Desmospongiae, Geodia morphotype
Euretidae, Chonelasma morphotype
Rossellidae, Bathydorus morphotype
Porifera Grant, 1836, commonly known as sponges, constitutes a rich and diverse phylum within the Animalia kingdom. Sponges are known as one of the oldest and most original animal taxon in Earth’s history, with a fossil record dating back to the Late Precambrian (Reitner & Mehl 1996). Sponges are successful colonizers of all marine environments and also many freshwater habitats. In the deep sea, Porifera have been observed down to the deepest hadal zones, and they belong to the dominant benthic fauna of abyssal environments world-wide.
Classification and taxonomic characterisations
The phylum Porifera is divided into four widely recognized classes (Van Soest et al., World Porifera Database): Hexactinellida Schmidt, 1870, Demospongiae Sollas, 1885, Homoscleromorpha Bergquist, 1978 and Calcarea Bowerbank, 1862; of which only the first two contribute significantly to global deep-sea benthic communities. Within the Porifera, the outer shape is rarely used as a classification criterion, but is often used for the first preliminary sorting into potential morphotypes. Traditional sponge taxonomy is almost exclusively based on the mineralogy, morphology and organization of their skeletal elements, spicules, or scleres. These are normally in the µm scope and have to be studied by means of light microscopy and/or SEM. Additionally, for documentation of the channel system and skeletal architecture, histological preparations of sections through the sponge body are made. In recent sponge research, molecular biological methods, e. g. bar-coding, and biochemical features, are being used for further taxonomicalcharacterization. Demospongiae, Hexactinellida and Homoscleromorpha, (also known as the siliceous sponges), are characterized by a collagenous skeleton supplied by mineral spicules of opaline silica. They are classified according to the symmetry and morphological features of the spicules, which occur in great diversity of morphologies.
- Hexactinellida sponges display a six-rayed, triaxial, symmetry of their spicules (hexactins), which are commonly reduced to simpler forms (e. g. tetractins, triactins, and diactins).
- Demospongiae, which comprise the vast majority (> 90 %) of all living Porifera species, are characterized by mainly tetraxonal or diactinal symmetry of their spicules. Some demosponges, notably of the order Dictyoceratida Minchin, 1900, have no spicules at all, but instead possess a strong skeleton of special collagen fibres, called spongin; this type of skeleton is best known from the species Spongia officinalis Linnaeus, 1759 (the bath sponge).
- Main spicules of the minor group Homoscleromorpha are regular tetractines, sometimes supplied by other types, e. g. diactins.
- The small taxon Calcarea is characterized by spicules of crystalline calcite with basic triactinal symmetry, with additional tetractins and/or diactins occasionally occurring.
Spicules of demosponges and hexactinellids are subdivided into larger (megascleres) and smaller (microscleres) categories, with the latter type especially important for species identification (further detailed information in Hooper & Van Soest 2000).
Porifera (lat. Pori = pores, fera= carrier) are characterized by three body layers, the entosome, ectosome and mesohyl. These layers are permeated by a complex system of channels, and the inner and outer body surfaces are often lined by special cells (pinacocytes) and covered with pores. Sponges are active filter feeders and use special flagellated cells, choanocytes, which are concentrated in choanocyte chambers, to generate a water current through their channel system, from which microbes and other tiny nutrient particles (picoplankton) are separated and digested. Depending on taxon and ecological conditions, sponges are very morphologically variable and may take on almost any body form, such as vase-, amphora-, plate- or fan-shaped, bulbous or cushion-formed or thinly crusting on a substrate. As a benthic sessile animal, most sponges are attached to firm substrates (rock, stones or sediment surface) by their basal parts, which may be characterized by a special skeletal construction, such as a basal plate or a root tuft made of long basal spicules.
Hexactinellida Schmidt, 1870 (junior synonym Hyalospongiae Vosmaer, 1887) comprises a mainly deep-water species today (unlike the fossil record, where numerous shallow water taxa were found). This is particularly true of the sub-class Amphidiscophora Schulze, 1886, which is mainly confined to bathyal-abyssal deep-sea environments.
Demospongiae are found in all marine zones from the intertidal to as far down as the abyssal plain, and are also widely distributed in many fresh water environments. Some taxa are cosmopolitan and occur at any water depth from the shallow shelf down to the abyss. This is true for the Polymastiidae Gray, 1867 and for many Poecilosclerida Topsent, 1928, sponges, such as the Cladorhizidae Dendy, 1922, family, which are carnivorous sponges ecologically specialized for deep-sea environments. These sponges are also in found submarine caves, polar and other marine environments where the waters are cold in temperature and nutrient-poor (e. g. Vacelet & Boury-Esnault 1996, Downey et al. 2012).
Homoscleromorpha and Calcarea comprise mainly of species with a preference for shallow to intermediate water depths. However, some deep-sea species, notably of the calcareous sponges, also exist (Rapp et al. 2011).
Limitations of photographic images
Due to the great morphological variability of the outer shape of many sponge species, a reliable taxonomical classification is almost exclusively only possible by identifying and measuring skeletal features, or by molecular methods. Without biological sponge samples, almost all species identifications based on images alone must be considered as highly uncertain. However, some sponge genera, especially of the Hexactinellida, and a small number of sponge species do have characteristic outer morphologies that can be used for identification. The success of photo-based taxonomy greatly increases, when combined with biological sampling, even when the samples are obtained from other specimens than those photographed (provided that the samples were taken in a nearby area, since the distribution of sponge species is normally patchy). By comparing the outer morphology and skeletal characteristics of the sampled sponges with those observed by visual documentation, it is often possible to succeed in a fairly exact deduction of the genera and species distribution within a studied ecosystem.
Downey, R. V.; Griffiths, H. J.; Linse, K. & Janussen, D. 2012: Diversity and distribution patterns in high southern latitude sponges.- PLoS One, 7 (7), 1-16: e41672. doi:10.1371/journal.pone.0041672
Hooper, J.N.A & van Soest, R W.M. (eds.), Systema Porifera: A Guide to the Classification of Sponges.- Kluwer Academic/Plenum Publishes, New York, 1708 pp.
Rapp, H. T., Janussen, D. & Tendal, O. S. (2011): Calcareous sponges from abyssal and bathyal depths in the Weddell Sea, Antarctica.- Deep-Sea Research, ser. II, 58 (2011) 58–67.
Reitner, J. & Mehl, D. 1995: Early Paleozoic diversification of sponges: New data and evidences.- Geol. Paläont. Mitt. Innsbruck, 20: 335-347.
Vacelet, J. & Boury-Esnault, N. 1996: A new species of carnivorous sponge (Demospongiae: Cladorhizidae) from a Mediterranean cave.- In: Willenz, Ph. (Ed.), Recent Advances in Sponge Biodiversity Inventory and Documentation. Pp. 109-115. Bulletin de l’Institut royal des Sciences naturelles de Belgique. Biologie, 66