Subtropical Sea Urchins: Skeletons in Humanity’s Closet

By Simone Burton

Everybody’s got skeletons in their closet, right? We all make mistakes. Maybe you went on a Tinder date, ghosted them, then found out you have a college class together. Yikes. Maybe you tried blue eyeshadow in junior high or idolized the “One Direction” band. No big deal. Push those regrets into the closet never to be seen again. But some skeletons need to be dealt with.

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Image taken by Amit Patel, Creative Commons Licensed

I’m talking about literal skeletons. Humans aren’t the only ones with skeletons. You’re thinking of monkeys, elephants, and dogs right about now. Think deeper. Past sea-level, organisms called sea urchins also have skeletons. But their skeletons are made of magnesium calcite. This mineral combination is soluble: it can dissolve in water. The lower the ratio of Magnesium (Mg) to Calcium (Ca) in sea urchin skeletons, the less soluble the combo is in water. This property is pretty important, as sea urchins live in the ocean.

Another piece of the puzzle is temperature. Previous studies have shown that temperature affects the overall growth rate of skeletons, particularly for organisms in intertidal and subtidal zones of the ocean. With global climate change upon us, tropical and subtropical regions of the northern hemisphere are particularly at risk of drastic ocean warming. So far, we don’t know how temperature could affect the Mg/Ca ratios in their skeletons, but sea urchins could literally “melt” under our watch.

So why should you care? They’re just sea urchins, right? The Legos® of the sea: you step on them and instantly wish you hadn’t. But when organisms near the bottom of the food chain are hurt, the whole chain suffers. It’s as if someone knocked over the first domino in a long line. Eventually, the whole thing will topple, including us.

Researchers at the University of Alabama at Birmingham (UAB) set out to test how near-future temperatures affect the sea urchins’ skeletons. Subtropical sea urchinswere collected from Eagle Harbor of Port Saint Joseph Bay, Florida. They were hurriedly transported to the laboratory back at UAB and randomly distributed into large aquarium tanks. Then the tanks were randomly split up to either be 26ºC or 30ºC. The 30ºC temperature is the predicted near-future temperature for seawater in the harbor the urchins were collected from. It’s also slightly below the lethal temperature limit for sea urchins, which is 32ºC. Statistical analysis of the sea urchins showed that both types of groups (26ºC vs. 30ºC), although they started with similar total wet weights (weights taken before the organism of interest has been dried in an oven), the 30ºC urchins showed lower wet weights than the 26ºC urchinsqafter ninety days. This pattern continued when both groups were tested on their diameters. When the scientists looked at the Mg/Ca ratios of both groups, the 30℃ group of sea urchins repeatedly had a lower ratio of Mg/Ca than the 26℃ group of sea urchins in all three tests of skeletal body components: the spines, the test (shells of the skeleton that fit together to protect the urchins in a hard-but-flexible, round way. Think of tectonic plates that fit together sitting on top of the Earth’s mantle), and Aristotle’s Lantern (the sea urchin’s mouth, with teeth arranged in a circle at the center that scrape away at the algae on the surrounding rocks). Remember how lower Mg/Ca ratios in the urchins will make them not melt? This is good news, people.

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Lytechinus variegatus, the species of sea urchin in the UAB study. Taken by James St. John, Creative Commons Licensed

Corresponding author on the paper, James B. McClintock from UAB, commented, “In this study, we found that as temperature increases with global warming, the levels of magnesium in the skeleton decline. This is a good thing, because it means that it will reduce vulnerability to ocean acidification.”

Fortunately, this sea urchin lucked out. But McClintock urges people not to get too hopeful. “In the really big picture – all calcified marine invertebrates regardless of what mineral they’re are made of – face HUGE challenges from global warming and ocean acidification. If we don’t address the burning of fossil fuels immediately, we stand to witness a massive species extinction event in the world’s oceans.” As global climate change increases as the consequences of human actions finally turn up, populations of all organisms in all habitats will be forced to move to new environments or adapt. Some marine invertebrates have dispersive larvae and are capable of escaping the warm areas of their homes. But other organisms are simply too slow to avoid climate change or are sedentary organisms.

This situation is regretful and preventable. Climate change is largely associated with human actions. Change our actions, change the lives of sea urchins, not to mention various other critters. There could be a few less skeletons in all of our closets if we all choose to make an effort.

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