THE SPONGES
The phylum Porifera (from the Latin porus: pore and ferre: to bear) comprises a group of animals (metazoans) widely regarded by scientists as the oldest still extant on Earth — the sponges. These organisms are sessile filter-feeders, characterized by a simple body structure that lacks true organs or tissues (except for the class Homoscleromorpha). Sponges possess a system of inhalant and exhalant channels and chambers, the aquiferous system, where specialized flagellated cells called choanocytes create a unidirectional water flow through their bodies. This water current is essential for their feeding, gas exchange, and waste removal processes.
Diversity of sponges in a cave, Fernando de Noronha.
Photo: Matheus Vieira Lopes.
Anatomy of Porifera. Modified from Fransozo & Negreiros-Fransozo (2016).
In addition to choanocytes, sponges have highly mobile and totipotent cells capable of differentiating into any cell type, including gametes, which provides them with remarkable morphological plasticity and reorganization capacity. These traits enable sponges to fragment and fuse their bodies, retract in size under adverse conditions, regenerate lost parts with impressive efficiency, reaggregate and reorganize their cells to form new individuals after cellular dissociation, and even move around a little through the amoeboid activity of cells in their adhesion areas. When present, their skeleton can consist of free or fused inorganic spicules made of calcium carbonate (CaCO3) or silica (SiO2). In the class Demospongiae, siliceous spicules may be supplemented or replaced by collagen fibers (spongin).
Diversity of siliceous spicules. Retrieved from Van Soest et al. (2012).
The phylum is currently divided into four extant classes: Calcarea, Demospongiae, Hexactinellida, and Homoscleromorpha, comprising over 8,700 valid species, though it is estimated that at least twice this number of species exist. Sponges inhabit a wide range of environments across all latitudes, including coral reefs, rocky shores, caves, abyssal and hadal zones, mangroves, estuaries, and freshwater habitats, such as high-altitude lakes. They exhibit diverse shapes and sizes, including encrusting, branched, tubular, and spherical forms, ranging from just a few millimeters to several meters in size. Depending on the nature and density of their structural components (spicules and/or spongin fibers), sponges can have a soft, compressible, fragile, friable, or hard consistency.
Matheus Vieira Lopes
Class Demospongiae
Matheus Vieira Lopes
Class Homoscleromorpha
NOAA Okeanos
Class Hexactinellida
Bárbara Ribeiro
Class Calcarea
THE FOUR CLASSES
DEMOSPONGIAE
Demospongiae is the most diverse class within the phylum Porifera, accounting for about 83% of all known sponge species. Demosponges inhabit both marine and freshwater environments, ranging from the intertidal zone to hadal depths. Some genera, such as Chondrosia Nardo, 1833, Halisarca Johnston, 1842, and Hexadella Topsent, 1896, lack a skeleton. When present, their skeletons may consist of siliceous spicules, which can be complemented or replaced by collagen fibers. Additionally, there are hypercalcified species (coralline sponges) that possess a basal skeleton of calcium carbonate associated with siliceous spicules. Demosponges have a leuconoid aquiferous system and exhibit diverse reproductive modes, including oviparous and viviparous.
HEXACTINELLIDA
Representatives of the class Hexactinellida, known as glass sponges, make up about 7% of the phylum Porifera. They are exclusively marine and predominantly found in deep waters (> 500 m), with occurrences in shallow waters recorded in only four locations: Antarctica, southern New Zealand, underwater caves in the Mediterranean, and the coastal waters of the North Pacific. Glass sponges exhibit variable body shapes and subtle coloration, with a skeleton composed of siliceous spicules, most of which have six rays (hexactinal symmetry) and can be fused by secondary silica deposition. Their entire body is formed by a continuous syncytial tissue called the trabecular reticulum, characterized by interconnected fused cells, allowing the passage of food and electrical signals throughout the organism. Species within this class have either a leuconoid or syconoid aquiferous system and are viviparous.
HOMOSCLEROMORPHA
The class Homoscleromorpha is the smallest group within the phylum Porifera, representing around 1% of its diversity. Until recently, it was considered a subclass of Demospongiae, but Gazave and collaborators (2012), using molecular tools, demonstrated that Homoscleromorpha is, in fact, a distinct clade, representing the fourth class of the phylum Porifera. This class consists of two families: Oscarellidae and Plakinidae. Homoscleromorpha species are primarily marine and found mostly in shallow waters, with only a few species occurring below 100 m depth. Most of them have an encrusting or lumpy form with a smooth surface, and their coloration can be quite variable, even intraspecifically, as seen in the polychromatic Plakina kanaky Ruiz & Pérez, 2015, which has yellow, red, green and blue chromotypes. They possess a leuconoid or sylleibid aquiferous system, and their skeleton, when present, is composed of siliceous spicules. Homoscleromorpha is unique among sponges for having a basement membrane composed of type IV collagen, which underlies the choanoderm and pinacoderm. They are viviparous.
CALCAREA
The class Calcarea represents about 9% of sponge species biodiversity. Its representatives are exclusively marine, occurring mainly in shallow waters, although they are also found in deep waters. These sponges are particularly found in cryptic environments, such as caves, crevices, and even under other organisms and rocks. It is the only class with spicules made of calcium carbonate, deposited in the form of Mg-calcite, and the only class that has all six types of aquiferous systems described for the phylum Porifera (asconoid, syconoid, solenoid, sylleibid, leuconoid and kladonoid). This class also includes hypercalcified (coralline) species, with a basal skeleton of calcium carbonate associated with calcium carbonate spicules. Regarding reproduction, calcareous sponges are viviparous.
