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 Figures 1 and 2).  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 Figure 3).  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).

The economically important sponges are in the group Demospongiae.  These have some silicaceous spicules but the bulk of the animal's "skeleton" of an elastic organic substance called spongin.  The porous, elastic nature of these sponges made them almost necessary as everyday devices for washing almost anything until the advent of the artificial sponge.  The demosponges are the most speciose of the three classes, and a few of them live in freshwater where they frequently form tan films over rocks and other hard substrates.  In some of the local streams in central Pennsylvania freshwater sponges like Spongilla can be so abundant that they can give a characteristic odor to the stream.  As the water cools through the fall, Spongilla begins to die back and form orange gemmules from which the next generation will emerge.

Glass sponges usually live at great depth and are formed by a highly ordered geometric pattern of spicules.  The delicate structure is covered by a syncytium of epidermis.  The calcareous sponges make spicules solely of calcium carbonate.  Thus, some of them make structures that resemble corals.  However, most are small and somewhat soft.

Most sponges can reproduce sexually in which case they usually make planktonic larva, which develops from a ball of cells, the blastula.  In general the sponge larvae are organized beyond the cellular grade of organization.  Thus, Nielsen (2008) suggests that the rest of the animal kingdom (the tissue grade) evolved from sponge larvae which lost the adult phase.

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. (2003) 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 Figure 4).  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. 2003).  

The analysis of Borchiellini et al. (2001) based on 18S rDNA sequences (see Figure 5) shows the Hexactinellida (H) as the sister group to all other metazoa.  Then, the Demospongiae is nested with the Calcarea + Eumetazoa (Placozoa-Cnidaria-Bilateria) clade.  A similar relationship is illustrated by Sperling et al. (2007, Figure 6).  Thus, Porifera is a grade of structure and not a monophyletic group.  Sperling et al. (2007) used the information to infer that the earliest form of feeding was by microphagy (filter-feeding by means of a water-canal system) and macrophagy (pulling larger objects through a gut) was derived.  






FIGURE 5. A figure from Borchiellini et al. (2001) based on 18S rDNA sequences.  H (the glass sponges) are sisters to the rest of the metazoa. The rest of the sponges are paraphyletic.

F = Fungi

C = Choanozoa

H = Hexactinellida

D = Demospongiae 

Ca = Calcarea

Cn = Cnidaria

Pl = Placozoa

Ct = Ctenophora


FIGURE 6. A figure from Sperling et al. (2007).  The classes of sponges are nested as paraphyletic groups.




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By Jack R. Holt and Carlos A. Iudica.  Last revised: 09/22/2015