Amphipoda

(Amphipods)

Photo: Skeleton shrimps (Caprella sp.) have raptor like claws, presumably used to capture prey (copepods, crustacea larvae, worms, amphipods, etc.) floating by in the current. (Photo ┬ęTony Wu/www.silent-symphony.com. Reproduced by permission.)

Evolution and systematics

The order Amphipoda is made up of three suborders (some scientists recognize the Ingolfiellidea as a suborder rather than a family of the Gammaridea), 155 families, and more than 6,000 species. The three suborders are Gammaridea, with 126 families; Caprellidea, with 8 families; and Hyperiidea, with 21 families.

The fossil record of crustacean arthropods is patchy, and fossils of amphipods are almost non-existent. The few that have been found can be traced back to the Cambrian period. The Gammarids appear to be the most primitive of the amphipods, with Hyperiids and Caprellids showing more specialization in body form, behavior, and ecological relationships.

Physical characteristics

Amphipods tend to have laterally compressed bodies that curve to form a "C." Although there is wide variation in body form, the general body type is made up of a head, thorax, and abdomen. The head has compound eyes of varying sizes and well-developed pairs of first and second antennae. The seven multisegmented thoracic appendages are made up of two pairs of claw-like gnathopods used for grasping and five pairs used for crawling, jumping, and burrowing. Gills are found on the thorax. The abdomen has three pairs of appendages (pleopods) used for swimming and moving water through a burrow, and three appendages (uropods) are used for jumping, burrowing, or swimming. Most amphipods are small, 0.2-0.6 in (5-15 mm) long, but deep sea benthic forms can reach over 9.8 in (25 cm) in length.

but 1,200 species are known to inhabit fresh water, and almost 100 species are terrestrial.

Habitat

Most amphipods are benthic, living in burrows of mud or among detritus. Some live in fresh water among decaying leaves. Others live among sand grains on beaches. Oceanic forms are found in the water column, living the majority of their lives associated with gelatinous Zooplankton (jellies, ctenophores, and thalicean tunicates).

Behavior

Gammarids live under decaying leaves or can make burrows in sand or mud. Hyperiids live at least part of their lives associated with gelatinous Zooplankton. Caprellids attach themselves to algae, hydroids, and other small structures. Cyamids live as ectoparasites on marine mammals in species specific relationships.

Feeding ecology and diet

Amphipods can be herbivores, carnivores, or scavengers. In many instances, amphipods help breakdown decaying animals and plants. Hyperiid amphipods live most of their lives attached to gelatinous Zooplankton, and Phronima eats the inside of thalicean tunicates, fashioning the remaining tunic into a barrel that it uses as a brood chamber. Cyamid amphipods eat the skin of the marine mammals they live on.

Distribution

Amphipods are a diverse group of crustacean arthropods found in virtually all habitats of the world. Most are marine

Reproductive biology

In many amphipods fertilization takes place when the male attaches to a female, transferring sperm to her genital duct.

Scuds mating. (Photo by Tom Branch/Photo Researchers, Inc. Reproduced by permission.)

Fertilized eggs are incubated in the female's ventral brood chamber formed by modified thoracic appendages. Development is direct so the newly hatched amphipods look much like their parents.

Conservation status

As a group, no amphipods are known to be in danger of extinction, and none are listed by the IUCN. Those that are ectoparasites in species-specific relationships with endangered marine mammals are at risk.

Significance to humans

In many habitats amphipods are important in breaking down decaying matter. They are an important part of the food chain for some commercially harvested species.

1. Cooper of the sea (Phronima sedentaria); 2. Skeleton shrimp (Caprella californica); 3. Gray whale lice (Cyamus scammoni); 4. Sperm whale lice (Neocyamus physeteris); 5. Hyperia galba; 6. Scina borealis. (Illustration by John Megahan)

1. Cystisoma fabricii; 2. Gammarus lacustris; 3. Rhabdosoma brevicaudatum; 4. Pleustes platypa; 5. Beach hopper (Orchestoidea californiana). (Illustration by John Megahan)

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