An Urchin in Hand

Red pencil urchin Heterocentrotus mamillatus in Hawaii

Red pencil urchin (Heterocentrotus mamillatus) in Hawaii

Sea urchin skeletons are frequent finds along the Maine coast. With luck, the shell-like skeleton is entire but denuded of the animal’s tube feet and protective spines, revealing orderly but mysterious patterns of tiny knobs and holes. Alive, a sea urchin is equally enigmatic. Legless, it slowly crawls. Eyeless, it “sees.” Without hands, it grasps. How does it accomplish these feats?

Behold the urchin

Nearly 1,000 species of sea urchins1 live in oceans around the world from the intertidal zone visible at low tides to nearly 3 miles depth. They are generally round, covered with spines, and have a hard internal skeleton, or test,2 that protects the urchin’s internal organs. Yes, the skeleton is internal because in life, it is covered with a very thin skin, unlike the true exoskeletons of arthropods like insects.
Large and small sea urchin
Tests of green sea urchins, Strongylocentrotus droebachiensis.3 The vertical rows of tiny holes are an ambulacrum.

Many animals (e.g., mammals, birds, insects, reptiles) are bilaterally symmetric. That is, they have two roughly equal halves, say, left and right sides. Sea urchins and their relatives are pentaradially symmetric. Think of a pizza divided into 5 equal parts, each of which houses tube feet arrayed in an ambulacrum from Latin meaning “walkway.” 

Pentaradial symmetry of sea urchin
A green sea urchin’s pentaradial symmetry. The area in the image on the bottom right is magnified to show paired openings for each tube foot in an ambulacrum.

Getting from here to there

Absence of legs notwithstanding, sea urchins can slowly crawl across rocks and other surfaces using coordinated movements of their tube feet and spines – two very different types of structures.

Spines are hard and rotate around a ball-and-socket joint on the outer surface of the test and are controlled by tiny muscular rings around the bases of each spine.

Sea urchin ball and socket joint between spine and skeleton
Magnified view of the ball-and-socket joint of a green sea urchin spine

Conversely, tube feet are soft and operate by changes in internal water pressure like a water balloon. The feet are part of an urchin’s hydraulic system, also called a water vascular system, which begins at a specialized array of holes or pores, the madreporite, at the urchin’s top and leads to the tube feet through a series of internal canals.

Sea urchin madreporite
Top view of a green sea urchin test showing the 2mm, heart-shaped madreporite. The asterisks indicate pores that release eggs or sperm during reproduction.

Together, tube feet and spines facilitate locomotion, burrowing in some species, food detection and capture, and predator avoidance.

Sea urchin tube foot apical disk

However, unlike the suction cups of octopuses, the disk-shaped tube feet tips have no suction function. They secrete natural adhesives to stick to surfaces. The internal muscles and hydraulics of the feet coordinate their attachment and release. By chemically analyzing the sea urchins foot prints,4 scientists study the nature of these adhesives.

These boots aren’t just made for walkin’

Tube feet and spines also coordinate with each other allowing more complex abilities. Sea stars are close relatives of sea urchins and possess true eyes at the tips of their arms. Conversely, sea urchins lack eyes, yet they demonstrate visually-guided behavior, such as preferring dark over light places (e.g., sheltering beside a rock). They are also able to detect an object larger than about 10 degrees in diameter. This corresponds to your fist held at arm’s length. In other words, very poor vision if you are a human but impressive if you have no eyes.

How do urchins do this? The tube feet contain photoreceptors5 – cells that respond to light – at their bases and tips and, together with directional light-screening by the spines, are thought to mediate sea urchins’ ability to visualize objects. Some researchers argue that the photoreceptor signals are integrated in such a way so that the urchin’s entire surface functions as a single, very unusual, compound eye.

Gimme shelter

Sea urchins often engage in so-called “covering behavior” in which the coordinated actions of tube feet and spines collect objects onto their tests so that they are often found adorned with various items. Functionally, this behavior may provide predator protection, mechanical defense, and shielding from sunlight.

Covering behavior by a sea urchin
A green sea urchin collecting a periwinkle.

Debate continues about whether this behavior involves a conscious decision or if it is simply reflexive. For example, urchins can choose when and with what to cover themselves depending on ambient light and the size of available objects. Some researchers consider covering behavior to be evidence of tool use in sea urchins.

Getting a grasp of it all – pedicellaria

Sea urchin tube feet and pedicellaria

Pedicellaria are like tiny, cartoon hands that protrude on stalks from sea urchins’ bodies – when first discovered they were thought to be parasites hitching a ride. However, despite their ability to grasp, pedicellaria are not used in locomotion or covering behaviors. Instead they are an integral part of the urchins’ grooming behavior in removing actual parasites as well as defending against predators like sea stars. One early researcher described the “spirited combat” between a sea urchin and an attacking sea star with the weapons of choice being the pedicellaria. The pedicellaria of some species can inject venom into a foe and can even be toxic to humans.

Hidden treasure

An uncommon find is Aristotle’s Lantern, the feeding apparatus of sea urchins, since it is often lost after the test is washed ashore. The lantern is an arrangement of five long, self-sharpening teeth within a boney cage that sea urchins use to scrape and eat algae and other foods. The teeth are on the underside of the body and aren’t visible unless the urchin is turned over – most of the lantern’s structure is internal. In the living sea urchin Aristotle’s Lantern is a complex structure of teeth, bones, muscles, and nerves.

Aristotle's lantern and sea urchin teeth
Aristotle's Lantern (left) and a sea urchin mouth (right)

"If I Only Had a Brain" – The Scarecrow in The Wizard of Oz

A Zombie sea urchin – spines move in an isolated skeleton.

Sea urchins don’t have a brain,6 but rather a distributed nervous system that has been likened to the internet’s interconnected system of small networks. Sensory information from tube feet, pedicellaria and spines are collected locally and help direct local responses – e.g., what should the nearby tube feet do? These local nodes communicate with neighboring regions and ultimately with a larger nerve net that serves an overall control and coordination function.

Profound thoughts may not arise from a sea urchin’s nervous system, however, an advantage of this “decentralization” is its resistance to catastrophic failure – it shows “graceful degradation.” In other words, a sea urchin may continue to function and thrive despite significant damage that might incapacitate an animal with a more centralized nervous system like a mammal.

Look in every tide pool

I remember as a child being entranced by The Burgess Seashore Book for Children.7 I expected every tide pool to be full of natural treasures – sea stars, chitons, whelk egg cases to name a few. I still have that expectation and can’t pass a tide pool without a quick look inside. Don’t assume that a common find like a sea urchin doesn’t hold a trove of riches – the closer one looks at nature, the more there is to find.
Seeing through sea urchins' eyes
Looking closely with sea urchin eyes

Notes

1 I will talk primarily about the green sea urchin, however, most of what I write is relevant to other species.

2 “Test” is the anatomical term for the urchins’ hard skeleton. The word comes from Latin for “tile” or “earthenware bowl”

3 This is not the longest species name, although it is the longest that I can reliably remember.

4 Each tiny tube foot leaves an equally tiny print on the surface it sticks to.

5 The skin also contains photoreceptors and plays a role in urchins’ overall light sensitivity.

6 And, unlike the Scarecrow, they probably don’t miss it.

7 By Thornton Burgess

References

Thanks

Special thanks to Zoe Weil’s editorial expertise

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Christine Rivard
Christine Rivard
11 months ago

Informative and humorous, EB.

Rae Sikora
Rae Sikora
11 months ago

Love this one so much! Sharing!
So much new information for me!
Ball and socket joint…who knew? I think this is my new favorite of all of your posts…Thanks for. All! 🥰👍

Steve Cochrane
Steve Cochrane
11 months ago

Fascinating and fun! Who knew the complexity and mystery of a seemingly simple sea urchin? Thanks for this Edwin!

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