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Glass sponges (phylum Porifera: class Hexactinellida) are exclusively marine sponges with mineral skeletons composed of silica (glass) spicules. They have a worldwide distribution but are mainly restricted to deeper waters (200 to > 6000 m) where they grow attached to hard or soft substrates. Occasionally, they also occur in shallower water accessible to divers, such as submarine caves in the Mediterranean Sea or off the coast of British Columbia, Canada, where they form massive structures analogous to Mesozoic sponge reefs (see below). These mostly inconspicuously coloured sponges are highly variable in body shape (e.g. sac-, vase-, blade-shaped, composed of branching tubes etc.) and often provide substrate and shelter (and sometimes food) for diverse invertebrates and fish. Glass sponges are viviparous and produce distinctive trichimella larvae. Like most sponges, they are filter feeders. To date, about 600 extant species are described, which is certainly an underestimate of their true diversity, given their remote habitats and very few taxonomic experts for the group. Iconic glass sponges include the venus flower baskets (Euplectella and related genera), which often enclose a pair of shrimps inside their bodies and were used as bridal gifts in ancient Japan, and Monorhaphis chuni, which anchors its body in the soft deep-sea floor with a single giant (up to 3 m long) spicule.
Hexactinellids are clearly distinct from other sponges (see the Porifera page for an introduction to general sponge design) in two main respects: 1) Their adult soft tissues are largely syncytial, i.e. the majority of cells are fused, resulting in one large multi-nucleated mega-cell that is wrapped around the mineral skeleton. It has been demonstrated that the sponges can use this syncytium to propagate electrical impulses to regulate their filtering activity, analogous to a nervous system in higher animals. 2) Their spicules have a triaxonic and cubic symmetry, i.e. they are composed of three axes that are arranged at right angles to each other. The basic spicule form is the hexactin, which has all six rays (two per axis) fully developed – hence the taxonomic name, Hexactinellida. By reduction, branching, and ornamentation of rays, glass sponges produce a staggering array of different spicule types that can be aesthetically highly appealing and form the basis to distinguish species and higher taxa. Spicules are categorized into two basic groups: megascleres, which provide structural support and are often visible with the naked eye, and the much smaller microscleres, the function of which – other than to please taxonomists – is largely unknown. Glass sponge skeletons have recently attracted the attention of materials scientists because they possess a number of interesting properties, including very good light-transmitting capabilities (if the living sponges actually use their spicules to transmit light is questionable, though).
Genetic and biochemical evidence suggests that glass sponges share an exclusive common ancestor with the largest of the four sponge classes, the Demospongiae. The exact time in Earth history when the two groups began to diverge from each other is unknown, but it certainly happened in the Precambrian (more than 541 million years ago [Ma]). The Precambrian fossil record of Hexactinellida is relatively sparse, but the group began to flourish in the Cambrian (541-485 Ma) and radiated into a number of taxa with diverse skeletal designs, most of which did not survive into the Mesozoic (252-66 Ma). In the Mesozoic, the modern orders and families developed their full diversity, which peaked in the Jurassic to Cretaceous (201-66 Ma). During that time, glass sponges, together with certain demosponges, built massive reefs along the coasts of the ancient Tethys Sea, which are still preserved as rock formations in many locations around Europe. After the Cretaceous, hexactinellid diversity gradually declined towards its present level.
Systematics and Phylogeny
Hexactinellida is divided into two subclasses, the Amphidiscophora, which have safety-pin like microscleres (amphidiscs), and the Hexasterophora, which have microscleres resembling stars or snowflakes (hexasters). Amphidiscophora contains a single order with three families; they exclusively have skeletons of unfused spicules. In contrast, Hexasterophora is divided into one order with mostly unfused spicules (Lyssacinosida with three families) and three orders characterized by fused (dictyonal) main skeletons (Hexactinosida with nine families, and Aulocalycoida and Lychniscosida with two small families each). Genetic analyses strongly support Hexactinellida and its two subclasses, as well as most families and genera investigated so far, as natural groups (monophyla). However, concerning the monophyly and interrelationships of the hexasterophoran orders, the situation is much less clear: there is strong evidence that Hexactinosida is not a natural group because one family (the Dactylocalycidae) appears to be more closely related to Lyssacinosida than to the remaining hexactinosidan families (which together form the monophylum Sceptrulophora); moreover, genetic data for Aulocalycoida and Lychniscosida are still missing and skeletal characters are inconclusive regarding their relationships to the other orders.