Sponges

Types of Sponges

There are currently around 5,000 known sponge species. ^[1] These include four main classes, glass sponges, calcareous sponges, demosponges, and homoscleromorpha.

Demosponges

Approximately 90% of all sponge species are in this class.

Often brightly colored, unlike the dull glass and calcareous sponges.^[2]

These can grow quite large, to over 2 meters in height.^[3]

Varied skeletal structure. Can be made up of spongin fibers or spicules of silica or calcium carbonate, or any combination of these. ^[4]

The common household sponge is a dried demosponge.^[5]

Calcareous sponges

Calcareous sponges can be found living on coral reefs in the shallow waters of tropical regions.

Scientists have identified around 400 species.

Their skeletal structure is made up of large spicules of calcium carbonate.^[6]

Glass Sponges

Glass sponges are not found in coral reefs. They exist mainly in deep Arctic waters.^[7]

Composed of silica, the same chemical compound that makes up glass.

Their spicules are fused in large complicated patterns. ^[8]

Homoscleromorpha

The class Homoscleromorpha is the most recent class identified. It used to be a part of the Demosponges.

There are less than 100 known species in this class.

Usually found in shallow waters.

Unlike the other classes, spicules are formed in both the sclerocytes and the epithelial cells and do not form a distinct pattern or structure.^[9]

Biology of Sponges

Sponges can be found in many different shapes and sizes. One distinction is between encrusting and free-standing sponges. An encrusting sponge is one the conforms to a hard surface. Free-standing sponges include the tube, vase, and barrel varieties. These are sponges that have an inner space with a larger volume than the amount of surface area on the outside.^[10]

Microbiology

Cell Layers

Sponges are made up of three layers.

The outside layer consists of pinacocytes.

The inside layer consists of choanocytes, or collar cells. These cells have flagellum that move water containing oxygen and nutrients throughout the sponge.

The space between these layers is called the mesenchyme. This is a layer containing the skeleton of the sponge and some cells.^[11]

Ostia and Osculum

Water flow is essential to the functioning of a sponge as they receive water and nutrients from this continual flow. Water comes into the sponge through ostia, small pores in the surface of the sponge. The water is moved through a series of canals by the flagellum of choanocytes, and then it exits through seperate pores, called oscula.^[12]

Skeleton

The skeleton of the sponge can be made up of a combination of either spongin fibers (collagen threads) , collagen filaments (complex protein bundles), or spicules.

Spicules can be composed of either calcium carbonate or silicon dioxide and are produced in sponge cells called sclerocytes.^[13]

Nervous System

The sponge has no nervous system but it does have genes that are comparable to those in human neurons.Researchers have discovered around 25 sponge genes that are comparable to those in human neurons and have observed protein behaviors that mimic interactions of proteins at human nerve synapses.^[14]

The life of a sponge

Respiration

Sponges gain oxygen from the water through diffusion. The water enters through small pores called ostia and then is distributed throughout the sponge from areas of high concentration to areas of low concentration. The water flows throughout the sponge with the help of choanocytes, cells with flagella for movement. This simple process works without a more complex respiratory system because all cells of a sponge are within close proximity of exchange sites. ^[15]

A study of the Red Sea coral reef sponge Negombata magnifica reveals the distribution of oxygen use. In this sponge, about 25.1% of oxygen was used for water pumping. Water pumping and maintenance together accounted for about 74% of all oxygen usage. The remaining 26% was used for other processes, mainly growth. This suggests that oxygen usage could contribute to controlling growth of sponges on the coral reef. ^[16]

Movement

Sponges are completely sessile past their first few days after fertilization. Once the sponge is attached it does not move from that surface. ^[17]

Reproduction

Sponges are hermaphroditic though they usually only release sperm or eggs at one time. Sperm is released and travels through the water until it lands within the interior of a sponge with eggs. The eggs are fertilized and then spend a short amount of time in the water until anchoring to a hard surface and beginning growth. ^[18]

Sponges can also reproduce asexually. This process is characterized by the creation of gemmules, tiny cells that branch off of the sponge and have the potential to form a genetically identical sponge. More complex gemmules, also contain an outer layer containing spicules that protects the cells and nutrients contained on the inside. This equips these cells to survive harsh conditions and only develop when the period of disturbance has passed. ^[19]

Feeding/Diet

Sponges gain basically all of their nutrients and sustenance from the water filtration process outlined in the respiration section.

They digest microorganisms and detritus in the water.

Any clogged ostia are cleaned by Archaeocytes.

Habitat

Sponges can survive in almost any habitat imaginable, due largely to their simple design and rudimentary requirements for subsistence, but they occur in the greatest proliferation on coral reefs.

Glass sponges rarely appear in reefs.

Because they are sessile, they have to compete for growing space. Because most competitors in this field are photosynthetic, sponges can often be found in shaded outcroppings of rock.

Most have to remain moored to a solid bottom, but some can moor themselves on soft bottoms (sand, etc) via a thick root system (looks a lot like a tap root in plants).

Defense

Sponges have a basic immune system that prevents cell movement in infected areas, and eventually kills all cells in the area by excreting toxin.

Sponges are very good at competing for space, which makes sense since they can't actually move. Some shed their spicules, making it hard for predators to approach them (think surrounding your tent with caltrops/upright nails/legos every night before going to sleep in the woods). They also produce toxins that make life difficult for other nearby sessile organisms.

Interactions

Microbes such as bacteria, archaea, microalgae, and fungi live in close contact with sponges. They can make up around 40% of a sponge's volume and interact with the sponge in a variety of ways.^[20]

Ecological Interaction of Sponges

Sponge filtration is actually a vital part of the coral ecosystem, as many sponges can cycle great amounts of nutrients back into the ecosystem very quickly after fish, corals, etc, die and degrade (normally disintegrated resources/floating detritivores might be carried away by the current, or any number of other things).

This is part of why Darwin's paradox is possible.

Many sponges form symbiotic relationships with photosynthetic algae. These sponges are great sources of Oxygen for the reef.

Common predators include nudibranchs, sea turtles, Wrasses, and parrotfish.

Human interactions

Humans used to harvest sponges for a variety of uses, from paint pigments to modern commercial sponge sales, but today most sponge-based products actually use synthetic sponges.

Reef sponges are often damaged by fishermen using weighted nets or even simply taut fishing lines. They are also often detached by storms.

Sponges are being affected by Climate change just like the rest of the reef.

Sponge pathogens seem to be on the rise. The largest sponges are the most vulnerable to these, since they filter the most water and most pathogens are caught by water filtration. This is especially notable because the largest sponges in a habitat are usually providing habitats for other creatures.