Sponge relationships


Sponges have been in the news lately, so how could I resist writing about a recent paper on sponge relationships? Sponges (phylum Porifera) are found in three classes: the Demospongia, the Calcarea, and the Hexactinellida, all of which are quite ancient with forms identified from the Cambrian. Fossil sponges can be identified by the arrangement of their skeletons, which consist of collections of spicules with characteristic shapes and chemical constitutions. Spicules of various sizes are organized into an interlocking meshwork that generates the supporting framework of the animal. Two characteristics of the spicules that are used to classify them are 1) shape, in particular the angle that the rays diverge from one another, and 2) the chemical structure, whether they are based on calcium or on silicon.

  • Demospongia. Spicules are siliceous, with a triradiate symmetry—the spicule rays typically diverge at 60° or 120°. Individual spicules may look like a caltrop, with points to the corners of an imaginary tetrahedron.
  • Calcarea. Spicules are made of calcite, with a triradiate or tuning fork shape.
  • Hexactinellida. Spicules are siliceous, with a hexactine or tetraradiate symmetry—the spicule rays diverge at 90° angles. Picture a child's set of jacks clustered together.

Our understanding of the relationships between these three, however, has been getting juggled about. My copy of Clarkson's Invertebrate Palaeontology and Evolution, for instance, groups the Demospongia with the Calcarea in the subphylum Gelatinosa on the basis of the organization of the soft tissues and spicule shape, and sets the Hexactinellida apart in the subphylum Nuda, while admitting that there are complications that make the groupings prone to radical revision. One alternative is to group them by whether their spicules are made of calcareous or silicaceous, which would mean that the Hexactinellida and Demospongia are sister lineages, with the Calcarea the odd man out.

Bitting and Butterfield are attempting to resolve these relationships by examining a Cambrian sponge, Eiffelia globosa. Eiffelia is a member of a somewhat problematic group of sponges called the heteractinids which have been classified in the Calcarea because they have spicules made of calcium carbonate, and hexaradiate spicules that are at least close in shape to those of calcareans. What the authors suggest, though, is that Eiffelia is actually a good transitional form that also has tetraradiate spicules and two mineralogically distinct layers to their spicules that may represent both a calcareous core and a silicaceous outer rind.

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