Redescription of the Rio skate Rioraja agassizii (Rajiformes: Arhynchobatidae) with notes on internal anatomy and intraspecific variation

Abstract

Rioraja agassizii belongs to the monotypic genus Rioraja and differs from the other Riorajini species of the genus Atlantoraja by the shape of anterior nasal flap, squamation, clasper skeleton, body measurements and color pattern. Although R. agassizii is a well-known species, commonly captured in inshore waters of the southwestern Atlantic, its external morphology and internal anatomy have never been fully described and little is known on intraspecific variation within the species. Juvenile and adult, male and female specimens were examined, measured and dissected. Herein, we redescribe the species and present detailed descriptions and illustrations of external morphology, neurocranium, jaws, hyoid and gill arches, and pelvic fin and girdle. Additionally, notes on sexual dimorphism and ontogenetic differences in coloration, body and cranial measurements, and dermal denticles are provided.

Phylogeny of the tribe Riorajini with comments on the morphological character evolution within Rajiformes (Chondrichthyes, Batoidea)

“…McEachran and Dunn (1998) coded two different states for the dermal denticle covering of the body dorsal surface in rajiforms (character 11, largely covered or largely naked). Both character states are not precise in expressing the condition observed in taxa examined herein and do not distinguish between thorns and prickles (Leible, 1988; Soares, Moreira, & Gomes, 2021). As defined by Leible (1988), prickles are diminute dermal denticles, usually smaller than 1 mm with narrow base and acute edges.…”

“…Leible (1988) and Soares, Moreira, and Gomes (2021) used the position of thorns along the body dorsal surface to name these structures and make comparisons among skate species. Well‐developed thorns along the rostral region and anterior to orbits were observed in A. radiata , C. rugosa , R. equatorialis and P. extenta (14, 1).…”

“…Morphological descriptions were based in our observations and data obtained from literature. Terminology for the neurocranium was based in Hulley (1972) and Leible (1988); scapulocoracoid follows McEachran and Compagno (1979); visceral skeleton, Gillis et al (2009); claspers, Moreira et al (2017); squamation, Soares, Moreira, and Gomes (2021).…”

“…Meristic and morphometric data were normalized and concatenated with the other characters (Goloboff et al, 2006). Fifty two characters were compiled from Hulley (1972), Hubbs and Ishiyama (1968), Gomes and Paragó (2005), Moreira et al (2017), Soares, Moreira, and Gomes (2021); Soares, Moreira, Silva, et al (2021) or modified from McEachran and Dunn (1998). 28 new characters (2, 3, 4, 6, 7–11, 14, 24, 26, 27, 30, 31, 33, 34, 38, 41–43, 49–51, 55, 57, 63 and 71) are proposed and cladistically analysed for the first time herein.…”

“…Moreover, some characters analysed in that study are not independent, or their state characters are not mutually exclusive, requiring a reexamination and reinterpretation of these. Detailed descriptions on anatomical structures and ontogenetic changes in species of Rioraja and Atlantoraja were recently provided (Moreira et al, 2017; Soares et al, 2020; Soares, Moreira, & Gomes, 2021; Soares, Moreira, Silva, et al, 2021) but phylogenetic relationships within the tribe remain not fully understood since no cladistic analysis including morphological characters has ever been performed.…”