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Creature Comparison: Corals and Sponges

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

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.

A variety of soft octocorals were found on the East “Wetmore” Seamount near Hawaii at around 2,065 meters deep (6,775 feet). (Image courtesy of the NOAA Office of Ocean Exploration and Research, 2017 Laulima O Ka Moana.)
At almost 1,800 meters (5,095 feet) deep on Mendelssohn Seamount, scientists came across this dense garden of unusually large pink corals. (Image courtesy of the NOAA Office of Ocean Exploration and Research, Deep-Sea Symphony: Exploring the Musicians Seamounts.)

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.

This image highlights the general anatomy of the polyps that make up a single coral. This cross-section of a singular polyp is an example of an organism which feeds by catching microorganisms drifting through the water column around it. (Image courtesy of NOAA’s Coral Reef Conservation Program)
A sea spider was seen climbing on this colony of bamboo coral on the Mendelssohn Seamount at 1,675 meters (5,495 feet). Some of the coral’s polyps are extended into the water column, while others are retracted into their casings. (Image courtesy of the NOAA Office of Ocean Exploration and Research, Deep-Sea Symphony: Exploring the Musicians Seamounts.)

Sponges

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.

This concentrated glass sponge community was found at 2,359 meters (7,740 feet) underwater on the “Ridge” Seamount around Johnston Atoll off Hawaii. Some deep-sea corals were present, but at a much lower abundance. (Image courtesy of the NOAA Office of Ocean Exploration and Research, 2017 Laulima O Ka Moana.)
Spicules and fibers bind together to compose the shape of a sponge, in this case, a glass sponge. Glass sponges are not made of glass itself, but of silica, which is the same material glass is derived from. (Image courtesy of G.P. Schmal)

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.

Scientists found this bright yellow glass sponge at a depth of 2,479 meters (8,133 feet) on the Sibelius Seamount. (Image courtesy of the NOAA Office of Ocean Exploration and Research, Deep-Sea Symphony: Exploring the Musicians Seamounts.)
In 2016, 2,133 meters (7,000 feet) down in the Papahānaumokuākea Marine National Monument off of Hawaii, the Okeanos Explorer discovered the largest sponge on-record, measuring 2 meters (7 feet) long and 3 meters (12 feet) across. At this size, this new species was estimated to be over 2,300 years old. (Image courtesy of NOAA)

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!

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Sources:

NOAA Coral Reef Conservation Program

NOAA Fisheries Alaska Fisheries Science Center

NOAA United States Department of Commerce

Discovering the Deep: Exploring Remote Pacific MPAs

On March 25th, the NOAA Ship Okeanos Explorer wrapped up an exciting cruise to explore the depths of remote Pacific Marine Protected Areas (MPAs). As scientists collected data and made discoveries over the course of the expedition’s 19 dives, the remotely operated vehicles collected amazing images of life in the deep ocean. Continue reading Discovering the Deep: Exploring Remote Pacific MPAs