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Phylum Echinodermata | Class Echinodea
Sea urchins
updated Apr 2020
if you learn only 3 things about them ...
Spines can inflict painful wounds. Don't touch!
Many sea urchins have a ring of 5 teeth.
The parts of the sea urchin eaten in sushi are the eggs.

Where seen? Sea urchins are sometimes seen especially on our undisturbed shores. But even on a well used shore such as Changi, they can be seasonally abundant.

What are sea urchins? Sea urchins belong to Phylum Echinodermata and Class Echinoidea which includes heart urchins and sand dollars.

Features: Like other echinoderms, sea urchins are symmetrical along five axes, have tube feet and spines.

And wow, do they have spines! Sea urchins are usually covered with lots of long, sharp spines that deter most predators. The spines also tend to break off inside their victim's flesh. Some sea urchins have venomous spines that can inflict serious pain.

Some sea urchins also have tiny jaw-like structures on stalks called pedicellariae. The main function of these is to keep the body of the sea urchin free of debris and parasites. They may also be used to collect food. Some sea urchins have larger venomous pedicellariae which painfully sting large creatures and can paralyse smaller ones.

Some have very long slender spines.
Cyrene Reef, Jun 08

Others have thick spiky spines.
Beting Bronok, Jun 03

Skeleton of a dead sea urchin
Changi, May 02
Test of Strength: Sea urchins have an internal skeleton (called the test) formed out of large ossicles fused together in plates. These form a pattern that resembles a sliced orange, in multiples of five. The test is a rigid, hollow sphere. To grow larger, each ossicle is enlarged, and new ossicles added near the anus.

Splendid spines: There are little knobs all over the outside of the test. The spines move on these little knobs, articulating somewhat like the ball-and-socket joint of our knees. Sea urchins usually have two kinds of spines; one larger and/or longer, and the other smaller.

Like other echinoderms, sea urchins have mutable connective tissue as well as muscles that move the spines. These moveable spines not only protect the sea urchin, but are also used for walking. Sea urchins can move in any direction because they are spherical. The spines can also be locked in place to wedge themselves in a safe hiding place.

Where do the spines of a dead sea urchin go?
Like us, sea urchins have a skin covering the spines and the test. When a sea urchin dies, the skin decays rapidly and all the spines fall off, leaving only the spherical test. The inside of a sea urchin is mostly empty except during mating season when it is full of sperm or eggs.

Tube Feet Too! Like other echinoderms, sea urchins also have tube feet. If we go back to the image of the test as a sliced orange, the tube feet emerge from holes along five 'slices'. Like other echinoderms, the tube feet are operated hydraulically with the water vascular system that all echinoderms have. These tube feet end in suckered discs that can stick to things and thus allow the animal to move, climb up vertical surfaces, dig or collect food. The tube feet are also used to sense chemicals, breathe, as well as excrete wastes! Some sea urchins 'carry' shells, seaweed and other debris. This behaviour may help camouflage them or shield them from sunlight.

A recently broken urchin.
Changi, Jul 11

Long tube feet.
Changi, Jun 05

Aristotle's lantern.
Changi, Jun 05
Toothy Urchins: The mouth of a sea urchin is on the underside facing the ground. They have a complex jaw structure made of a circle of five plates that meet in the middle to form a beak-like structure. The entire structure can be extended outwards to chomp on their food. It is called the Aristotle's Lantern after the Greek philosopher Aristotle who first described it. New 'teeth' grow to replace those that are worn down. A sea urchin's anus is on the opposite side of its mouth, on the upperside of its body.

What do they eat? Most sea urchins graze on seaweed, detritus from hard surfaces or on immobile creatures such as sponges or encrusting animals.

The crunching of feeding sea urchins can be very loud! As sea urchins scrape on algae and other encrusting stuff growing on hard surfaces, their hollow bodies amplify the sound so that they contribute to most of the noise on some reefs. This noise may have a role in reef health! A study has found that reef fish and crab species swim towards underwater sound, thus noise generated around the coast plays an important role in guiding baby fish and crustaceans to a suitable habitat in which they can settle. Here's more about this study.

Baby sea urchins:
Sea urchins have separate genders and are usually either male or female. They practice external fertilisation, releasing eggs and sperm simultaneously into the water. Each female sea urchin can release millions of eggs at a time! This is probably why we sometimes see many sea urchins gathered together.

Sea urchins undergo metamorphosis and their larvae look nothing like their adults. The form that first hatches from the eggs are bilaterally symmetrical and free-swimming, drifting with the plankton. At this stage, they have several long 'arms' which are believed to funnel food particles into the central mouth. They eventually settle down and develop into a more sea urchin-like shape.

Sometimes gathered in large groups.
Changi, Jul 04

Sometimes gathered in large numbers.
Cyrene Reef, Apr 07

Some sea urchins 'carry' stuff.
Beting Bronok, May 06
Prickly home: Sea urchins with their prickly spines offer a safe hiding place for various animals. Including some fishes such as the Razorfishes (Family Centriscidae) and a strange worm-like thing often found around the mouth of some sea urchins. Sometimes, tiny colourful brittle stars are seen on the spines of some sea urchins.

Role in the habitat: Grazing sea urchins keep seaweed growth in check. An excessive seaweed 'bloom' can deplete oxygen, smother life forms and upset the ecological balance. Thus sea urchins help to maintain the balance. If there are too many seaweeds on a reef, for example, baby corals can't find a place to settle down. More about the role of seaweeds, baby corals and animals that eat seaweeds on the wild shores of singapore blog

Despite their spines, sea urchins are preyed upon by many creatures including crabs, fishes and birds.

Hole with 'burn' mark suggests the urchin was attacked by a Helmet snail.
Changi, May 08

Worm-like animal often seen around the
mouth of the Black sea urchin
Changi, Jun 05

Fishes among the spines of an urchin.
Terumbu Hantu, Jun 13
Photo shared by Loh Kok Sheng on his blog.
Humans uses: The roe (egg mass) of some sea urchins are relished as a Japanese delicacy and sea urchins are commercially harvested for this reason in various parts of the world. Sea urchins have been extensively studied to better understand egg fertilisation and embryo development for other applications. This is because their eggs are large and easy to study. Sea urchins are also harvested for their attractive skeletons which are made into souveniers.

Status and threats: In Singapore, the main threat is habitat loss due to reclamation or human activities along the coast that pollute the water. Like other creatures of the intertidal zone, they are affected by human activities such as reclamation and pollution. Trampling by careless visitors also have an impact on local populations.

Some Sea urchins on Singapore shores

Sea urchins recorded for Singapore
from Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.
*additions from Lane, David J.W. and Didier Vandenspiegel. 2003. A Guide to Sea Stars and Other Echinderms of Singapore.
in red are those listed among the threatened animals of Singapore from Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore.

**from WORMS
+Other additions (Singapore BIodiversity Record, etc)

  Family Cidaridae
  Prionocidaris sp. (Thorny sea urchin) with species recorded for Singapore

  Family Diadematidae
  *Chaetodiadema granulatum (EN: Endangered)

(Long-spined black sea urchin) with species recorded for Singapore

*Echinothrix diadema

  Family Echinothuriidae
  *Asthenosoma varium (VU: Vulnerable)

  Family Temnopleuridae
  +Mespilia globulus

Paratrema doederleini

Salmacis sp. (White sea urchin) with species recorded for Singapore.

Temnopleurus sp. (Black sea urchin) with species recorded for Singapore.

Links References
  • Lane, David J.W. and Didier Vandenspiegel. 2003. A Guide to Sea Stars and Other Echinoderms of Singapore. Singapore Science Centre. 187pp.
  • Jeffrey K. Y. Low. 29 May 2015. Globe urchin in the Singapore Strait, Mespilia globulus. Singapore Biodiversity Records 2015: 65
  • Chou, L. M., 1998. A Guide to the Coral Reef Life of Singapore. Singapore Science Centre. 128 pages.
  • Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore. Nature Society (Singapore). 285 pp.
  • Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore. National Council on the Environment. 163pp.
  • Miskelly, Ashely. 2002. Sea Urchins of Australia and the Indo-Pacific. Capricornia Publications. 180pp.
  • Gosliner, Terrence M., David W. Behrens and Gary C. Williams. 1996. Coral Reef Animals of the Indo-Pacific: Animal life from Africa to Hawai'i exclusive of the vertebrates Sea Challengers. 314pp.
  • Allen, Gerald R and Roger Steene. 2002. Indo-Pacific Coral Reef Field Guide. Tropical Reef Research. 378pp.
  • Edward E. Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate Zoology Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
  • Pechenik, Jan A., 2005. Biology of the Invertebrates. 5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
  • Hendler, Gordon, John E. Miller, David L. Pawson and Porter M. Kier, 1995. Sea Stars, Sea Urchins, and Allies: Echinoderms of Florida and the Caribbean. Smithsonian Institution Press. 390 pp.
  • Schoppe, Sabine, 2000. Echinoderms of the Philippines: A guide to common shallow water sea stars, brittle stars, sea urchins, sea cucumbers and feather stars. Times Edition, Singapore. 144 pp.
  • Coleman, Neville. undated. Sea Stars of Australasia and their relatives. Neville Coleman's World of Water, Australia. 64pp.
  • Allen, Gerald R and Roger Steene. 2002. Indo-Pacific Coral Reef Field Guide. Tropical Reef Research. 378pp.
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