There is only one species of amphionid, Amphionides reynaudii. Adult males are unknown; females are about 1 inch (2.6 centimeters) long. Their bodies are divided into a head, thorax, and abdomen. The head and thorax are completely covered by a thin, membranelike carapace. There is a considerable amount of space between the carapace and the underside of the body. The head has a pair of compound eyes on stalks that are believed to produce light. Compound eyes have multiple lenses. Both pairs of antennae are branched, or biramous (BY-ray-mus). The second pair of antennae also has a single, large, leaflike flap on their bases.
The thorax has seven segments. Segments 3 and 7 have feathery gills. Juvenile males have seven pairs of thoracic limbs, but the last pair is absent in females. Pairs 1 through 6 are biramous. Only the first pair of limbs, the maxillipeds, is used for swimming. The remaining limbs are sticklike and useless for swimming. In females, the fifth pair of thoracic limbs is very long, and the sixth pair has the openings to the reproductive system. The last pair of thoracic limbs (juvenile males only) is unbranched, or uniramous (YU-neh-RAY-mus).
The abdomen is divided into six segments. The first five segments each has a pair of appendages underneath called pleopods (PLEE-oh-pawds). In females, the first pair of pleopods is long, ribbonlike, and uniramous. They reach toward the head and are about half the length of the carapace. They are used to close off the underside of the carapace to form a brood chamber when protecting eggs. The remaining four pairs of pleopods are much shorter and biramous. The tip of the abdomen has a pair of long appendages called uropods (YUR-oh-pawds). The uropods are found on either side of a flaplike tail, or telson. The uropods and telson together form a fanlike tail.
Amphionids live in all oceans. Because they are found throughout the world, no distribution map is provided.
Amphionids are marine and are most common near the equator. Young animals live with other plankton at depths of 90 to 300 feet (30 to 100 meters). Plankton is made up of free-floating, often microscopic, plant and animal life. Adult females have been found at depths of 5,577 feet (1,700 meters).
Immature amphionids probably eat algae (AL-jee) and other microscopic organisms. Adult females have reduced mouthparts and digestive tracts. This suggests to scientists that the amphionids must rely on energy from food they ate before reaching adulthood.
BEHAVIOR AND REPRODUCTION
There is very little information on behavior and reproduction. Younger animals live with other plankton in the upper layers of the ocean, while adults live and breed in deeper waters. The light-producing eyestalks may be used to attract or locate mates.
The larvae (LAR-vee) molt, or shed their exoskeletons, up to 13 times before reaching the postlarval stage. Postlarvae resemble the adult in shape and behavior, but are not able to reproduce. The number of larval stages varies from region to region and among individuals living in the same place.
AN ORDER IS BORN
Amphionids were formerly grouped with coral and snapping shrimps on the basis of their similar larval forms. In 1973, they were reclassified in the new order Amphionidacea. This placement was based on the adult female's unique brood chamber and the ribbonlike structure of the pleopods. Before anything was known about their development, scientists thought the various larval stages of the only known species were distinct species and described them as new.
Nothing is known about their mating habits. The eggs pass from openings at the bases of the sixth pair of thoracic limbs into the brood chamber underneath the thorax. It is likely that they are fertilized in the chamber. The developing eggs probably remain in the chamber until they hatch. Hatchlings probably escape through the gap created when the female loosens or removes her ribbonlike pleopods.
AMPHIONIDS AND PEOPLE
These unique crustaceans are of interest to scientists who study crustaceans and how they survive in their environment.
This species is not considered endangered or threatened.
FOR MORE INFORMATION
Holthuis, L. B. The Recent Genera of the Caridean and Stenopodidean Shrimps (Decapoda); with an Appendix on the Order Amphionidacea. Leiden, The Netherlands: Nationaal Natuurhistorisch Museum, 1993.
Heegaard, P. "Larvae of Decapod Crustacea: The Amphionidae. Dana Expedition," Report 77 (1969): 1-67.
Lindley, J. A., and F. Hernández. "The Occurrence in Waters Around the Canary and Cape Verde Islands of Amphionides reynaudii, the Sole Species of the Order Amphionidacea (Crustacea: Eucarida)." Revista de la Academia Canaria de las Ciencias 11, nos. 3-4 (1999): 11-119.
Williamson, D. I. "Amphionides reynaudii (H. Milne Edwards), Representative of a Proposed New Order of Eucaridan Malacostraca." Crustaceana 25, no. 1 (1973): 35-50.