You may think you know the Aquarium, but take a look at any exhibit and you'll see there's a lot more going on than you may think. Take the Living Corals exhibit, for example. Everyone loves to watch the clownfish cuddle with its anemone, or the Banggai cardinalfish posing for pictures with their distinctive stripes. But take a closer look in the sand, the rocks the quiet corners of the exhibit—you never know what you'll see.

For example, a hermit crab found itself in a bit of a sticky situation when a passing urchin latched on to its shell. These two invertebrates are not the focal points of this exhibit, but they certainly exhibited some fascinating behavior worthy of a closer look! Here's the whole interaction sped up in a timelapse video.



One reason urchins carry around shells and seaweed may be for camouflage. The hermit crab's shell would have made for a dashing addition. But the crab was able to eek out of its shell enough to pluck the tube feet from its shell so it could scurry away.

So the next time you visit, get macro. Pause to look at the tiny animals around the Aquarium and get ready for a little bit of awesome.

Here's more about these humble heavyweights in our ocean communities:

• Meet the urchins in our cold-water tidepool touch tank
• Aquarists actually caught one of our hermit crabs hatching her eggs
• The BBC filmed hermit crabs in Belize while researcher Randi Rotjan was there
• Check out this unbelievable video of hermit crab migrations
• See more hermit crabs in Belize, plus other amazing ocean animals
• Here's a hermit crab with a most unusual shell

The green sea urchin (Strongylocentrotus droebachiensis) comes from the class of animals called  Echinoidea, which means "like a hedgehog." Can you guess why?

Green sea urchin at the New England Aquarium

Young hedgehog (Photo: By Calle Eklund/V-wolf, via Wikimedia Commons)

Sea urchins are related to sea stars, sand dollars and sea cucumbers. You can find urchin species in oceans around the world (and in the Edge of the Sea Touch Tank here at the Aquarium!). The green sea urchin prefers shallow, rocky areas. They snack on algae and dead matter by scraping with five pointed teeth.


Urchins can destroy kelp beds and are sometimes seen as pests! An urchin's mouth structure is also called an Aristotle's Lantern. It contains the five teeth and also has 40 different skeletal parts and 60 muscles.


Urchins can move at a rate of six to seven feet per hour. They use their spines and tube feet, also called podia, to get around. Each spine is attached with ball and socket joint.

 Look closely and you can see the podia in this picture.

Podia are long and slender with terminal suckers at the end. Tube feet operate hydraulically by means of a water vascular system.


 Urchins often cover themselves with pebbles, shells, and bits of seaweed for camouflage.

Sea urchins have a long list of predators, and people are tops on that list. Marine birds, arctic foxes, sea otters and starfish also eat urchins. Wolffish swallow urchins whole. Triggerfish blow them over with a jet of water. Gulls pick the urchins from a tide pool, fly overhead, and drop the animals onto the rocks. The shell breaks open on impact and the gull flies down for its meal.

 Group of green sea urchins in the Aquarium's Edge of the Sea exhibit

Conservation Notes: Sea urchin roe (actually both the male and female gonads), called uni in Japan, is considered a delicacy. During the "Green Gold Rush" of the 1980's and 1990's, millions of pounds of urchins were harvested for their roe. Overharvesting amid a lack of regulation caused the urchin population to crash. Regulations now prevent overharvesting of urchins, but populations have been slow to recover.

On your next visit to the Aquarium, look for a sea urchin at the Edge of the Sea touch tidepool. You can also see urchins in the cuttlefish exhibit and the Northern Waters gallery.