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SYSTEMATIC BIOLOGY					KINGDOM ANIMALIA
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PHYLUM PORIFERA

Porifera (por- i -fe-ra) is a combination of two Latin roots that mean bearing pores (pore-porus; bear-fero). The name is a reference to the porous nature of the sponge animal.

The sponges are sessile, mostly upright filter-feeding animals (see A and B above). Although they are metazoans, they exist at the cellular grade of organization, and, as such, lack true tissues. The body of the animal has a series of water canals, parts of which are lined with choanocytes or collar cells (see image C). The collar cells trap and ingest food particles and create the one-way flow of water by their beating action. Amoeboid cells occur as a covering (in the case of the Hexactinellida) and as internal carrier cells within the mesohyl, the "internal" portion of the sponge body between the two cell layers. The sponges show the greatest structural and molecular affinities with the choanoflagellates, the non-metazoan animals. According to Bergquist (2001) the sponges appeared more than 550 mya and had differentiated into the three classes by the early Cambrian (550-525 mya).

A. A large basket sponge. The whole animal is more than a half meter across.

B. Glass sponge showing lattice-like organization of the silicaceous spicules.

C. A Transmission Electron Micrograph of a sponge choanocyte.

D. Silicaceous spicules of Spongilla.

Images A, B, & D from Systematics Biodiversity collection.
Image C from: http://www.niwa.co.nz/pubs/wa/09-2/evolution.htm

SYNOPTIC DESCRIPTION OF THE PORIFERA

The following information came from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984), Bergquist (2001), Brusca and Brusca (2003), Hickman (1973), Nielsen (2001), Storer and Usinger (1965), and Tudge (2000).

I. SYNONYMS: Porifera was defined by Grant in 1836. Sponges.

II. NUMBER: >10,000 species known.

III. PHYLUM CHARACTERISTICS:

A. Structure

Symmetry: Generally none. Symmetry may be radial.

Body Cavity: Not present in a strict sense. Body of a mesohyl containing amoebocytes, spicules, and mesenchyme. Organization of the animal based on an interconnected tubular system lined wholly or in part by choanocytes follows three basic body plans: asconoid, synconoid, and leuconoid.

Body Covering: Exterior of the animal covered with epidermis-like cells called pinacoderm (strictly they are not epidermis because they lack a basement membrane). Inner portion of animal porous with tubes lined with choanocytes.

Support: Support by spicules of calcium carbonate or silica. Sometimes with an organic matrix.

Digestive System: Animal is a filter-feeder. Particles are trapped and taken into choanocytes.

Circulatory System: None.

Locomotion: Adults are sessile. Some have larvae that move by ciliated epithelium.

Excretory System: None.

Nervous System: None.

Endocrine System: None.

B. Reproduction:

Reproductive System: No special organs. Eggs and sperm are produced. Asexual reproduction occurs; freshwater forms produce over wintering asexual structures called gemmules.

Development: Some with a planktonic larva that resembles a blastula.

C. Ecology: Found mainly in marine environments. One freshwater group.

SYSTEMATICS OF THE PORIFERA

Classic taxonomic systems like Brusca and Brusca (2003) and Bergquist (2001) divide the sponges into three major groups: silicaceous sponges, calcareous sponges, and sponges with an organic spongin matrix. Borchiellinni et al. (2004) and Adl et al. (2005) place the silicaceous and spongin taxa together in the same major group. This seems reasonable on structural grounds because those with a spongin matrix usually have silicaceous spicules, too (see D above). Other work (summarized by Ruppert et al., 2004) suggests that the glass sponges only superficially resemble the true sponges. The main difference lies in the cellular nature of the animal body: glass sponges form syncytial tissues (multicellular without separation by cell membrane) while the other sponges retain the cellular structure. Thus, Leys (1995) and Reiswig and Mackie (1983) suggested that the glass sponges should be separated from the other sponges at the level of subphylum or higher (Subphylum Symplasma for the glass sponges and Subphylum Cellularia for all of the others). More recently, Leys (2003) presented observations of the development of the syncytium that support a strong relationship between the glass sponges and the other sponges and that the syncytium is a derived trait. Thus, the silicon-metabolizing sponges (Hexactinellida and Demospongiae) may be sister groups and the calcareous sponges may be so different as to be paraphyletic (Rokas et al., 3002). Thus, until strong evidence for any of the hypotheses is presented, I will give the following taxa without apparant sister-group relationship.

HIERARCHICAL CLASSIFICATION OF THE PORIFERA

Taxonomy of the Phylum after the systems of Brusca and Brusca (2003) and Bergquist (2001). Descriptions of the following taxa were taken from: Barnes (1984), Bergquist (2001), Brusca and Brusca (2003), Hickman (1973), Storer and Usinger (1965), and Tudge (2000).

CLASS HEXACTINELLIDA (ALSO CALLED SYMPLASMA; 4 ORDERS DISTRIBUTED IN 2 SUBCLASSES)

Skeleton of silicaceous spicules; usually 6-rayed. No epidermis but covered by syncytium of amoebocytes. Choanocytes in finger-like chambers. Spicules large (megascleres) and small (microscleres). >400 extant species.

SUBCLASS HEXASTEROPHORA

Microscleres 6-rayed (hexasters), megascleres fused into a skeleton.

Aphrocallistes, Caulophacus, Euplectella, Hexactinella, Leptophragmella, Lophocalyx, Rosella, Sympagella.

SUBCLASS AMPHIDISCOPHORA

Microscleres 2-rayed, megascleres never fused into a skeleton.

Hyalonema, Monorhaphis, Pheronema.

CLASS CALCAREA (7 ORDERS DISTRIBUTED IN 2 SUBCLASSES)

Spicules of calcite or aragonite, free or fused. Spicules variable, megascleric, but not of two distinct size classes. Epidermis of pinacocytes. >500 extant species.

SUBCLASS CALCINEA

3-rayed spicules usually not fused into a skeleton. Larva forms a flagellated ball of cells (hollow or solid).

Clathrina, Dendya, Leucascus, Leucetta, Murrayona, Soleniscus.

SUBCLASS CALCARONEA

Spicules may be fused or not. Larva forms hollow ball of cells, about half of which are flagellated.

Amphoriscus, Grantia, Leucilla, Leucoselenia, Petrobiona, Scypha (Sycon).

CLASS DEMOSPONGIAE (15 ORDERS DISTRIBUTED IN 3 SUBCLASSES)

Skeleton of spongin; sometimes with silicaceous spicules in megasclere and microsclere size classes; some genera without skeleton. Epidermis of pinacocytes. Important as source of commercial sponges. >8,000 extant species.

SUBCLASS HOMOSCLEROMORPHA

Spicules numerous and very small. Not differentiated into mega and microscleres.

Corticium, Oscarella, Plakina, Plakortis, Pseudocorticium.

SUBCLASS TETRACTINOMORPHA

Very large group. Distinct differences between mega and microscleres. Megascleres organized into distinct patterns.

Acanthochaetes, Asteropus, Chondrilla, Chondrosia, Cliona, Cryptotethya, Geodia, Merlia, Polymastia, Rhabdermia, Stelletta, Superites, Tethya, Tetilla.

SUBCLASS CERACTINOMORPHA

Very large group. Distinct micro and megascleres, but megascleres not organized into distinct patterns. Spongin present in most.

Adocia, Agelas, Aplysilla, Aplysina (Verongia), Asbestopluma, Astrosclera, Axinella, Axociella, Calcifibrospongia, Callyspongia, Ceratoporella, Clathria, Coelosphaera, Goreauiella, Halichondria, Haliclona, Halisarca, Hispidopetra, Hymeniacidon, Ircinia, Lissodendoryx, Microciona, Mycale, Myxilla, Spongia, Spongilla, Stromatospongia, Tedania, Valceletia.

This page is maintained by Jack R. Holt. Last modified: 01/05/08