Gulf Sun Sea Star

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.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.
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Purple Sea Star Eating a Mussel
Courtesy of Rosario Beach Marine Laboratory, via Invertebrates of the Salish Sea, CC-BY-NC-SA

About Sea Stars (Class Asteroidea): Feeding

Most sea stars are carnivores (eating live animals) or scavengers (eating dead animals). They feed on prey that move slowly such as polychaete worms or not at all such as oysters. Most sea stars turn their own stomachs inside out onto the bodies of their prey to spread digestive juices. As the prey's body softens, the sea star pulls its stomach back into its mouth. The food travels to another stomach for further breakdown. To eat an oyster, the sea star uses its tube feet to pry open the shells, then pokes its stomach inside to begin digestion. Populations of sea stars can damage coral reefs by inching along and digesting everything they come across. Some sea stars keep their stomachs in their bodies and swallow whole prey. After digestion, shells and other waste are expelled out their mouths. While they do not look fierce, sea stars are top predators in many ecosystems.

Body plans of types of fossil echinoderms
Photo by Mary Parrish, Department of Paleobiology, Smithsonian Institution

About Echinoderms (Phylum Echinodermata): Body Plan

Echinoderms stand out as the only organisms on Earth with five arms or other elements spaced evenly around a central point. This pentaradial symmetry (penta = “five;” radial = “around a central point") is obvious in some echinoderms, such as sea stars or brittle stars that have five arms. It’s more subtle on others, such as the five rows of tube feet on a sea cucumber or five grooves on a sand dollar. This unusual symmetry of adult echinoderms is not found in juveniles. In fact, echinoderm larvae have two-sided (bilateral) symmetry like humans, and must undergo a metamorphosis to become pentaradial adults. Body plans of animals during their development often say something about their ancestry. The bilateral bodies of developing echinoderms are a reminder that, aside from other back-boned animals (vertebrates), echinoderms may be our closest relatives. Unlike most living echinoderms, many fossil echinoderms did not have pentaradial symmetry.

Fossil echinoderm (Anartiocystis foerstei) showing attachment stalk
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.