Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Arthropoda
Subphylum Crustacea
Class Ostracoda

Common names: ostracods, seed shrimps, mussel shrimps

Overview

Ostracoda, or seed or mussel shrimps, are small aquatic crustaceans that resemble tiny bivalve molluscs. Ostracods appear similar to conchostracans (clam shrimps) and can be separated from them by the presence of relatively few thoracic appendages. Like conchostracans, ostracods have a modified bivalved carapace that totally encloses the body in a clam-like fashion. The carapace halved hinge dorsally, where the body is attached to the carapace. Adductor muscles attach laterally to the carapace halves and allow for the carapace to be closed tightly. Ostracods have significantly reduced bodies with two pairs of antennae, two pairs of mouthpart appendages, three pairs of leg-like thoracic appendages, and a pair of furcal claws. Both the first and second antennae (antennules, antennae) are well developed and are the most conspicuous appendages, protruding beyond the carapace. The rest of the body is very short and ends in a multi-spiked tail (furca). Most ostracods are very small, typically 0.5�2.0 mm long however, some interstitial forms are as small as 0.2 mm and some freshwater species attain 8 mm. Although most appear to be translucent or pale in colour, the texture and patterns of the carapace can be quite variable and is an important taxonomic feature.

Distribution and diversity

Ostracods are common and widespread but often overlooked because of their small size. They are found worldwide in most aquatic habitats with the highest abundance and diversity in the ocean. Most species live on the sea floor but a few are planktonic. Freshwater species occur in lakes, pools, streams and groundwater and a few terrestrial (or semi-terrestrial) species are found in damp leaf litter and soil in forests in Australia, New Zealand, western Pacific Islands, Africa and South America. Ostracods are considered to be one of the most speciose groups of living Crustacea with an estimated >20,000 species, of which around 8,000 have been described. Around 2,000 of these are non-marine. Australia has many hundreds of species (many undescribed) with some of the highest levels of freshwater species endemicity. Identification of species, or even families, requires excellent dissection skills and a powerful microscope.

Life cycle

Most ostracods reproduce sexually although some freshwater species are parthenogenic. Males deposit sperm into the female�s seminal receptacle. Females only need to mate once to produce several broods during their lives. They typically deposit the eggs into the posterior of the carapace, where they are brooded. Non-brooding species typically release eggs freely into the water column, or deposit them onto suitable surfaces. Some freshwater species that inhabit temporary water-bodies produce desiccation-resistant, dormant eggs. An unusual feature of ostracod development is the presence of the carapace in all life stages. Development follows a single nauplier stage (with three pairs of appendages), six metanauplier stages and the adult phase, appendages are usually added with successive moults. Despite being relatively small animals, ostracods can live for more than 2 years but only moult four or five times.

Feeding

Ostracods are mostly filter feeders. A water current is typically formed by the legs, with water being drawn into the carapace anteriorly and exiting the carapace posteriorly. Edible particles are filtered from the water current and are passed to the mouth. Some species are predators and have strong claws for grasping small prey. Others are scavengers on fish carcasses.

Ecology

Ostracods can be found in deep oceans, shelf seas, estuaries, lagoons, salt marshes, freshwater and saline lakes, temporary ponds, streams, springs, groundwater, water-filled bromeliads and (semi-) terrestrial habitats such as damp mosses, soil and leaf litter in forests. Several freshwater habitats that are hotspots for ostracod diversity and endemicity include ancient lakes, groundwater, temporary pools and Australian salt lakes. Ostracods have a long and well-documented fossil record from the Cambrian (over 500 Ma) to the present day. Ostracods are highly diverse and most species exist in only a narrow range of environmental parameters. This sensitivity to environmental parameters has made them useful biological indicator species of environmental changes.