Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca
Order Thermosbaenacea

Common names: thermosbaenaceans, cave microshrimps

Overview

Thermosbaenaceans, or cave microshrimps, are a rare group of small, slender crustaceans found in subterranean waters. They may or may not have eyestalks, but never have eyes (they are blind). The first pair of antennae (antennules) is branched (biramous), with a long, whip-like exopod, and the second pair of antennae are uniramous (unbranched). A short, shield-like carapace covers the head and segmented thorax. The carapace of adult females has a large, bulging dorsal area that forms a brood chamber. The first thoracic segment of both males and females is fused to the head. The thorax has five to seven pairs of leg-like biramous pereopods, usually used for walking and sometimes swimming. The abdomen has six segments, but only the first two have pleopods, which may be well developed, narrow and rounded, or very small. At the end of the abdomen is a pair of slender, cylindrical uropods and a telson (tail). The telson and uropods do not join together to form a fanlike tail. Like the similarly subterranean, blind spelaeogriphaceans, thermosbaenaceans are unpigmented (colourless and typically transparent), small crustaceans >5 mm in length.

Distribution and diversity

Thermosbaenaceans are known from the Mediterranean, the Caribbean, Cambodia, Southern Japan and Australia. Worldwide there are about 34 described species, in seven genera from four families. There is only one Australian thermosbaenacean, Halosbaena tulki, a tiny species (1�1.5 mm long) recorded from a cave on the North West Cape Peninsula, Western Australia.

Life cycle

Little is known about the reproduction of Thermosbaenaceans. Males and females are known and the female carries the eggs until hatching in a dorsal brood pouch formed by the swollen carapace. The larvae resemble adults when they hatch and development into adults is via several moults.

Feeding

Like spelaeogriphaceans, thermosbaenaceans are thought to use their brush-like mouthparts to sweep up into their mouths small plant particles plants and other materials off rocks and the bottom that are washed into caves and underground springs. Species living in hot springs graze on cyanobacteria and other organisms growing underwater on rocks.

Ecology

Thermosbaenaceans occur in fresh or brackish underground springs or limestone caves, along the seashore, in anchialine waters (with a subterranean connection to the ocean) and in thermal springs. A few species live in hot springs at temperatures of 44 to 48�C. Thermosbaenaceans do not burrow and rarely swim, instead mostly using their pereopods for walking over the sediment and rocks. Because of their distribution, rarity and geological age, thermosbaenaceans (like spelaeogriphaceans) are of particular interest to scientists studying endemic species and the evolution of crustaceans. Like many groundwater invertebrates, thermosbaenaceans may prove functionally important by maintaining water quality and flow by breaking down plant and animal fragments that wash into the ground from the surface, and accordingly, are threatened by water abstraction and contamination.