Despite being frequently encountered by scientists aboard the NOAA Ship Okeanos Explorer, E/V Nautilus, and other exploration vessels, much is left to learn about corals and sponges. Both are sessile (non-moving) organisms, serve as vital resources for other marine life, and can indicate the health of oceanic ecosystems. Learn more about these fascinating animals below!
Corals exhibit some plant-like characteristics, but are actually animal relatives of jellyfish and anemones. They are all within the phylum Cnidaria. All corals are classified as either “hard corals” or “soft corals”. Hard corals have a limestone skeleton, and make up the foundation of a coral reef. They can take a rounded, branching, or flat appearance. Soft corals bind together on a softer structure, and can take the shapes of whips, spirals, and trees. Hard corals can grow as much as ten centimeters per year, the same rate of growth as human hair, but most only grow up to three centimeters each year. Soft corals grow at a rate of two to four centimeters per year. When a coral reef is damaged by a storm, pollution, or by other factors, it may take a significant amount of time before it is able to recover and grow to its former size.
All corals are comprised of polyp colonies. Many reef-building corals with access to sunlight also share a symbiotic relationship with a single-celled algae called zooxanthellae. The zooxanthellae live within the polyps and produce organic material, most of which is transferred back to the coral tissue (providing energy to the coral). In return, the waste produced by the coral feeds the zooxanthellae, and the coral’s structure also provides them with shelter. Some corals feed directly via their polyps, using their tentacles to extend out and grab prey (typically microscopic zooplankton) from the water column. To reproduce, groups of corals may simultaneously spawn gametes into the water column; larvae drift and develop until they find a place to settle. Other corals are able to bud off polyps to begin new colonies, or even regrow from a severed branch.
In contrast to corals, sponges are the simplest multicellular animal, and one of the most ancient animals on Earth. They dominate the phylum Porifera, and have existed for over 500 million years. Most sponges typically have a skeleton of limestone, silica, or collagen. These skeletons are made up of small, fiber-like spicules which often aid scientists in identifying species. Unlike most animals, sponges are asymmetrical, and this varied shape optimizes water flow through passages in their bodies. Some sponges are even stalked, with their bodies elevated above the seafloor. This is more often the case for deep-sea sponges, rather than those in shallower, more turbulent waters.
The small currents created by a sponge’s body allows it to draw in plankton and other organic material from the surrounding waters. These materials then get caught inside the sponge’s fibrous body and are digested. Sponges can even be carnivorous, using hook-like protrusions to capture prey and secrete enzymes to break down the nutrients. The growth of sponges is highly dependent on the amount and quality of available nutrients, although they generally grow at a higher rate than corals. To reproduce, some sponges produce larvae which develop within their bodies. When ready to reproduce, the larvae will exit the sponge to drift for a short period before anchoring themselves to the seafloor substrate. Some sponges are even able to release chemicals that inhibit the growth of other sponge cells in their vicinity. Since sponges are sessile organisms, this strategy allows them to prevent space and food competition with other sponges settling in the same area. Similar to coral, severed portions of a sponge may also be able to regrow if they are reattached to the substrate.
Corals and sponges can each be precious resources for humans. Precious corals are vital to national economies all over the world, as they are harvested for fertilizer, consumption, and even jewelry. While sponges are also utilized for some of the same purposes, some scientists believe their potential can be expanded to aid in the fight against cancer. The composition of the growth-prohibiting chemicals some sponges secrete may lead scientists to a breakthrough in cancer research, since these sponges are able to kill other cells of the same species without harming their own cells. These innovations are just a few reasons to continue investigating these amazing creatures!