A reconsideration of the relationship between polyaxonid and monaxonid spicules in Demospongiae: new data from the genera Crambe and Discorhabdella (Porifera)
15 páginas y 7 figuras.The relationships between different spicule lineages of Demospongae are revised through the
ontogenetic study of the main spicules of the genera Crambe and Discorhabdella. The presence
of terminal orifices in the basal spines of the asterose acanthostyles of Discorhabdella, and the
actines of the desmas of Crambe have been shown by examining young spicules under high
magnification. Thus, the polyaxonid origin of both spicule types is hereby supported by the
ontogenetic information, and their homology is also supported by their equivalent arrangement
in the skeleton. The current differences in shape between both spicule types are considered
the result of a divergent morphological evolution from an ancestral polyactinal corpuscle, by
the atrophyihypertrophy of a different number of actines. Arguments are also presented to
support the homology of these two spicule types with the sphaeroclons of Vetulina, and other
fossil genera. Moreover, the presence of axial canals inside the tubercles of the tuberose
tylostyles of Discorhabdella and Crambe tuberosa indicates that the tubercles are actually atrophied
actines as in the case of the hadromerid genus Terpios. According to the ontogeny, the
tuberose morphology of these spicules may correspond to the retention of an ancestral
characteristic in the Poecilosclerida and Hadromerida; in this case, a monophyletic origin, is
suggested between both taxa. From the overall results here presented, the tetraxonid spicule,
presently considered by most authors as the primitive morphotype, as well as some monaxons,
could be considered as evolving from a polyaxial form.Research was partially sponsored by projects CEE MAS2 CT91-004 (Marine
Sciences and Technologies, MAST-11), CICYT MAR9 1-0528, PB89-008 1
(Marine Resources and Aquaculture National Program), CICYT PB89-008 1,
and a ‘Generalitat de Catalunya’ fellowship to M. Maldonado.Peer reviewe