Introduction
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.
Design
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).
Evolution
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 (Amphidiscosida) with three families; Hexasterophora contains three orders – Lyssacinosida with four families, Sceptrulophora with nine families, and Lychniscosida with two small families – as well as one family (Dactylocalycidae) and five genera with uncertain taxonomic assignment (Hexasterophora incertae sedis). Genetic analyses strongly support Hexactinellida and its two subclasses, as well as most families and genera investigated so far, as natural (monophyletic) groups. Concerning the deeper interrelationships of Hexasterophora, genetic data strongly suggest that Dactylocalycidae is more closely related to Lyssacinosida than to Sceptrulophora. Genetic data for Lychniscosida are still missing but there is some morphological evidence that this order is also more closely related to Lyssacinosida.
Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers consider them sufficiently distinct to deserve their own phylum, Symplasma. Some experts believe glass sponges are the longest-lived animals on earth;[2] these scientists tentatively estimate a maximum age of up to 15,000 years.
Glass sponges are relatively uncommon and are mostly found at depths from 450 to 900 metres (1,480 to 2,950 ft) below sea level. Although the species Oopsacas minuta has been found in shallow water, others have been found much deeper. They are found in all oceans of the world, although they are particularly common in Antarctic and Northern Pacific waters.[3]
They are more-or-less cup-shaped animals, ranging from 10 to 30 centimetres (3.9 to 11.8 in) in height, with sturdy lattice-like internal skeletons made up of fused spicules of silica. The body is relatively symmetrical, with a large central cavity that, in many species, opens to the outside through a sieve formed from the skeleton. Some species of glass sponges are capable of fusing together to create reefs or bioherms. They are generally pale in colour, ranging from white to orange.[3]
Much of the body is composed of syncitial tissue, extensive regions of multinucleate cytoplasm. The epidermal cells characteristic of other sponges are absent, being replaced by a syncitial net of amoebocytes, through which the spicules penetrate. Unlike other sponges, they do not possess the ability to contract.[3]
Their body comprises three parts: the inner and outer peripheral trabecular networks, and the choanosome, which is used for feeding purposes. The choanosome acts as the mouth for the sponge while the inner and outer canals that meet at the choanosome are passages for the food, creating a consumption path for the sponge.[4]
All hexactinellids have the potential to grow to different sizes, but the average maximum growth is estimated to be around 32 centimeters long. Some grow past that length and continue to extend their length up to 1 meter long. The estimated life expectancy for hexactinellids that grow around 1 meter is approximately 200 years (Plyes).
Glass sponges possess a unique system for rapidly conducting electrical impulses across their bodies, making it possible for them to respond quickly to external stimuli.[5] Species like "Venus' flower basket" have a tuft of fibers that extends outward like an inverted crown at the base of their skeleton. These fibers are 50 to 175 millimetres (2.0 to 6.9 in) long and about the thickness of a human hair.
Glass sponges are different from other sponges in various other ways. For example, most of the cytoplasm is not divided into separate cells by membranes but forms a syncytium or continuous mass of cytoplasm with many nuclei (e.g., Reiswig and Mackie, 1983).
These creatures are long-lived, but the exact age is hard to measure; one study based on modelling gave an estimated age of a specimen of Scolymastra joubini as 23,000 years (with a range from 13,000 to 40,000 years). However, due to changes in sea levels since the Last Glacial Maximum, its maximum age is thought to be no more than 15,000 years,[6] hence its listing of c. 15,000 years in the AnAge Database.[7] The shallow-water occurrence of hexactinellids is rare worldwide. In the Antarctic, two species occur as shallow as 33 meters under the ice. In the Mediterranean, one species occurs as shallow as 18 metres (59 ft) in a cave with deep water upwelling (Boury-Esnault & Vacelet (1994))
Staurocalyptus sp.
Various hexactinellid sponges.
Hexactinellid sponge on a xenophorid gastropod.
Pattersonia ulrichi Rauff, 1894; an Ordovician hexactinellid sponge from near Cincinnati, Ohio.
The sponges form reefs (called sponge reefs) off the coast of British Columbia, southeast Alaska and Washington state,[8] which are studied in the Sponge Reef Project. Reefs discovered in Hecate Strait, British Columbia, have grown to up to 7 kilometres long and 20 metres high. Prior to these discoveries, sponge reefs were thought to have died out in the Jurassic period.[9][10]
Reports of glass sponges have also been recorded on the HCMS Saskatchewan and HCMS Cape Breton wrecks off the coast of Vancouver Island.
The earliest known hexactinellids are from the earliest Cambrian or late Neoproterozoic eras. They are fairly common relative to demosponges as fossils, but this is thought to be, at least in part, because their spicules are sturdier than spongin and fossilize better. Like almost all sponges, the hexactinellids draw water in through a series of small pores by the whip-like beating of a series of hairs or flagella in chambers which in this group line the sponge wall.
The class is divided into two subclasses and several orders:[11]
Class Hexactinellida
Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers consider them sufficiently distinct to deserve their own phylum, Symplasma. Some experts believe glass sponges are the longest-lived animals on earth; these scientists tentatively estimate a maximum age of up to 15,000 years.
