Sponges, or sea sponges, are primarily marine invertebrates that constitute the phylum Porifera, and are thus also referred to as poriferans. The name Porifera derives from the words porus, meaning ‘pore,’ and -fer, meaning ‘bearing or carrying.’ This etymology reflects the numerous tiny pores or openings that characterize members of this phylum.
They represent some of the oldest known multicellular animals. Most sponges are asymmetrical and lack true tissues and organs, though a few, such as those in the genus Leucosolenia, exhibit radial symmetry. They are acoelomates, lacking a true body cavity or coelom.
A defining feature of sponges is their water canal system, which allows them to carry out essential functions such as feeding, respiration, and excretion. As water enters through the pores, specialized cells trap suspended food particles, especially bacteria, and pull them in for intracellular digestion.
Their bodies consist of numerous undifferentiated cells that can transform into other types, exhibiting totipotency.
Most sponge species range from under 0.5 in (12.7 mm) to about 12 ft (3.6 m) in length. However, according to Guinness World Records, the smallest species, Leucosolenia blanca, is just 0.11 in (3 mm) tall. In contrast, one of the largest species, the giant barrel sponge (Xestospongia muta), is at least 3 ft (0.9 m) tall and reaches a diameter of 6 ft (1.8 m).[1]
A sponge’s body comprises 3 functional layers: the outer pinacoderm, the middle mesohyl, and the inner choanoderm.
Since sponges depend on a highly specialized water canal system, they possess numerous pores and canals that regulate the inflow and outflow of water. The surface of the body contains many tiny incoming pores called ostia, each controlled by valve-like porocyte cells. These cylindrical cells penetrate the pinacoderm and contain a canal-like passage that allows water to enter the incurrent canals. From there, water moves into the choanocyte chamber, where flagellar beating drives it toward the excurrent canals. These canals finally direct the water to the central exit pore, the osculum.
The mesohyl functions as the endoskeleton, comprising spicules and spongin fibers.
The spicules are either composed of silica (in most members of classes Demospongiae and Hexactinellida) or calcium carbonate (members of class Calcarea). These structures may exist in different forms, ranging from rods to shapes like stars with up to six rays.
One particular group of sponges, the sclerosponges, produces a hard exoskeleton of limestone beneath their living tissue, a feature unique among members of Porifera.
Depending on the complexity of the arrangement of the water canal system, sponges are classified into 3 body types: asconoid, syconoid, and leuconoid.
These animals lack distinct circulatory, respiratory, digestive, and excretory systems. It is their water canal system that encompasses the role of each organ system.
They absorb oxygen directly from the surrounding water through diffusion as it flows into their bodies. In the same way, carbon dioxide and soluble wastes such as ammonia diffuse out into the excurrent water.
Food particles are trapped by the ostia and the cellular components of their bodies. Larger particles, over 50 μm, are too big to enter the ostia and are instead engulfed and digested by pinacocytes (phagocytosis). Particles between 0.5 and 50 μm become trapped in the ostia, where they are consumed either by pinacocytes or by archaeocytes that extend through the ostial walls to capture them. The smallest particles, mainly bacteria less than 0.5 μm, pass through the ostia and are directly captured by choanocytes. Since the most minute particles dominate the water column, choanocytes are estimated to supply about 80% of the sponge’s total nutritional intake.
Although sponges lack true neurons, studies suggest that they possess the genetic toolkit for building neural synapses. A study on the sponge Amphimedon queenslandica revealed that the species has most of the genes that construct the post-synaptic density (PSD) in higher animals. These genes are expressed in a specific larval cell type called the flask cell, hinting that sponges likely represent an early evolutionary step toward the first true neurons.[2]
Around 9,000 species of sponges have been identified and classified into 4 classes, depending on the composition of their skeletons.
Around 400 species are glass sponges, and about 500 are calcareous, while all remaining species belong to the demosponges. Members of every class except Calcarea possess silica-based spicules, whereas calcareous sponges have spicules made of calcium carbonate. Moreover, members of Homoscleromorpha and Hexactinellida lack the spongin fibers that are present in the other two classes.
Sponges have a cosmopolitan distribution, found in all oceans, ranging from shallow coastal waters to depths greater than 8,800 m (5.5 mi). They typically live in quiet, clear waters, since sediment clogs the pores of their bodies. Their highest diversity is found in the tropics, but they are more abundant in the temperate regions.
Glass sponges are commonly found at considerable depths of cold, polar waters, whereas demosponges and calcareous sponges are found in shallower, non-polar waters.
Unlike most sponges, members of the family Spongillidae thrive in freshwater lakes and rivers.[3]
These animals filter bacteria, dissolved organic matter, viruses, and detritus for nutrition.
Some sponge species, particularly those belonging to the family Cladorhizidae (and a few from Guitarridae and Esperiopsidae), are carnivorous. They feed on small crustaceans, such as amphipods and copepods, as well as other tiny aquatic invertebrates. This carnivorous diet has evolved in response to the scarcity of suspended food particles in their deep-sea habitats, as well as the reduction or complete loss of functional filtering structures.[4]
The adults of most sponges are sessile, though a few members of the genus Hymeniacidon are capable of moving across the seabed at speeds of 0.039 to 0.157 (1 to 4 mm) per day.[5]
Their larvae and juveniles, however, are free-swimming.
They rely on several defensive strategies to ward off their predators. For instance, many species shed large numbers of spicules, creating a dense, needle-like carpet around them that can extend for meters, keeping predators away. Additionally, a few, such as the brown tube sponge (Agelas conifera), produce compounds, like ageliferin, which protect against harmful microorganisms.[6]
Many freshwater sponges harbor photosynthetic endosymbionts, particularly bacteria and dinoflagellates, in their archaeocytes and other body cells. These symbionts perform photosynthesis, producing glucose and other organic molecules, which are stored and utilized as chemical energy within the body of the sponge.
Typically, sponges survive around 10 to 20 years. However, a few species have been estimated to live way longer. For example, according to the Guinness World Records, the longest-lived species of sponge, the giant volcano sponge (Anoxycalyx joubini), is estimated to survive around 15,000 years, given its exceedingly slow growth rate in the Antarctic waters.
Sponges reproduce sexually during favorable seasonal conditions, typically annually, but shift to asexual modes during the rest of the year.
Most sponges are hermaphroditic, with a single individual producing both sperm and eggs, though the timing of gamete release varies to prevent self-fertilization.
Some chaonocytes transform into sperm-producing cells and release sperm into the surrounding water through the osculum. Driven by water currents, the sperm travel to neighboring sponges. The receiving sponge captures the incoming sperm using choanocytes, just like it captures food. The choanocyte that captures the sperm transforms into an amoeboid carrier cell, which moves through the mesohyl and delivers the sperm to an egg cell (archaeocytes modified to produce an ovum). Eventually, the gametes fertilize in the mesohyl.
The fertilized egg, typically retained within the body, hatches into a free-swimming, non-feeding (lecithotrophic) larva, typically within 5 days.
Sponges produce different kinds of larvae because embryonic development differs among the various groups. For example, calcareous sponges (and a few demosponges) have a hollow oval larva, the amphiblastula, with two distinct halves: a front half of cylindrical, flagellated cells used for swimming and a rear half of non-flagellated, rounded cells. Similarly, most demosponges have parenchymella, a compact larva with a flagellated outer layer and an inner mass of non-flagellated cells. Other larval forms include the trichimella of glass sponges, characterized by an elongated, ciliated body with an apical tuft of long cilia and a syncytial cellular organization.[7]
The larva exits the parent’s body, swims for a few hours to 3 days (depending on the species), and finally settles on a substrate. After attachment, the larva metamorphoses into a young sponge, losing its flagella in the process.
All sponges are capable of reproducing asexually, especially after an injury or when conditions in the surroundings become harsh. Depending on the species, they undergo three types of asexual reproduction: fragmentation, budding, and gemmule formation.
Sponges form the primary dietary component of various spongivorous animals. For example, the hawksbill sea turtle specializes in sponges. Similarly, several fish living in reefs, such as the emperor angelfish (Pomacanthus imperator), the spotted trunkfish (Lactophrys bicaudalis), and the planehead filefish (Stephanolepis hispidus), also feed on sponges.[10]
Some species of nudibranchs feed on sponges. For instance, the species Hypselodoris purpureomaculosa most likely specializes in consuming sponges of the genus Euryspongia.[11]
Spongeflies (family Sisyridae) parasitize freshwater sponges, particularly those of the genus Spongilla.[12]
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