Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca
Order Amphipoda

Common names: amphipods, groundhoppers, landhoppers, beachhoppers

Overview

Amphipoda are a diverse order of small, shrimp-like crustaceans that mostly occur in the ocean, but also in freshwater and some terrestrial habitats. Although there is wide variation in body form, they are typically laterally compressed with chewing mouthparts and well-developed pairs of first and second antennae with a long, whip-like exopod on the first pair (antennules). The eyes usually are well developed and may be stalked or sessile, but in some species they may be reduced or absent (ie. they are blind). They do not have a carapace and the thoracic segments make up most of the body with 7 (rarely 6) of the segments distinct, bearing the gills and leg-like appendages. The thoracic legs of amphipods (pereopods) are modified into three types. The first two pairs (gnathopods) and are generally used for grasping. The remaining periopods are used for walking, jumping and burrowing with pereopods 3 and 4, facing posteriorly and the last 3 pairs, which are usually stouter, are directed backwards. The abdominal segments are typically fused and there are only three pairs of brush-like pleopods (abdominal legs). The fourth and fifth pairs are modified to form short uropods so that amphipods have three pairs of uropods. Freshwater and terrestrial species are typically are small, 5�15 mm long, and translucent, pale or brown.

Distribution and diversity

Amphipods occur worldwide with most being benthic (living on the ocean floor). There are also many species found in a variety of freshwater habitats and a few terrestrial species that are restricted to tropical to temperate forests in the southern hemisphere where they inhabit moist leaf litter. Amphipods are an extremely abundant and diverse order with over 7,000 described worldwide from over 200 families with probably tens of thousands of species in total. Around 1,200 of these are known to inhabit freshwater, and almost 100 species are terrestrial. There over 700 species described from Australian waters with estimates that this may be less than 50% of the actual number of species.

Life cycle

In aquatic amphipods there are many different types of copulatory behaviour. For example, the mating pair can adopt a �face-to-face� position, the male attached to a female, transferring sperm to her genital duct. In other species the male is attaches his gnathopods to the female�s anterior coxae. The fertilised eggs are then deposited in a ventral brood pouch on the underside of the female�s thorax. Amphipods are unusual among crustaceans in that the entire naupliar (larval) development takes place within the egg. The eggs hatch in one to three weeks and juveniles emerge directly from the eggs. Young amphipods resemble the adults and leave the pouch within 1�8 eight days. Some species show parental care of their young after they leave the brood pouch. Terrestrial amphipods also transfer sperm by direct mating and the female deposits the eggs into her fluid-filled brood pouch, which then hatch and the young are released.

Feeding

Amphipods have chewing mouthparts and feed on algae, bacteria on the surface of particles and dead vegetation and animals, with the vast majority being scavenging omnivores. Food items are manipulated with the front two pairs of pereopods. Predatory species capture other small invertebrates using these legs. Terrestrial amphipods feed mostly at night to avoid desiccation and predation.

Ecology

Freshwater amphipods are found in many freshwater environments such as the littoral zones of lakes and rivers and groundwater where there are many endemic species. They are often found under decaying leaves or in burrows in sand or mud. Similar to isopods, terrestrial amphipods dehydrate easily through their cuticle, and are restricted to damp environments such as moist leaf litter and under logs in forests and woodlands with some species known to occur in moist native grasslands. Several species dig burrows in the soil, or use burrows made by other soil invertebrates. Some species of landhoppers can be found in parks, gardens and around houses in mulch, compost or under dense plantings. Semi-terrestrial amphipods such as sand fleas can often be seen amongst sand and pebbles, under driftwood or on beaches. When disturbed, these species often jump erratically to escape: by digging their uropods into the substrate and flicking their abdomens, hence their common names. Amphipods frequently occur in large numbers, however their cryptic nature often results in them being overlooked. They are almost always a significant component of freshwater environmental surveys and, as omnivorous detritivores, they play an important role in nutrient cycling in the environments that they inhabit.