Tusk Shells: Scaphopoda

views updated

TUSK SHELLS: Scaphopoda

TUSK SHELL (Antalis entalis): SPECIES ACCOUNT

PHYSICAL CHARACTERISTICS

The tubelike shells of scaphopods (SKAF-oh-pods) measure up to 7.8 inches (200 millimeters) in length. They are long, tusk-shaped, and open at both ends. The broadest end of the shell is considered the front. A muscular foot and slender feeding tentacles reach out from the front opening. The mouthparts are made up of a radula (RAE-jeh-leh) that is short, thick, and has five extremely hard teeth. The opening at the narrow end, or at the rear of the shell, is where oxygen-carrying water flows in and waste, eggs, or sperm flow out. The shell is usually white, but in some species the shells are green or have red or yellow bands. The shells of other species are clear, and the internal reproductive organs are sometimes clearly visible.

GEOGRAPHIC RANGE

Tusk shells are found in cool and warm water oceans worldwide, from seashores to depths down to about 23,000 feet (7,000 meters).

HABITAT

Tusk shells are found only in soft, muddy ocean bottoms where they burrow to search for food.

DIET

Tusk shells eat microscopic organisms, especially foraminiferans (fo-re-mi-NIH-fer-ehns), single-celled organisms that have a nucleus. The nucleus is a structure that contains the genetic information. Other species feed on tiny crustaceans, as well as clams and their relatives.

BEHAVIOR AND REPRODUCTION

All tusk shells burrow into soft ocean bottoms. Some species completely bury themselves, as much as 16 inches (400 millimeters). Others are shallow burrowers and leave the tips of their shells sticking out of the mud.

Most species require males and females to reproduce. Only a few species are hermaphrodites, with individuals having both male and female reproductive organs. Eggs and sperm are released into the water, where fertilization takes place. The eggs hatch into free-swimming larvae (LAR-vee) that resemble the immature stages of other mollusks.

TUSK SHELLS AND PEOPLE

Tusk shells were used as money by Native Americans in the Pacific Northwest and circulated throughout western Canada south to California. Tusk shells were also worn as displays of wealth. Today they are sold to shell collectors and made into jewelry. The eggs of tusk shells, especially those of Antalis entalis are used to study the early development of fertilized eggs.

WHAT IS IN YOUR POCKET?

Native Americans living along the Pacific Coast from Alaska to California used the tusk shell Dentalium pretiosum as a form of money. The value of the shells was determined not by their numbers but by length. They were usually measured by comparing them to the length of finger joints. The ligua was the most valuable and was made up of twenty-five shells strung together, end-to-end. It measured about 6 feet (1.83 meters) long.

CONSERVATION STATUS

No species of scaphopod is considered threatened or endangered.

TUSK SHELL (Antalis entalis): SPECIES ACCOUNT

Physical characteristics: The shell is smooth and white and sometimes has faint, lengthwise ribs toward the narrow rear.

Geographic range: Tusk shells live in the northeastern Atlantic, from Ireland to the Canary Islands; another population lives in the northwestern Atlantic, from Nova Scotia to Cape Cod.

Habitat: They burrow in soft ocean bottoms at depths down to about 6,600 feet (2,000 meters).

Diet: Tusk shells eat all kinds of microscopic organisms, but prefer foraminiferans.

Behavior and reproduction: They burrow into soft ocean bottoms, leaving only the tips of their shells exposed. Males and females are required for reproduction. Both eggs and sperm are released into the water, where fertilization and development take place.

Tusk shells and people: This species is not known to impact people or their activities.

Conservation status: Tusk shells are not considered threatened or endangered. ∎