Belonging to the class Hexactinellida, these sponges possess mineral skeletons made up of silica spicules which give them their common name. These are made of amorphous opal (Wang et al., 2009). About two thirds of glass sponge species are found below 1000m with half of these species surviving below 2000m (McClintock et al., 2005). The largest siliceous glass sponge is Monorhaphis chuni which can grow to 3m high (Wang et al., 2009). A specimen was collected from a depth of 1100m in the East China Sea and was aged at about 11,000 years ± 3000 years (Jochum et al., 2012). For more information on this sponge see sponge-date 3.
Glass sponges are important ecosystem engineers as communities are capable of growing on both hard and soft substrate (Dohrmann, 2012). The spicule mats that glass sponges form acts as a three-dimensional habitat for a diverse range of invertebrates (Barthel and Gutt, 1992). Sizeable glass sponges such as Rosella and Anoxycalyx are a refuge for a selection of fauna and have been seen to harbour higher species richness than demosponges (Barthel and Tendal, 1994). A classic example of symbiosis in sponges is the Venus flower-basket (Euplectella) (Figure 7) which typically houses a mating pair of shrimps (see sponge-date 1 for more details about this particular sponge species). A pair of mating shrimps (Spongicola japonica) live encased within this sponge until death (Saito et al., 2002).
The Venus flower-basket (Euplectella), a deep-sea glass sponge, has a lattice of fused spicules that provides enhanced structural support (Sundar et al., 2003). Made up of amorphous hydrated silica, these spicules typically reach 5-15cm long and 40-70mm in diameter (Sundar et al., 2003). Interestingly, these spicules have been observed to be akin to the man-made fibre optics utilised in commercial telecommunication, except with added benefits. For example, this naturally made spicule lattice is crack-resistant and created in ambient temperatures giving them beneficial impurities. It has also been suggested that these sponges distribute light on the deep-sea floor (Sundar et al., 2003).
Burton (1928a) made a few observations about differences between shallow-water sponges and deep-sea sponges. These notes have since been reinforced by further studies. One difference is that deep-sea sponges tend to display bilaterally symmetrical, a characteristic usually absent in Demospongia (Tabachnick,1991). Similarly to most species of Demospongia, glass sponges tend to filter feed by pumping water through their body (Reiswig, 1990). In the deep sea where food supply is low and water flow is unpredictable, bilateral symmetry enables some deep-sea sponges to passive feed (Vogel, 1977). This mechanism is advantageous at low water flow when active feeding is more costly than beneficial. The tube-like shape of Hyalonema (Figure 5) uses this method as it can curve under the influence of water flow to maximise efficient feeding (Tabachnick,1991). For more information on this sponge, read the case study ‘Abyssal Station (STA, N 4100m)’. The development of anchoring structure and the lack of pigment are further differences possessed by deep-sea sponges when compared to their shallow-water counterparts (Tabachnick,1991).
Light absence, low organic matter flux, poor food supply and low water flow are all characteristics of the deep-sea habitat (Vacelet et al., 1994). Submarine caves can often simulate these conditions (Vacelet et al., 1994), while also having the advantage of being a shallower depth which enables a greater ease of research. Organisms in both environments rely on organic input sinking down from the surface. For this reason, the faunal composition at each environment have shown similarities and some deep-sea species have been discovered surviving in the darkest areas of littoral caves (Riedl, 1966). Temperature and pressure, however, still differs (Vacelet et al., 1994). An example of this phenomenon is a Mediterranean cave discovered which has been described as a “bathyl island”. It is home to Oopsacas minuta (Figure 8), an otherwise exclusively deep-sea glass sponge (Vacelet et al., 1994). This species has since been found in four more littoral caves of the Adriatic Sea (Bakran-Petricioli et al., 2009).
Deep-sea sponges are adapted to the environment in which they live through their structure, feeding mechanisms and life tactics. Go to the case studies section for information on particular species of deep-sea glass sponges and to the conclusions page for an overall summary of how sponges survive at abyssal depths.