Loss, decay and bleaching: Why sponges may be the ‘canary in the coal mine’ for impacts of marine heatwaves

Subject :Environment

Context-

Marine sponges were thought to be more resilient to ocean warming than other organisms. But earlier this year, New Zealand recorded the largest-ever sponge bleaching event off its southern coastline.

About sponges-

Like corals, sponges contain symbiotic organisms.
Cymbastella lamellata is unusual in that it hosts dense populations of diatoms, small single-celled photosynthetic plants that give the sponge its brown colour.
These diatoms live within the sponge tissue, exchanging food for protection.
When the sponge bleaches, it expels the diatoms, leaving the sponge skeleton exposed.
Tissue loss occurs when sponges are stressed and either have to invest more energy into cell repair or when their food source is depleted and they reabsorb their own tissue to reduce the body volume and reallocate resources.
Tissue decay or necrosis is generally associated with changes in the microbial communities living within sponges and the growth of pathogenic bacteria.

Significance of sponges-

Sponges are among the most ancient and abundant animals on rocky reefs across the world.
They filter large quantities of water, capturing small food particles and moving carbon from the water column to the seafloor where it can be eaten by bottom-dwelling invertebrates.
These invertebrates are consumed by organisms further up the food chain.
Provides habitat for a range of other species such as crabs, shrimps and starfish.

Recent event of Sponge bleaching-

Observed on the southern coastline of the Newzealand, while the bleaching is not being observed in central areas of New Zealand’s coastline.
One species viz the cup sponge Cymbastella lamellata was affected due to a prolonged marine heatwave.
A marine heatwave is defined as an event when seawater temperatures exceed a seasonally-varying threshold (usually the 90th percentile) for at least 5 consecutive days. Successive heatwaves with gaps of 2 days or less are considered part of the same event.