sabellariidae
close to Sabellida is uncertain and further research is highly demanded.
Org. Divers. Evol. 5, Electr. Suppl. 13 (2005)