In the Water: Sponges

Giant barrel sponge

Xestospongia muta

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About

These incredible, cylindrical sponges can grow to be 2 m/6 feet in diameter and almost 2.5 m/8 feet in height, making them the largest sponges in the Caribbean. They also have incredibly long lifespans, contributing to their nickname, “redwoods of the reef.” Giant barrel sponges can be red, brown, and purple on the outside, but the inside of their body cavities are often tan-colored. While they don’t have spines or bones, they do have a “porifera skeleton” — a delicate, internal glass-like matrix that helps them to maintain their massive shapes.

Giant barrel sponges, much like other sponges, are passive filter feeders. As water enters their bodies through pores all along their sides, the sponge’s cells catch and consume the bacteria and plankton that are suspended in the water. The water circulates back out to the ocean through a large opening at the top of their bodies — the osculum — and the cycle repeats. These gigantic organisms and their filter-feeding habits are critical to keeping bacteria and algae populations in check.

Giant barrel sponges can live from hundreds to even thousands of years. They often inhabit waters around 9 m/30 feet deep, though they’ve been documented living at depths of nearly 120 m/400 feet. They’re able to produce both sperm and egg cells, and to reproduce they release both from their osculum to combine either in the water column, or in the bodies of other nearby sponges. The larvae travel ocean currents to new places, and those that survive find a place to settle, where they spend the rest of their lives growing until they reach their full size.

Just like reef-building corals, giant barrel sponges can experience bleaching events. Sponges sometimes have symbiotic organisms living within their tissues, and when water temperatures are too warm, the sponges may expel those symbionts. While bleaching is not always fatal for sponges, scientists continue studying the impacts of these stressful events on giant barrel sponges in hopes of better understanding how the sponges may be able to recover. The giant barrel sponge is one of the best-studied species of sponges in the ocean, but there’s still lots to learn about this magnificent invertebrate.

Header image by @zahnerphoto (CC-BY-NC).

This species is:
Native

Why that matters:
Native species are those that evolved in the region naturally, without human influence. That means they’re specifically adapted to Saba’s habitat, and play a key role in island biodiversity. When we lose native species, gaps appear in the ecosystem. That leads to cascades of additional extinctions, and to the loss of the ecosystem services (food, clean air and water, flood and coastal protections, and more) that we humans rely on.

Credit: @djscho, iNaturalist (CC-BY).

iNaturalist Observations

Giant barrel sponge

iNaturalist user @zahnerphoto (CC-BY-NC)

Species_giantbarrelsponge_obvs2_zahnerphoto.jpg.jpg

iNaturalist user @zahnerphoto (CC-BY-NC)

iNaturalist user @michaelbakkerpaiva (CC-BY)

Where locals, researchers, and visitors have seen this species.

Saba Island Map and Pins: iNat Taxonomy ID 130046

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Ask a Scientist

Question: What role do giant barrel sponges play in their ecosystem?

Answer: A mature giant barrel sponge can filter a massive volume of seawater every day, which contributes to the general health of a coral reef by straining nutrients and organic waste out of the water column, cycling floating nutrients back into the reef, and maintaining seawater clarity. Despite their massive size, these “redwoods of the reef” are very slow growing and can live for hundreds to even 2,000 or more years.