Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Arthropoda
Subphylum Crustacea
Class Branchiopoda
Order Anostraca

Common names: anostracans, brine shrimps, fairy shrimps

Overview

Anostraca, or brine or fairy shrimps, are mostly small, aquatic crustaceans. The head consists of two sections, one with the antennae and eyes, and the second bearing the jaws (mandibles and maxillae). The first pair of antennae (antennules) are typically long but unsegmented. The second pair is more muscular, especially in males that use their antennae to grasp females while mating. In some males these have numerous, odd-shaped outgrowths. At the sides of the head is a pair of stalked compound eyes and an unpaired naupliar eye at the top of the head. In most anostracans the thorax consists of 11 segments (a few have 17-19 thoracic segments) with each segment bearing a pair of paddle- or leaf-like legs (phyllopodia). They are all similar in structure, usually only vary slightly in size, becoming smaller posteriorly and every leg has two or three lobes on the outer side, which contain the breathing organs, and one lobe for paddling. On the inner side are six lobes that push the water to the mouth opening. On the ventral side of the body is a deep groove inside which the water flows to the mouth. Although most anostracans are small (less than 10 mm), the largest species are over 150 mm long.

Distribution and diversity

Anostracans occur worldwide in fresh and saltwater ephemeral pools. There are around 270 described species and the Australian fauna comprises three families and around 30 species that occur from freshwater to inland saline lake habitats.

Life cycle

During mating, the male swims under the female and grasps her with his antennae. Fertilisation is internal and he may hold on to her for minutes to several days. Females release the eggs into paired egg sacks on their abdomen 2-3 days after mating, after which they are released into the water where they sink to the bottom. Due to their marginal, ephemeral habitats, females can produce two types of eggs: thin shelled �summer� eggs and thick-shelled �winter� eggs. Summer eggs hatch rapidly and the young form while still inside the brood sac which enables anostracans to utilise even short periods of flooding. The young from these eggs will rapidly populate the water body during the same season they are laid. Winter eggs remain in the mud and dry out as their environment does. The eggs will remain dormant until their habitat is re-flooded and can be dispersed by animals and wind. Though the resting period usually varies between 6 to 10 months, eggs have been hatched in a laboratory after 15 years.

Feeding

Anostracans are thought to be mostly filter feeders, feeding on bacteria, algae, rotifers and other crustacean larvae (nauplii). Food particles are filtered from the water by the constant motion of the thoracic legs. Larger species are known to be predatory, using raptorial phyllopodia or large antennae to capture and feed on smaller aquatic invertebrates, including other anostracans.

Ecology

Anostracans are usually found in freshwater and saline temporary pools and salt lakes which often exhibit extreme ranges of salinity and temperature. Anostracans are mostly planktonic and swim in a characteristic upside-down orientation with the legs directed dorsally beating in a continuous series of waves. They do not compete well with other invertebrates and are readily consumed by fishes. As such, they typically do not occur in water bodies where fishes and predatory invertebrates are numerous. Anostracans can be an important food supply for migratory water birds that breed near flooded salt lakes. The cysts (winter eggs) of species of Artemia are harvested, cleaned, dried, packed and sold as aquarium and aquaculture fish food. Adult brine shrimp are also harvested as a food for aquarium fishes. Anostracans (also usually Artemia) are also sold as short-lived aquarium pets (�sea monkeys�).