Differences between two sympatric species of Tethya (Porifera, Demospongiae) concerning the growth and final form of their megasters

The megasters of T. aurantium and T. citrina consist, in their mature stage, of spherasters and spheroxyasters respectively. A study of the ratio between the ray length and the diameter of the centre (R/C), which determines the megaster shape, has been carried out on spicules of different size.
While the small megasters of these two species have a similar shape, successive stages show in T. aurantium an isometric growth and in T. citrina an allometric growth with rays growing faster than the centre. The two species differ also in the mean size, size frequency distribution and number of rays of the asters. Differences in the mean total diameter and shape were also detected among spicules coming from different regions of the sponge body. The taxonomic and evolutionary meaning of these phenomena are discussed.