Rhina ancylostoma, the bowmouth guitarfish, shark ray or mud skate, is a species of ray and a member of the family Rhinidae. Its evolutionary affinities are not fully resolved, though it may be related to true guitarfishes and skates. This rare species occurs widely in the tropical coastal waters of the western Indo-Pacific, at depths of up to . Highly distinctive in appearance, Rhina ancylostoma has a wide and thick body with a rounded snout and large shark-like dorsal and tail fins. Its mouth forms a W-shaped undulating line, and there are multiple thorny ridges over its head and back. It has a dorsal color pattern of many white spots over a bluish gray to brown background, with a pair of prominent black markings over the pectoral fins. This large species can reach a length of and weight of .
Usually found near the sea floor, Rhina ancylostoma prefers sandy or muddy areas near underwater structures. It is a strong-swimming predator of bony fishes, crustaceans, and molluscs. This species gives live birth to litters of two to eleven pups, which are nourished during gestation by yolk. The International Union for Conservation of Nature (IUCN) has assessed Rhina ancylostoma as Critically Endangered because it is widely caught by artisanal and commercial fisheries for its valuable fins and meat. It is viewed as a nuisance by trawlers, however, because its bulk and thorny skin cause it to damage netted catches. Habitat degradation and destruction pose an additional, significant challenge to this ray's survival. Rhina ancylostoma adapts well to captivity and is displayed in public aquariums.
Taxonomy and phylogeny
German naturalists Marcus Elieser Bloch and Johann Gottlob Schneider described Rhina ancylostoma in their 1801 Systema Ichthyologiae. Their account was based on a long specimen, now lost, collected off the Coromandel Coast of India. The genus name Rhina comes from the Greek rhinos ("snout"); the specific epithet ancylostoma is derived from the Greek ankylos ("curved" or "crooked") and stoma ("mouth"). Although Block and Schneider wrote the epithet as ancylostomus and that form appears in some literature, most modern sources regard the correct form to be ancylostoma. Other common names for this species include shark ray, mud skate, shortnose mud skate, bow-mouthed angel fish, and bow-mouthed angel shark.
The evolutionary relationships between Rhina ancylostoma and other rays are debated. Morphological evidence generally points to a close relationship between Rhina, Rhynchobatus and Rhynchorhina, which are a group of rays known as the wedgefishes that also have large, shark-like fins. Morphological analyses have tended to place these two genera basally among rays, though some have them as basal to just the guitarfishes (Rhinobatidae) and skates (Rajidae) while others have them basal to all other rays except sawfishes (Pristidae). A 2012 study based on mitochondrial DNA upheld Rhina and Rhynchobatus as sister taxa related to the guitarfishes, but also unexpectedly found that they formed a clade with the sawfishes rather than the skates. Following the description of Rhynchorhina in 2016, a study of mtDNA found that it is part of the same group and their phylogenetic relationship is ((Rhynchobatus+Rhynchorhina)+Rhina).
In terms of classification, Bloch and Schneider originally placed the bowmouth guitarfish in the order Abdominales, a now-obsolete grouping of fishes defined by the positioning of their pelvic fins directly behind the pectoral fins. Modern sources have included it variously in the order Rajiformes, Rhinobatiformes, Rhiniformes, or the newly proposed Rhinopristiformes. The placement of the bowmouth guitarfish in the family Rhinidae originates from the group "Rhinae", consisting of Rhina and Rhynchobatus, in Johannes Müller and Jakob Henle's 1841 Systematische Beschreibung der Plagiostomen. Later authors have also assigned this species to the family Rhinobatidae or Rhynchobatidae. Joseph Nelson, in the 2006 fourth edition of Fishes of the World, placed this species as the sole member of Rhinidae in the order Rajiformes, which is supported by morphological but not molecular data. More recent authorities have placed it in Rhinidae together with Rhynchobatus and Rhynchorhina, reflecting both genetic data and the morphologically intermediate position of Rhynchobatus between Rhina and Rhynchorhina.