Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca
Order Bathynellacea

Common name: bathynellids

Overview

Bathynellacea are small, aquatic crustaceans that occur in ground water and among sediments and sand. They have a worm-like (vermiform) body and, as an adaptation to their subterranean habitats, no eyes or pigmentation. The first pair of antennae (antennules) are uniramous (unbranched) and the head of some species may have a beak-like projection (rostrum). The thorax and abdomen are differentiated with the eight-segmented thorax having 7 or 8 pairs of simple biramous (branched) legs. The six-segmented abdomen may or may not have one or two pairs of small limbs on the end of the body called uropods that are on either side of a central projection (telson). They range in size from 0.5 to 3.5 mm.

Distribution and diversity

Bathynellaceans occur worldwide with around 150 described species. However, they are not known from Antarctica, Central America, from islands that are volcanic in origin, and from some other islands, such as New Caledonia, Fiji, and the Caribbean islands. There are two families, Bathynellidae and Parabathynellidae, both recorded from Australia, with around 10 described species and many more undescribed. Many species are known to have very limited distributions.

Life cycle

Although both male and female bathynellaceans are known, their mating behavior has never been observed. Unlike most crustaceans that carry their eggs or young for at least some period of time, female bathynellaceans lay 1 or 2 large eggs in the surrounding sand or sediment. Larval development is anamorphic, where the napulis (larvae) hatches with only working antennae and mouthparts and additional appendages are added with each molt to reach adulthood. The number of molts varies among species.

Feeding

Bathynellaceans eat plant and animal material washed in from the surface as well as other microscopic aquatic animals and bacteria. There is great variation in types of mouthparts making it likely that some species may be specialists and feed on just one or two of these groups. One species has mouthparts with setae to scraping off material from sand grains.

Ecology

Bathynellaceans are interstitial organisims living amongst sediments in brackish to fully saline water in lakes, groundwater or in caves, with a few species found in beach sand. At least one African species lives in hot springs. Although they are very awkward swimmers in open water, bathynellids are extremely agile moving via a combination of swimming and walking through the constricted spaces between sediments where they live. Like many groundwater invertebrates, bathynellaceans may prove important in maintaining to water quality and flow through the filtering sands that surround underground springs by breaking down plant and animal fragments that wash into the ground from the surface.