The periodicity of sexual elements and soft tissue modifications during
the life cycle of the hypercalcified sponge Petrobiona massiliana was
investigated monthly from June 2006 to November 2007. Sexual
reproduction, likely regulated by seawater temperatures, occurred during
more than half of the year (from early April to late October); 70% of
the samples appeared reproductively active. Specimens of P. massiliana
displayed a high plasticity of tissue organization, allowing modulation
and rearrangement of their aquiferous systems in response to life cycle
phases and environmental changes. Permanent changes were observed in the
basal region of the choanosome in non-reproductive specimens, such as
disorganization/restructuring events leading to remodeling of the
aquiferous system. Periodic modifications occurring during sexual
reproduction included the transformation of choanocytes from a typical
form to hourglass and vespiform shapes, and more global disorganization
of the basal region of the choanosome during provisioning of oocytes and
embryos, followed by restructuring after release of the larvae. Finally,
episodic disorganization/reorganization phenomena occurred in a few
specimens after unfavorable environmental conditions (e. g., decreasing
seawater temperatures). Histological and ultrastructural observations of
storage cells, located in peculiar trabecular tracts, suggest a
transdifferentiation capacity that allows such soft tissue dynamics.