Key to Australian Freshwater and Terrestrial Invertebrates
Phylum Annelida
Class Clitellata
Order Arhynchobdellida
Common names: freshwater and terrestrial leeches, �proboscisless leeches�, �jawed leeches�
Overview
Arhynchobdellida are freshwater and terrestrial leeches without an eversible proboscis. Many have three jaws armed with sharp teeth used for attaching to prey and feeding on their blood (�Gnathobdellae�). The group also includes freshwater predatory leeches that are jawless (�Pharyngobdellae�). Arhynchobdellids are relatively stout, slightly flattened annelids that are broader posteriorly, tapering anteriorly. They have a small anterior sucker filled by the mouth, and a larger posterior sucker. Depending on the family, they have three to eight pairs of eyes, along the dorsal margin of head area. Unlike most annelids, leeches have reduced and consistent numbers of body segments, though these are usually obscured by additional annulations (rings) on the epidermis. They have a clitellum, which only becomes conspicuous during reproduction and egg laying and are usually brown, olive green or blackish in colour, and can sometimes possess striking pale, red or yellow longitudinal stripes. Most terrestrial leeches are small and rarely exceed 30mm in length when resting, however aquatic members of this order are frequently larger (up to 150 mm). Despite their relatively small size, they are capable of extreme stretching and contraction. Many leeches have a characteristic mode of moving, looping using their suckers. Many aquatic species are also able to swim by flattening their body and using dorsoventral undulations of the body to propel the leech in an 'eel-like' motion.
Distribution and diversity
Arhynchobdellida occur worldwide in a variety of tropical, subtropical and temperate freshwater and forest/woodland habitats with 10 families comprising around 200 freshwater and 90 terrestrial species. The Australian fauna consists of three freshwater families found throughout the continent: Erpobdellidae (three genera each with a single species; Barbronia weberi introduced from southeast Asia), Hirudinidae (nine genera with 12 species) and Ornithobdellidae (one species); and one terrestrial family, Haemadipsidae (12 species in 10 genera) that occurs across eastern Australia.
Life cycle
Arhynchobdellida are sexually reproducing hermaphrodites, with some species producing sperm and eggs at the same time, while others are protandrous hermaphrodites, first functioning as a male producing sperm and then as a female for egg production. Like oligochaetes, arhynchobdellids lay their eggs in a chitinous cocoon formed by the clitellum. Aquatic species bury their cocoon in the substrate or attach it to submerged rocks, logs or plants, while some Hirudinidae species deposit their cocoons in terrestrial habitats to protect them from aquatic predators such as snails, insect larvae and fish. Terrestrial species produce cocoons with a special protein coat that helps protect from desiccation and are usually placed under a rock or in moist soil or leaf litter. Development is direct with juveniles growing continuously until adulthood with sexual maturity reached within a few months up to one or two years depending on the species. Reproductive structures are important characters for delineating families, genera and species.
Feeding
Sanguivorous leeches (Hirudinidae, Ornithobdellidae and Haemadipsidae) are temporary ectoparasites that feed on the blood of fish, turtles, crocodiles, amphibians, waterfowl and mammals. Most are not host specific and a few species are also known to feed on invertebrates including oligochaetes, snails and other leeches. All terrestrial leeches (Haemadipsidae) are blood feeders and feed mostly on warm-blooded vertebrates. The mouth, surrounded by the anterior sucker, possesses two or three sharp �jaws� that are used to make an incision in soft skin. When blood begins to flow, the leech drinks through contractions of the muscular pharynx. The saliva contains anticoagulants to promote blood flow and an anesthetic that allows leeches to complete feeding without detection. Symbiotic bacteria occur in the intestine that aid in the digestion by producing enzymes that aid in the breakdown of blood. Leeches can consume several times their weight in blood in a single feed, however they feed infrequently and many can sustain growth with as few as two or three feeds a year. Erpobdellidae are macrophagous predators that feed on chironomids, oligochaetes, amphipods, and molluscs. They swallow prey organisms whole or in pieces and secrete digestive enzymes to break down the fluids and tissues of their prey.
Ecology
Terrestrial leeches occur in moist, forested habitats where they are often found in great numbers, clinging to plants or under rocks and attaching themselves to passing warm-blooded animals. They are active only when ground moisture is suitable, remaining inactive at other times. Freshwater leeches occur in both lotic (running water) or lentic (standing water) habitats as a part of the benthic (bottom-dwelling) community on mud or silt. Predacious leeches are found attached to submerged objects such as wood, stones and aquatic vegetation. They are nocturnal and often cryptically coloured. In general, leeches can tolerate a broad range of salinities, temperatures, and oxygen concentrations. Leeches are vectors of blood parasites of fishes, turtles and frogs, but as humans are only incidental hosts for leeches, these cannot survive in humans. However, bacteria, viruses, and parasites from previous human blood sources (including HIV and hepatitis B) have been found to survive within a leech for a short period. Members of the Hirudinidae have a long history of use in medicine and in modern times, leeches are used in the rehabilitation of re-attached small body parts and plastic surgery. The European medicinal leech (Hirudo medicinalus) are sometimes encouraged to feed on the re-attached part as the anticoagulant properties in the saliva allows blood to move through the re-attached part, restoring blood flow and oxygen.
