- Source: Sewellia
Sewellia is a genus of fish in the family Gastromyzontidae. They are commonly called hillstream loaches (a common name shared with family Balitoridae) and are found in Laos and Vietnam. According to Roberts (1998) a possible defining characteristic of the genus Sewellia is their posterior pelvic valve is highly complex and different from other rheophilic loaches (loaches adapted to fast flowing water), their posterior pelvic valve involves the overlapping of the second to last ventrally exposed rays. Adapted to high velocity streams, Sewellia have depressed body shapes that are laterally expanded, thus increasing their hydrodynamical properties and allowing them to better stay attached to rocks in swift current. Sewellia also have paired fins with a single simple ray and numerous branched rays allowing them to grip rocks in swift current.
Distribution
Fish in the genus Sewellia occur in the Sekong River, a tributary of the Mekong river. The Sekong river drains large parts of central and southeast Laos as well as southern Vietnam.
Habitat and ecology
Sewellia are found in the rapids and riffles of steep gradient 'hill streams', hence the common name Hillstream Loach. Due to their steep gradients these streams have high current velocities, Sewellia are known to inhabit streams with current velocities exceeding m/s in some habitats. Sewellia feed mainly on micro to macro sized aquatic invertebrates that inhabit the periphyton attached to submerged rocks.
Sexual dimorphism
Sewellia are sexually dimorphic, males have soft elevated patches of fine tubercles on the anterior part of pectoral-fin rays 1-6, females do not. Sewellia lineolata are common in the aquaria trade, they are sexed based on differences in shape, at sexual maturity males tend to be more streamlined while females are somewhat broad; the beginning of the pectoral fins occurs at a much sharper angle in the male than females, females have pectoral fins that begin in a more rounded fashion (softer angle).
Species
There are currently 13 recognized species in this genus:
Sewellia albisuera Freyhof, 2003
Sewellia analis H. D. Nguyễn & V. H. Nguyễn, 2005
Sewellia breviventralis Freyhof & Serov, 2000
Sewellia diardi T. R. Roberts, 1998
Sewellia elongata T. R. Roberts, 1998
Sewellia hypsicrateae Endruweit & T. D. P. Nguyễn, 2016
Sewellia lineolata (Valenciennes, 1846)
Sewellia marmorata Serov, 1996
Sewellia media H. D. Nguyễn & V. H. Nguyễn, 2005
Sewellia monolobata (H. D. Nguyễn & V. H. Nguyễn, 2006)
Sewellia patella Freyhof & Serov, 2000
Sewellia pterolineata T. R. Roberts, 1998
Sewellia speciosa T. R. Roberts, 1998
Sewellia trakhucensis H. D. Nguyễn & V. H. Nguyễn, 2005
Conservation status
Sewellia albisuera - IUCN Critically Endangered
Sewellia analis - IUCN Not Evaluated
Sewellia breviventralis - IUCN Critically Endangered
Sewellia diardi - IUCN Data Deficient
Sewellia elongata - IUCN Near Threatened
Sewellia hypsicrateae - IUCN Not Evaluated
Sewellia lineolata - IUCN Vulnerable
Sewellia marmorata - IUCN Endangered
Sewellia medius - IUCN Not Evaluated
Sewellia patella - IUCN Endangered
Sewellia pterolineata - IUCN Endangered
Sewellia speciosa - IUCN Least Concern
Sewellia trakhucensis - IUCN Not Evaluated