Amphionidacea
(Amphionids)
Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca Number of families 1
Thumbnail description
Single-species order of small crustaceans with carapace covering head and thorax; females carry developing eggs in a brood chamber; development is multi-staged and complex
Illustration: Amphionides reynaudii.
Evolution and systematics
Amphionids have been studied very little, and as a result, the amount of information about them is very limited. No adult males or brooding females have yet been found.
Until the 1970s, the amphionidacea were classified as a family among the 30-odd families of caridean shrimps of the in-fraorder Caridea, because of similarities between the early larval stages of amphionidacea and those found among caridean shrimp species. Larval amphionids at various stages were sometimes classed as distinct species. In 1973, Williamson proposed reclassifying the amphionidacea as a separate, single-species order within the superorder Eucarida. He based his proposal on characteristics unique to the amphionids, including the brood chamber of the female and the structure of the first pair of pleopods in the female.
For the purposes of this chapter, the taxonomic breakdown for the Amphionidacea is: phylum Arthropoda, subphylum Crustacea, class Malacostraca, and order Amphionidacea. There is one family, Amphionididae, one genus, Amphionides, and one species, Amphionides reynaudii (H. Milne Edwards, 1832).
Amphionides reynaudii is the single extant species in its order that would have once contained others all descended from an ancestor in which females shed eggs directly into the water before evolving the distinctive brood chamber.
Physical characteristics
Amphionids are marine crustaceans with a number of unique, identifying characteristics. An adult amphionid is
about 1 in (2.6 cm) long. The head and thorax are covered by a thin, shell-like carapace that encloses the dorsal and ventral surfaces and about half the animal’s total length, with considerable space between the ventral body surface and the bottom of the carapace. There are gaps in the front and rear of the carapace. The thorax and abdomen are distinct and differentiated.
The head bears a pair of stalked, multifaceted eyes, the stalks each carrying a structure that is probably a photophore, or light-producing organ. There are two sets of biramous (two-branched) antennae, each antenna of the second pair bearing large, leaf-shaped accessory structures, the antennal scales, or scaphocerites. There are seven pairs of thoracic appendages, the paraeopods, all but the seventh being biramous. Only the first pair, close to the mouth, is functional, the max-illipeds being used for swimming. There is a wide gap between the first and second pair of appendages. The second to sixth pairs are thin and sticklike. They are useless for swimming and probably are not remnants of true maxillipeds. In the female, the fifth thoracic paraeopods are much longer than the rest, while the coxae, or proximal segments of the sixth pareaopods, carry genital pores. The seventh thoracic segment lacks appendages in the female, but bears a pair of uni-ramous (unbranched) appendages in juvenile males. Pairs of feathery pleurobranchs (gills) are mounted on the third to seventh thoracic somites (segments).
The heart, placed dorsally in the rear part of the thorax, is well developed. In females, ovaries run from the heart region to oviducts that lead to the genital pores in the sixth thoracic appendages.
Two pairs of longitudinal muscles link the posterior thorax and the abdomen. The abdomen divides into six somites, of which five bear appendages, the pleopods. In the female, the pleopods on the first abdominal segment are very long, thin, and flattened, able to reach forward to about half the length of the carapace. When extended forward, these specialized appendages are able to cover the rear and part of the forward gap of the carapace, thus rendering it a brood chamber. The unusually long fifth thoracic pareaopods may serve to clean and tend the eggs. In the male, judging by the larval stages, the first pair of pleopods is stoutly constructed and capable of vigorous propulsion.
Following the first pair of pleopods are four more pairs of shorter, biramous pleopods. The last segment carries a pair of uropods, or tail appendages.
Distribution
The Amphionidacea live throughout the oceans of the world, with some concentration in the equatorial regions. (Specific distribution not known; no map available.)
Habitat
Larval and juvenile forms live in a shallow, planktonic layer at depths of 90-300 ft (30-100 m). Adult females have been found in a depth zone of 21,000-111,000 ft (700-3,700 m).
Behavior
Judging from incomplete information, free-swimming larval and juvenile forms consume microorganisms as food; the pre-adult forms reside within swarms of nutrient-rich oceanic plankton. Adult forms sink to deeper ocean levels for breeding. Newly hatched individuals rise into the shallow plank-tonic layer to start feeding and maturing through the larval and juvenile stages. Males are able to swim faster than females, by virtue of the males’ strong first pair of abdominal segments, giving them a survival edge, yet the only purpose of the males may be to mate once.
Feeding ecology and diet
What amphionids eat and how they feed is poorly known. Larval and juvenile forms probably feed on algae and microscopic fauna. Adult females have only a degenerate mandible and maxillule (jaw parts) and reduced digestive tracts with stomachs that are still full of food ingested during the pre-adult stage. They may not feed at all as adults, but live off their stored food.
Reproductive biology
Young pass through up to 13 larval stages, then molt to a juvenile (postlarval) form, and then developed into the adult form. The number of larval stages may vary among regions and even among individuals in one region. Forms intermediate between known larval stages are often found.
The eggs form within the ovaries, pass through the oviducts to the genital aperture in the coxae of the sixth thoracic appendages, and then into a brood chamber defined by the carapace, the ventral thorax, and the first pair of pleopods, which can extend forward and over the bottom of the carapace. Although mating details are still not known, fertilization most likely takes place in the brood chamber and the developing eggs are kept there until hatching. Hatchlings are presumably released by the female pulling her first abdominal appendages loose enough to provide a gap through which the young may escape.
The photophores of the eyestalks may be used for signaling and attracting the opposite sex.
Conservation status
This species is not listed by the IUCN, but it is probably not threatened because of its wide distribution.
Significance to humans
The value of these little-studied crustaceans is mainly scientific. They are probably destined to attract more attention and study by marine biologists due to their unique array of characteristics that offer new insights into evolution and adaptation.
