18 April 2018

Tohoku tsunami impacted home-building habits of eco-engineer heart urchins

Posted by Nanci Bompey

By Brendan Bane

Heart urchins, also known as sea potatoes, build their homes along almost every coast. Though they are small, they are ecosystem engineers, digesting organic material along the seafloor and building burrows that other sea creatures call home.
Photo credit: Ecomare/Oscar Bos

Tsunamis bring destruction. The massive waves can rip homes from beachside perches and wipe coasts clean of life. But tsunamis also bring change in their wake and can influence the lives of creatures large and small.

In a new study published in the Journal of Geophysical Research: Oceans, a journal of the American Geophysical Union, researchers explore how tsunamis impact shallow marine environments, also known as benthic environments, and the small, burrowing animals that dwell there.

To investigate the ecological footprint of tsunamis in detail, lead researcher Koji Seike from the Geological Survey of Japan at the National Institute of Advanced Industrial Science and Technology in Japan, looked to one potato-sized creature, the heart urchin, Echinocardium cordatum, and how it was impacted by the 2011 Tohoku tsunami in Japan.

“They look like a small-sized mammal, like a hedgehog or a mouse,” Seike said, who also compared the invertebrates to manjū, a type of Japanese sweet bun.

Seike also refers to heart urchins as “ecosystem engineers” for their influential roles in benthic environments. As bioturbators — mixers of seafloor sediment — they free nutrients once bound in soil and build burrows other creatures can use.

The new study found that because tsunamis leave sandy layers of varying grain sizes in their wake, they can influence the distribution of heart urchin burrows. Burrows made in larger-grained sand deposits tend to collapse, so the urchins were largely restricted to areas where the 2011 Tohoku tsunami deposited fine-grained sand. 

By controlling the distribution of coarse and fine-grained sandy deposits, the tsunami influenced where the urchins burrowed and, ultimately, where they perform their ecologically significant job as bioturbators. Understanding the heart urchin’s ecological role and how it’s influenced by tsunamis is imperative in understanding the health of coastal ecosystems, Seike added.

“The tsunami disturbance alters fundamental environmental components of coastal ecosystems like topography or substrate grain size,” Seike said. “Revealing the effect of the tsunami event is essential for our understanding the natural history of regions where tsunami disasters occur.”

When a tsunami strikes, it can wipe entire sections of the seafloor (and the creatures that dwell there) clean, leaving untouched patches of fine and coarse sand in their place. In Japan’s Funakoshi Bay, the heart urchin was among the first to recolonize the freshly laid tsunami deposits, where they burrowed only as deep as the fine sedi-ment allowed.
Credit: Koji Seike

Digging down

When the Tohoku earthquake struck offshore Japan on March 11, 2011, it produced huge tsunami waves.  Some of the waves were 40.5 meters (133 feet) tall and reached up to 10 kilometers (six miles) inland.

To stay alive, the heart urchin maintains a breathing tube that reaches to the seafloor surface. If the seafloor becomes too coarse, it can collapse the urchin’s breathing tube.
Credit: Koji Seike

The tsunami also wiped away many of the benthic sea creatures from Funakoshi Bay, a subsection of Japan’s northeastern coast. The huge waves laid down mixed patches of fine and coarse sand. This provided a clean slate for Seike and his team to explore how the tsunami’s freshly deposited sand would influence benthic creatures, like the heart urchin, when they recolonized the bay.

Seike monitored how many heart urchins returned, how their body size changed over time, which type of sand they chose to burrow in and how deep they dug. Aquarists suspected heart urchins tend to burrow only in soft, fine sand, but the new study provides confirmation that wild heart urchins show a clear preference for finer sand.

By sticking sediment cores in a computed tomography scanner, the researchers produced X-ray images revealing clear pictures of the burrows and how deep they ran. The scans showed heart urchins tend to burrow through fine sand, stopping when they meet coarser granules. Heart urchins burrow a few centimeters into the sandy seafloor, where they collect and eat organic debris. To survive underground, the urchins maintain a set of tubes: a couple of tubes for excreting waste and one tube to the surface for breathing.

Heart urchins maintain the structural integrity of these tubes by emitting a mucus adhesive that keeps their burrow walls roughly in place. The urchin’s mucus holds up well enough against fine sand and mud, but coarse grains collapse under pressure like a miner’s tunnel, according to the new study. By depositing layers of coarse sand, tsunamis can dictate the distribution of heart urchin burrows, the researchers found.

Although the urchins of Funakoshi Bay returned to their pre-tsunami body sizes a few months after the tsunami, bays and benthic creatures elsewhere may not fare as well. Some tsunamis leave behind many more coarse deposits that can persist for hundreds or thousands of years, according to the study. Understanding the interplay between natural disasters like tsunamis and the ecological impact they leave behind is essential for stewarding and understanding coastal ecosystems, Seike said.

— Brendan Bane is a freelance science writer based in Santa Cruz.