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Sponges originated before the Cambrian period. Two groups of calcareous spongelike organisms occupied early Paleozoic reefs. The Devonian period saw rapid development of many glass sponges. Phylogenetic studies 2 using sequence data from large subunit rRNA, small subunit rRNA, and protein kinase C, indicate that sponges with calcareous spicules in the class Calcarea belong to a separate clade from those with spicules made of silica in classes Demospongaie and Hexactinellida. Two potential placements emerge for calcareous sponges: In one, calcareous sponges are the sister taxon to a clade of siliceous sponges, and in another, the phylum Porifera is paraphyletic because the calcareous “sponges” are more closely related to other metazoan taxa than they are to siliceous sponges.
Porifera are a highly successful group that includes several thousand species in a variety of marine and freshwater habitats. Their diversification centers largely on their unique water-current system and its various degrees of complexity. However, within the silicious Demospongaie, a new feeding mode has evolved for a family of sponges inhabiting nutrient-poor deep-water caves. These deep-water sponges have a fine coating of tiny hooklike spicules over their highly branched bodies. The spicule layer entangles the appendages of tiny crustaceans swimming near the surface of the sponge. Later, filaments of the sponge body grow over prey, enveloping and digesting them.
These sponges are carnivores, not suspension feeders, although some of them may augment their diets with nutrients obtained from symbiotic methanotrophic bacteria. The presence of the typical silicious spicules clearly identifies these animals as sponges, but they lack choanocytes and internal canals.
The loss of choanocytes in these species is doubtless disturbing for students learning to identify sponges, but students of evolution should be fascinated by it. The convoluted pathway taken by one branch of the sponge lineage clearly illustrates the non-directional nature of evolution. To colonize such a nutrient-poor habitat initially, the ancestors of this group must have had at least one alternative feeding system, either carnivory or chemoautotrophy, already in place. Presumably, after the alternative method of food capture was in use, the choanocytes and internal canals were no longer formed. If there are further body modifications in this lineage, we might eventually not recognize the descendants as sponges. Imagine how the lineage would look if spicules were lost in favor of a greater reliance on the bacterial symbionts, and you will begin to understand why it is sometimes hard to trace morphological evolution or to identify the closest relatives of certain animals.
Useful External Links
- Deep Phylogeny and Evolution of Sponges (Phylum Porifera) by ResearchGate
- Deep phylogeny and evolution of sponges (phylum Porifera) by NIH
- Phylogeny and Adaptive Radiation by BioCyclopedia
