Sea urchins are echinoderms that look like rounded mounds of spines. Centrally located on their ventral surface is the mouth, which is equipped with jaws and teeth that scrape up algae. All sea urchins have tube feet and are able to move slowly across the floor of the reef.
The spines on different species of sea urchins show adaptations for unique environments. Urchins with thick spines use them to wedge their bodies between rocks, preventing predators from pulling them from their hiding places. Those that have flattened spines are able to tolerate the strong energy of waves. Sharp, venomous spines are adaptations to protect against predators. Some species have movable spines that help them crawl.
Savigny’s sea urchin (Diadema savignyi) is a large, black reef species armed with sharp spines that can cause painful punctures. Mathae’s sea urchin (Echinometra mathaei) is brown or pink with purple spines. The flower urchin (Toxopneustes pileolus) looks innocent, but its “flowers” are actually very small spines that are highly modified. Each little spine is a venomous pincer that carries enough toxin to cause paralysis or death in humans. The red pencil urchin (Heterocentrotus mammilatus) has flattened spines that help it wedge among rocks and survive in high-energy regions of the reef.
 |
| Heterocentrotus mammilatus |
Sea cucumbers differ significantly from most other echinoderms. Instead of having the round or star shapes of their relatives, sea cucumbers, like the one in the upper color insert on page C-6, are cylindrical and look very much like the vegetable for which they are named. Some species are small and wormlike, but others reach impressive lengths of up to 3 feet (2 m).
During the day, sea cucumbers hide among rocks, but at night they emerge and work their way across the reef floor, “vacuuming” up sediment as they go. Inside their digestive systems, sand is filtered of its organic material, then the cleaned sand is excreted. The mouth, which is located at one end of the body, is surrounded by tentacles. Instead of sucking up sand, some species strain plankton from the water with their tentacles. The mouth does not contain teeth, but one species has teeth in its anus. In these animals, the anus also serves as the opening through which water is drawn into the body for gas exchange.
Sea cucumbers move sluggishly, crawling slowly on the sandy reef floor on their tube feet. These feet also enable sea cucumbers to cling very tightly to solid surfaces. Many divers who have tried to collect specimens have been disappointed to find that they will not budge. A few species are lacking tube feet and capable of swimming.
Of all reef inhabitants, sea cucumbers use one of the most unusual defense strategies. To protect themselves sea cucumbers project sticky threads of their intestines onto the attacker. The threads are coated with toxins that stop most predators in their tracks. When the threat has moved away, the intestinal structures begin to regenerate themselves.
The beaded sea cucumber (Euapta lappa) is a large one, measuring up to 3 feet (1 m) in length. Each thick, beadlike body segment is brown with thin yellow and black stripes. Beaded sea cucumbers have long, thick tentacles that are shaped like feathers. Members of this species do not have tube feet.
 |
| Euapta lappa |
The five-toothed sea cucumber (Actinopyga agassizii), 1 foot (30 cm) long, ranging from gray to brown in color, is an unusual creature. The teeth of this echinoderm are located in its anus and can be seen when the sea cucumber “exhales” water that has been circulated over the gills. The anus is also the home of a commensal organism, the pearlfish (Carpus).
In commensal relationships, one organism is benefited, and the other is not harmed or helped. In this case the adult pearlfish has a safe place to live inside the sea cucumber, which neither benefits nor is hurt by the pearlfish’s presence.
As juveniles, pearlfish are parasites, feeding on the sexual organs of their hosts, but as adults, they forage for food on the reef. For a foraging adult pearlfish to return to its host, it must swim into the cucumber’s anus. The cucumber keeps its anal opening squeezed closed, so the pearlfish waits until the cucumber “exhales” and relaxes the opening, then the fish darts inside.