Sponges (phylum Porifera) are ancient and unique multicellular animals. These aquatic organisms, often resembling plants or inanimate structures, have a simple body plan yet exhibit remarkable diversity. Their ability to filter large volumes of water and their varied forms have long fascinated scientists. The sheer number of species within this group reflects their widespread presence and ongoing scientific discovery.
The Estimated Number
The estimated number of known Porifera species is constantly evolving as new discoveries are made and classification methods improve. Currently, approximately 8,500 to 9,000 formally described species are recognized. While over 11,000 species have been described, around 9,000 are considered valid. Most species inhabit marine environments, but a notable number, around 150 to 400, are found in freshwater.
What Defines a Poriferan?
Poriferans are distinct from other animal groups due to several fundamental characteristics. They are sessile organisms, meaning adults remain attached to a surface and do not move freely. Sponges filter-feed by pulling water through numerous tiny pores (ostia) on their body surface. This water circulates through internal canals and chambers, where specialized cells called choanocytes capture food particles before the water exits through larger openings called oscula. Unlike most other animals, sponges lack true tissues and organs, instead exhibiting a cellular level of organization where different cell types perform specific functions. Their body structure is often supported by a skeleton composed of mineral spicules, protein fibers called spongin, or both.
Why the Number Changes
The variability in Porifera species counts stems from several factors. New species are continually discovered, especially in underexplored environments like the deep sea or cryptic habitats. Many species exhibit morphological plasticity, meaning individuals can look very different depending on their environment, making identification challenging. Taxonomic revisions frequently occur as new genetic and molecular data clarify relationships.
This often reveals “cryptic species,” which are distinct but morphologically indistinguishable. Molecular markers, like DNA barcoding, are instrumental in distinguishing these hidden species.
The difficulty in sampling certain habitats and the labor-intensive nature of sponge taxonomy also contribute to the dynamic count. The true diversity of sponges is likely underestimated, with total species potentially much higher than currently described.
Major Divisions of Sponges
The phylum Porifera is categorized into four main classes, distinguished by their skeletal elements.
Demospongiae
This is the largest and most diverse group, encompassing over 90% of all known sponge species. They possess skeletons made of siliceous spicules, spongin fibers, or both, and include all freshwater species.
Calcarea
Also known as calcareous sponges, these are characterized by spicules composed of calcium carbonate. They are generally smaller and predominantly found in shallow marine waters.
Hexactinellida
Commonly known as glass sponges, they are distinguished by intricate, often fused, six-rayed siliceous spicules that create a glass-like framework. These sponges typically inhabit deeper or polar waters.
Homoscleromorpha
This class was recently recognized as a distinct group. These sponges have very small, simple siliceous spicules, or sometimes lack spicules entirely, and often have a compact or encrusting growth form.