Sponges are animals of the phylum Porifera, despite their plant-like sessile life-style and branched body shapes. Similar to plants, they utilise their surface area to capture resources with which to live (Ruppert et al., 2004). Their expanded surfaces allow them to maximize their ability to capture food particles in the water column. Sponge body shapes are specialized for filter feeding (Leys and Hill, 2012). They have natural meshes that filter the water passing through, leaving the organic food particles behind. Sponges have the incredible ability to change shape in order to filter-feed more efficiently.
Sponges are diverse, both in size and body shape (Leys and Hill, 2012). They can range from a mere few millimetres to greater than one metre in diameter and height. The feature that stands true for most sponges is that they lack standard symmetry. However, many are radially symmetrical (i.e. shaped like a cone, sphere or cyclinder) (Ruppert et al., 2004). Species of sponge and the local conditions can determine whether a sponge grows to be thick, erect, branching or encrusting. Deep-sea glass sponges tend to be anchored to the sea floor for support (Tabachnick, 1991).
Sponges seem to come in three main designs: asconoid (Fig1a), syconoid (Fig1b) and leuconoid (Fig1c). Asconoid, the simplest design, entails a cylinder attached to some form of substrate by its base. Within this cylinder is a hollow area known as the atrium which is lined with flagellated cells called choanocytes (Ruppert et al., 2004). Pores, or ostia, are present throughout the walls of the sponge. Numerous choanocytes make up a layer called the choanoderm (Ruppert et al., 2004). At the unattached end of this cyclinder shape, there is a larger opening called the osculum. The previously mentioned choanocytes produce a water flow that enters through the pores and to the atrium, before passing out through the osculum. This is known as an aquiferous system.
Regardless of growth form, sponges need a tough skeleton for support. This tends to be in the form of a mesophylar endoskeleton, though exoskeletons can be localised to certain parts of sponges or occur over the entire body (Ruppert et al., 2004). The composition and rigidity varies from growth form to growth form. Encrusting sponges possess a gelatinous covering (mesophyl) with collagen fibres that acts as their skeleton. Spicules occur within the mesophyl, but often extend beyond this, offering the sponge protection, sometimes by guarding the sponge’s openings (ostia, oscula etc.) (Ruppert et al., 2004).
Spicules can be formed from one of two compounds; Silcon oxide or calcium carbonate. They have a stiffening affect on the mesophyl. The strength of this affect relies on their density, arrangement and how much the spicules interlock. Spicules can form an immense lattice such as in the glass sponge Euplectella aspergillum (Sundar et al., 2003) (Figure 2).
Generally, a sponge can pump a volume of water equal to its own body volume, every 5 seconds (Ruppert et al., 2004). Water cannot be compressed, so the volume of water in to a sponge must equal the volume of water flowing out. The water volume is created by a pressure gradient, initiated by the low pressure generated at the base of the sponge by the beating of flagellum (Larsen and Riisgard, 1994).
Here is a link to a flash animation of water flow through the three body designs of sponge species: http://www.biology.ualberta.ca/facilities/multimedia/uploads/zoology/Porifera.swf
Sponges use both sexual and asexual reproduction. Most sponges are hermaphrodites capable of both releasing sperm and being fertilised by it (Ruppert et al., 2004). The sperm gets transported through the water by currents. Sponges also use asexual reproduction which is carried out in a variety of ways such as fission. A piece of sponge, dislodged by wave action or grazing organisms, attaches to a substrate and transforms in to a fully functional sponge. Furthermore, rare sponges use “budding”. For example, buds swell from the asconoid tubes of sponge species Clathrina (Figure 3). These buds are released, attach to substrate and become another individual sponge (Ruppert et al., 2004). Deep-sea glass sponges tend to have very long time intervals between reproduction (Dayton et al., 1974).
In order to feed, most sponges (including glass sponges) filter the water that passes through their body cavities (Reiswig, 1990). The food particles that they filter can vary from <1mm to 50mm in size. The seawater is a soup of single celled plankton which is what the sponges need to survive and grow. This concoction includes dinoflagellates, bacteria, miniscule organic debris and viruses. Sponge cells ingest these food particles through phagocytosis which is when a cell engulfs a food vesicle (Ruppert et al., 2004). Similarly to Demospongiae, ultraplankton is the main food source for glass sponges (Reiswig, 1990).
Sponge feeding mechanisms have been observed to be shaped by the environment. Sponges surviving in environments with low food levels develop methods to process a greater volume of water (Maldonado and Young, 1998; Leys et al., 2011) or, in some cases, use an alternative food source. Carnivorous sponges are an example of this (Vacelet and Boury-Esnault, 1995).
Harp sponge footage uploaded by youtube user: MBARI
For example, found at a depth of 3316m, the harp sponge (Chondrocladia lyra) is capable of catching crustaceans, covering it in membrane and digesting it whole. Its harp-like structure is believed to aid catching as much prey as possible by maximizing the surface area of the sponge (Crew, 2012). A video of this extraordinary sponge is above and more information can be found on “Sponge-date 2”.
The form, skeleton and feeding of glass sponges enable them to survive at abyssal depths and contribute to their role as ecosystem engineers. To learn about how they can increase the diversity of the local area read the page “Role of Sponges“. Furthermore for a more in-depth knowledge of the members of the class Hexactinellid, go to the page “Glass Sponges“.