Rhynchobdellida
Encyclopedia
Rhynchobdellida, the jawless leeches, are classified as an order
of the Hirudinea. But leech taxonomy
and systematics
will eventually be revised in due time, not because there are many uncertainties about their phylogeny, but because the major clade
s of clitellate
annelid
s – and whether the clitellates are themselves a clade – have not been fully elucidated. For example, the "true leeches" (Euhirudinea
) might actually be synonymous with the Hirudinea, as all other leech-like annelids might not be very closely related to the true leeches.
Compared to their sister taxon, the Arhynchobdellida
(proboscisless leeches), rhynchobdellid systematics
has always been rather straightforward as regards the main clades contained therein. The common name is actually misleading however, as jawlessness is a primitive trait among leeches in general – most Arhynchobdellida are in fact jawless too. It is rather the proboscis that is the characteristic apomorphy of the Rhynchobdellida.
Two or three families
are recognized: The Glossiphoniidae
are freshwater
leeches, flattened, and with a poorly defined anterior sucker. The Piscicolidae occur in seawater
, have cylindrical bodies, and a usually well-marked, bell-shaped, anterior sucker. The Ozobranchidae are not universally accepted as distinct; some authors prefer to merge them with the Piscicolidae.
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of the Hirudinea. But leech taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
and systematics
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
will eventually be revised in due time, not because there are many uncertainties about their phylogeny, but because the major clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
s of clitellate
Clitellata
Clitellata is a class of Annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycle. The clitellates comprise around 8,000 species...
annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s – and whether the clitellates are themselves a clade – have not been fully elucidated. For example, the "true leeches" (Euhirudinea
Euhirudinea
Euhirudinea, the true leeches, are an infraclass of the Hirudinea. These clitellate annelids are of somewhat unclear relationships; namely the relationships of Hirudinea with oligochaetes are in need of revision...
) might actually be synonymous with the Hirudinea, as all other leech-like annelids might not be very closely related to the true leeches.
Compared to their sister taxon, the Arhynchobdellida
Arhynchobdellida
The proboscisless leeches, Arhynchobdellida, are classified as an order of the Hirudinea. But leech taxonomy and systematics will eventually be revised in due time, not because there are many uncertainties about their phylogeny, but because the major clades of clitellate annelids - and whether the...
(proboscisless leeches), rhynchobdellid systematics
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
has always been rather straightforward as regards the main clades contained therein. The common name is actually misleading however, as jawlessness is a primitive trait among leeches in general – most Arhynchobdellida are in fact jawless too. It is rather the proboscis that is the characteristic apomorphy of the Rhynchobdellida.
Two or three families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
are recognized: The Glossiphoniidae
Glossiphoniidae
Glossiphoniidae are the family called freshwater jawless leeches or glossiphoniids. They are one of the main groups of Rhynchobdellida, true leeches with a proboscis. These leeches are generally flattened, and have a poorly defined anterior sucker...
are freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
leeches, flattened, and with a poorly defined anterior sucker. The Piscicolidae occur in seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
, have cylindrical bodies, and a usually well-marked, bell-shaped, anterior sucker. The Ozobranchidae are not universally accepted as distinct; some authors prefer to merge them with the Piscicolidae.