Purpled-spined Sea Urchin

Associated Smithsonian Expert: David L. Pawson, Ph.D.

Dave Pawson

Dr. David Pawson is a senior researcher and an expert on echinoderms, sea cucumbers, sea urchins, and sea stars at the Smithsonian National Museum of Natural History. He studies echinoderms to understand how they live in different ocean ecosystems, how they are grouped together or classified, and how they reproduce. He grew up on the North Island of New Zealand, but came to work at the Smithsonian in 1964 after getting his doctorate degree. His research has taken him across the world and deep into the ocean. He has travelled to Antarctica, Australia, New Zealand, Chile, the Galapagos Islands, the Caribbean, and the east and west coasts of the United States. He has gone on 100 dives in deep-sea submersibles such as the Alvin and the Johnson-Sea-Link. In the 1980s Pawson was a member of a team of four scientists who made more than 150 submersible dives off Florida and in the Caribbean, to study echinoderms. They discovered about 200 species, about thirty percent of which were new to science.

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This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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Underside of Green Sea Urchin showing Jaw
Courtesy of Malcolm Storey, via BioImages - the Virtual Fieldguide (UK), CC-BY-NC-SA


About Urchins (Class Echinoidea): Feeding

Most urchins are grazers, scraping animal and plant material off surfaces as they move slowly along. Algae, bryozoans, sponges, and even sea cucumbers may be food to an urchin. A complex jaw apparatus called the Aristotle's lantern allows grazing urchins to eat just about anything. The lantern contains five teeth which are used for chewing, but can also be pushed out through the mouth for scraping. Feeding by urchins makes a star-shaped imprint from the lantern. When fossilized, the imprint leaves a record of urchin activity that can be interpreted by paleontologists. Some urchins, such as sand dollars and heart urchins, have evolved to burrow. While grazing urchins tend to have sharp spines that protect them from predators as they move along the seabed, burrowing urchins do not. Some burrowers lack the Aristotle's lantern, but others use the lantern and strong teeth to crush and ingest sand grains.

California Sea Cucumber
Courtesy of Ken-ichi Ueda, via iNaturalist.org, CC-BY-NC-SA

About Echinoderms (Phylum Echinodermata): Locomotion

Echinoderms move using rows of small tube feet powered by a fluid-filled (water vascular) system. The fluid travels from central, circular canals out through radial canals like the spokes of a wheel. Along the spokes are the tube feet, which typically end in adhesive pads. Each tube foot is kept firm by internal fluid pressure. By sticking down and unsticking in coordinated waves, the tube feet inch the animal along a surface. The water vascular system used for locomotion can also function for feeding. Some echinoderms pass small pieces of food along from foot to foot until it reaches their mouths. The otherwise soft echinoderm body gets the support it needs from a skeleton made of calcified pieces (ossicles). Ossicles often have spiny projections that give the overlying skin a prickly appearance (echino = spiny; dermis skin).

Fossil echinoderm (Geocome sp) from the Jurassic period
Photo by Smithsonian Institution, Department of Paleobiology

About Echinoderms (Phylum Echinodermata): Paleobiology

Echinoderms appear in the fossil record more than 500 million years ago, during the early Cambrian. What are usually left behind are hard mouthparts or parts of their skeletons, made of calcite plates. Rarely, an entire skeleton fossilizes, for example in a situation where it was quickly buried in sand. Even a piece of skeleton can provide information, because echinoderms have specific patterns in their skeletons. The echinoderms you see today have five-point (pentaradial) symmetry, often noticeable in five arms. While some of the earliest echinoderms were pentaradial, others had unusual body shapes. The “helioplacoids” had long, oval-shaped bodies with no arms, and a spiral pattern on the surface from tube feet wrapped around a central core. Helioplacoids went extinct even before the end of the Cambrian, as did a variety of other echinoderms, including the star-shaped Somasteroidea. Some echinoderms survived and diversified, becoming dominant in the oceans of the Paleozoic era.