Sea Sponges & threats to them

Published: 9th Dec, 2022

Context

Earlier this year, New Zealand recorded the largest-ever sponge bleaching event off its southern coastline.

Now, the latest research shows the most severe impacts on sponges occurred in areas where the marine heatwave was most intense.

What are sea sponges?

Sea sponges are among the most basic multicellular organisms on the planet. They come in a variety of sizes, colours, and textures.
Sea sponges feature a shell-like or glass exterior layer with small pores (ostia) that penetrate deep into the soft interior structure.
Water can flow in and out of the sponge through these pores. The sponge collects food and oxygen while also releasing waste which serves as food for other animals. These sponges also create tree-like, cup, and tube shapes, creating a forest-like structure. These so-called "sponge gardens." serve as shelter for a lot of other animals.
The approximately 8,550 living sponge species are scientifically classified in the phylum Porifera, which is comprised of four distinct classes:
- Demospongiae(the most diverse, containing 90 percent of all living sponges)
- Hexactinellida(the rare glass sponges)
- Calcarea(calcareous sponges)
- Homoscleromorpha(the rarest and simplest class, only recently recognized, with approximately 117 species)

Difference from Corals
Corals are complex, many-celled organisms	Sponges are very simple creatures with no tissues.
All corals require saltwater to survive	While most sponges are found in the ocean, numerous species are also found in fresh water and estuaries.

Important ecological functions played by Sponges

Filtering water: They filter large quantities of water, capturing small food particles.
Moving carbon: They help in moving carbon from the water column to the seafloor where it can be eaten by bottom-dwelling invertebrates.
Maintaining food chain: These invertebrates in turn are consumed by organisms further up the food chain, including commercially and culturally important fish species.
Habitat for marine species: Sponges also add three-dimensional complexity to the sea floor, which provides habitat for a range of other species such as crabs, shrimps and starfish.

In New Zealand, they occupy up to 70% of the available seafloor, particularly in so-called mesophotic ecosystems at depths of 30-150m.

What causes bleaching?

The sea sponges have tiny organisms that capture sunlight and make food through photosynthesis. The sponges depend on this food for survival.
When water temperature rises, much like corals, these sponges push out these photosynthesizing organisms.
In addition to food, these organisms are also a source of their colour. Hence, when sponges push them out, the sponges lose their colour (and turn white) as well as their source of food.

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