What’s that pointy shrub thing?

Kaelyn Lemon, Macalester College/ Dr. Bob Podolsky at Grice Marine Lab

Arbacia punctulata is.. a shrub? Right? Arbacia sounds like some kind of plant, maybe.

Close. A. punctulata is short and round, like a shrub, but it lives in the ocean. And instead of branches sticking out from its center, it has spines. You may be more familiar with one of its nicknames: Purple-spined sea urchin, or Brown rock urchin (1). So A. punctulata is an animal, not a plant. Rather than a shrub, a better analogy would be the initial English meaning of sea urchin: sea hedgehog (2).

Purple-spined sea urchins, somewhat obviously, have purple spines. The second common name of A. punctulata, though, gives a clue that these urchins are not always purple. They can actually vary in color from a range of purple hues to brown or black and even red (1). While some urchins have both long and short spines, a unique feature of A. punctulata is having only long spines. The spines are the easiest body part of an urchin to identify- they are the sometimes incredibly sharp protrusions that are also the reason Taylor Swift has an irrational fear of sea urchins (see: https://www.youtube.com/watch?v=yPGrJCbaKB4). Spines can break off, but they can re-grow (2). However, sea urchins are not immobile pincushions waiting to stab things. Their spines can move around in joints where they connect to the test. The test is main shell-like skeleton of the urchin, the part in the center that can make urchins look like rocks rather than animals. For sand dollars, the closest relatives of sea urchins, the test is the white shell-like structure that you collect dried on the beach.

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The two most basic parts of a sea urchin, shown on A. punctulata. Photo credit: Aaron Baumgardner

Sea urchins have radial symmetry and are symmetrical in five parts. Their mouths are on the bottom, where they have five plate-like “teeth” that scrape against rocks or whatever they are on top of, even other urchins (2). Sea urchins release both their poop and their gametes (sperm or eggs) from the top of the test (though out of different locations!). Finally, while urchins pretty obviously have tons of spines, what you can’t see unless you look closely are the tons of tube feet. Tube feet look like extremely thin fleshy strings with little suctions cups at the ends. They come out of pores in the test and can extend beyond the length of the spines. They are controlled by increasing and decreasing the amount of water inside them, and are how sea urchins move (yes, they do move, sometimes surprisingly rapidly).

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Sea urchins can be broken into five parts that are symmetrical to each other. Photo credit: Aaron Baumgardner


This is a view looking from a bottom angle at A. punctulata. If you look where the urchin meets the side of the bin, you can see tiny tube feet, like little strings with suction cups at the ends, holding on to the wall. Three white spots near the top of the dark underside view are teeth plates that surround the mouth. Photo credit: Kaelyn  Lemon

There are male and female sea urchins, which release sperm and eggs, respectively, into the water where they hope to meet, fertilize, and create an embryo that will develop into a larva. Unlike humans, where usually one egg is released at a time (into the uterus rather than out of the body), urchin females will release millions of eggs at once (2). Sea urchin larvae are bilaterally symmetrical, just like dogs, fish, and people. Unlike adult urchins, which are less mobile and stick to the ground, urchin larvae are free swimming in the ocean water.

A. punctulata is found along the entire Atlantic coast of the US and west to Texas, as well as in the Caribbean around Cuba and Panama. These urchins are usually found in water that is less than 164 feet deep (1). A. punctulata is mainly an herbivore, eating mostly algae, though it can also eat animals such as sponges, corals, and sand dollars, as well as other urchins, even those of its own species.

Sea urchins, besides being incredibly beautiful feats of nature, are important to people in two main ways. Nearly every species of sea urchin is eaten by people- either the meat inside the test, or the urchin’s eggs (2). Second, urchins are incredibly important to science because they are used as a model organism to study reproduction and development.

A. punctulata specifically will be contributing greatly to my research by providing gametes that I fertilize and develop into larvae while being exposed to normal or high carbon dioxide levels. I greatly admire the incredible, and somewhat variable, body structure and coloration of these purple animals.


While this individual may look very different from the urchins in previous photos, it is actually still the same species. Photo credit: Aaron Baumgardner

Cited References:

1. Smithsonian Marine Station at Fort Pierce: http://www.sms.si.edu/irlspec/Arbaci_punctu.htm

2. Animal Diversity Web: http://animaldiversity.org/accounts/Arbacia_punctulata/

Funding and support for my research is provided by the National Science Foundation and the College of Charleston

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