The Choanosome of Hexactinellid Sponges

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Three-dimensional images of the choanosome of hexactinellid sponges can help in explaining how these sponges mix cellular and syncytial tissues in a fine cobweb-like network. Plastic replicas of the water canals show that Rhabdocalyptus dawsoni has large, highly branched incurrent canals, and equally large, although less branched, excurrent canals that are studded with 60 (im-long flagellated chambers. Freeze-fractured, fixed specimens show the syncytial tissue, known as the trabecular reticulum, envelops cells in a thin collagenous mesohyl at the flagellated chambers in both R. dawsoni and Aphrocallistes vastus. Groups of archaeocytes are especially abundant in the mesohyl of R. dawsoni during summer months. A branch of the trabecular reticulum, the secondary reticulum, surrounds the collars in the flagellated chambers, effectively forcing water to be drawn through the collar microvilli. Another branch, the inner membrane, occupies up to 10% of flagellated chambers of sponges during all months of the year, but is especially prevalent in specimens which have been kept in sea water aquaria for several weeks. This three-dimensional view of hexactinellid tissues reinforces the conclusion that hexactinellid sponges be separated from other sponges at the subphylum level. Additional key words: Hexactinellida, Porifera, syncytium, sponge Sponge morphology has been studied for well over a century. These animals are considered to be simple, radially symmetric or asymmetric, diploblastic metazoans that carry out all the basic functions of ingestion and egestion, growth and reproduction with a very small complement of cell types. The Hexactinellida, however, differ fundamentally from other sponges in having a syncytial tissue (the trabecular reticulum) rather than cells, as their major tissue component (Reiswig 1979; Mackie & Singla 1983; Leys 1995). The uniqueness of the construction of the Hexactinellida has been recognized since these animals were first examined in the late 1800s. Bidder (1929) first proposed separating the Porifera into two phyla based partly on the difference in tissue organization between cellular and syncytial sponges. Later, Reid (1963) argued that this difference should be recognized at the subphylum level, but the lack of ultrastructural proof of syncytial tissues prevented sponge workers from unanimously accepting the proposal (Bergquist 1978). Because hexactinellids are predominantly deep water sponges, specimens were usually obtained by dredging, and consequently were in poor condition. The discovery of shallow-water populations accessible by a Present address: Department of Zoology, University of Queensland, Brisbane, QLD 4072, Australia. SCUBA in the late 1970s allowed the first ultrastructural examination of tissues and confirmed that the major tissue component in the sponge constitutes a single giant syncytium (Reiswig 1979; Mackie & Singla 1983). Advances in our understanding of the physiology (Perez 1996; Wyeth et al. 1996; Leys & Mackie 1997), development (Boury-Esnault & Vacelet 1994; BouryEsnault et al. 1999), and molecular biology (Koziol et al. 1997; Kruse et al. 1998) of hexactinellids have confirmed the unusual status of this group within the phylum Porifera. However, recent invertebrate texts have yet to acknowledge a proposal for subdivision of sponges into two subphyla (Reiswig & Mackie 1983). Because hexactinellid tissue is unusually difficult to preserve well for either transmission or scanning electron microscopy (TEM, SEM), it is possible that discrepancies in interpretation of preserved tissue by specialists in this field have been the reason why no text carries a clear description of the tissue organization of these sponges. Despite several excellent articles describing fixed tissues from some eight species of hexactinellids (e.g. Mackie & Singla 1983; Reiswig & Mehl 1991; Boury-Esnault & Vacelet 1994), a clear, three-dimensional picture of their tissues is still lacking. In other sponge groups the flagellated chambers and This content downloaded from 157.55.39.153 on Mon, 19 Sep 2016 04:45:05 UTC All use subject to http://about.jstor.org/terms