Leeches

In their mode of locomotion and hermaphrodite reproductive arrangement leeches resemble monogeneans and some leeches ventilate their bodies by means of body undulations (Mann, 1962) in a similar way to the monogenean Entobdella soleae (Chapter 3). However, this resemblance is superficial and leeches and monogeneans are unrelated and structurally very different. The body of a leech consists of a series of segments (metameres), each of which contains the same basic components (gut, muscles, nerves, excretory organs). This repetitive segmental arrangement is a feature of the Annelida, the phylum to which the leeches belong. It has long been recognised that leeches are closely related to the oligochaetes, a group of annelids that includes the familiar earthworm Lumbricus terrestris. In fact, the Hirudinea (leeches) and the Oligochaeta were grouped together in the same taxon, the Clitellata by Sawyer (1986ab), on the basis that members of both groups possess a specialised annular region or clitellum on the body surface that secretes the cocoon (see below). However, recent molecular studies by Siddall et al. (2001) have taken this further, placing the leeches firmly within the Class Oligochaeta. Separate status in the Oligochaeta is indicated for the Hirudinea, the branchiobdellidans (found predominantly on freshwater crustaceans and of no concern to us here) and the acanthobdellidans (leech-like fish parasites regarded as primitive – see below). According to Siddall et al. (2001), all three of these groups should be regarded as orders of the Oligochaeta, equal in status to their closest relatives the Lumbriculida. However, these changes have not yet been formally proposed. Leeches are divided into two groups, the Arhynchobdellida and the Rhynchobdellida. As well as blood feeders, both groups contain predatory leeches that do not feed on blood. However, there are fundamental differences in the way in which blood-feeding arhynchobdellidans and rhynchobdellidans extract blood from their hosts. Arhynchobdellidans include the familiar medicinal leech, Hirudo medicinalis, which is known to feed occasionally on fishes (see below). Hirudo uses jaws to gain access to blood and prevents clotting while feeding by injecting saliva containing a nonenzymatic polypeptide called hirudin, which specifically inhibits the host’s clotting enzyme thrombin. The rhynchobdellidans include all other British leeches that feed on fishes and a giant Amazonian leech Haementeria ghilianii that readily feeds on humans as well as on cattle. Rhynchobdellidans extract blood via a tubular proboscis lacking jaws and employ different biochemical methods to prevent clotting. H. ghilianii does this by secreting an enzyme, hementin, which prevents clots from forming and also dissolves new clots after their formation. According to Sawyer (1986b), these fundamental morphological and biochemical differences between bloodsucking arhynchobdellidans and rhynchobdellidans indicate that the bloodsucking lifestyle has evolved independently in the two groups. However, Apakupakul et al. (1999) found evidence to suggest that the common ancestor of all the leeches was a blood feeder and that the blood-feeding habit has been lost at least four times during the course of leech evolution, twice in the rhynchobdellidans and twice in the arhynchobdellidans.