Comparative study of putative conspecific sponge populations from both sides of the Isthmus of Panama

A morphological, cytological and genetic comparison of putative conspecific populations of Spirastrella SD Cf. mollis (Porifera: Demospongiae) from bo th sides of the Isthmus of Panama revealed a very high level of genetic dif ferentiation together with morphological and cytological differences. The m ain differences were the distribution of the spirasters within the choanoso me, the size and shape of spirasters 1, and the size and shape of inclusion s within type I cells. Consequently these two populations clearly belong to different biological species. The Atlantic one, S. hartmani sp, nov corres ponds to what previous authors have called S. cunctatrix in the Caribbean. The finding of this new species raises to three the number of Spirastrella spp. known from the Caribbean Sea. The Pacific species is named here S. sab ogae sp. nov. The levels of gene divergence found between S. hartmani and S sabogae (Nei's genetic distance D = 2.30) were as high as those found betw een different genera in other groups of invertebrates. Similarly exceptiona lly high values of gene divergence have been found between other congeneric sponge species and may be indicative either of a higher rate of molecular evolution or a very slow rate of morphological evolution in the Porifera co mpared to other metazoans.