Summary
Highlights
Sponges, belonging to the phylum Porifera, represent some of the earliest animal life, emerging around 600 million years ago. Their name, meaning "pore bearers," aptly describes their bodies, which are covered in tiny holes called ostia. Unlike most animals, sponges are largely asymmetrical, lack organized tissues or organs like a mouth or anus, and instead have unique body shapes optimized for efficient water flow through a central cavity, where nutrients are filtered and water exits via an osculum.
A sponge's body consists of two thin cell layers with a collagen-rich mesohyl in between. The outer layer is made of pinacocytes, which protect the sponge and can digest organic material, and also anchor it. The most crucial cells for feeding are choanocytes, which possess flagella. Their whip-like motion creates a current that draws water and tiny organic matter into the sponge, making them filter feeders. Other cells in the mesohyl include porocytes that control pores, sclerocytes or spongocytes that excrete spicules for structural support and predator deterrence, and various mobile cells like oocytes, spermatocytes, grey cells (immune-like), amoebocytes (totipotent), and myocytes (signal conduction).
Lacking a true digestive system, almost every sponge cell consumes organic matter. They are primarily filter feeders, consuming phytoplankton and detritus from the water. However, some species, like the ping-pong tree sponge, are carnivorous, using hooked spicules to capture and digest small crustaceans. Many sponges are sessile as adults. Interestingly, some sponges host photosynthesizing endosymbionts, allowing them to produce more food and oxygen than they consume, similar to plants, although they don't produce these symbionts themselves.
Sponges primarily reproduce sexually by releasing sperm into the water, with eggs either released or retained for internal fertilization. Most sponges are hermaphrodites but lack gonads, producing reproductive cells from individual cells. Fertilized eggs develop into flagellated larvae that are mobile and can even have light-sensitive "pigment ring eyes" to guide their locomotion before they settle and grow into adults. Asexual reproduction occurs through budding (less common), fragmentation (more common, requiring totipotent amoebocytes), and gemmules. Gemmules are dormant survival pods of unspecialized cells produced in harsh conditions, which can later form new sponges.
There are three main aquiferous systems or body structures in sponges: asconoid (simple tube), syconoid (pleated body wall with inner pockets), and leuconoid (complex network of choanocyte-lined chambers, the most common type). Less common are sylleibid and solenoid structures. The four extant taxonomic classes are Demospongia, Calcarea, Hexactinellida, and Homoscleromorpha. Demospongiae are the largest and most diverse class, soft-bodied, leuconoid, and have silica spicules. Calcarea are smaller, come in all three main body structures, and have calcium carbonate spicules. Hexactinellida, or "glass sponges," are deep-sea leuconoids known for rapid electrical impulse conduction. Homoscleromorpha are typically massive or encrusting leuconoid or sylleibid sponges with little spicule variation and are phylogenetically distinct from Demospongiae.